TITLE PAGE

AlGaN-GaN Single- and Double-Channel High Electron Mobility Transistors

by

Rongming CHU

A Thesis Submitted to The Hong Kong University of Science and Technology

in Partial Fulfillment of the Requirements for the Degree of Master of Philosophy

in Electrical and Electronic Engineering

August 2004, Hong Kong

AUTHORIZATION

I hereby declare that I am the sole author of the thesis. I authorize the Hong Kong University of Science and Technology to lend this

thesis to other institutions or individuals for the purpose of scholarly research. I further authorize the Hong Kong University of Science and Technology to

reproduce the thesis by photocopying or by other means, in total or in part, at the request of other institutions or individuals for the purpose of scholarly research.

________________________ Rongming CHU

ii

AlGaN-GaN Single- and Double-Channel High Electron Mobility Transistors

by

Rongming CHU

This is to certify that I have examined above MPhil thesis and have found that it is complete and satisfactory in all aspects,

and that any and all revisions required by the thesis examination committee have been made.

_________________________________________ Prof. Kevin J. CHEN

Thesis Examination Committee Member (Thesis Supervisor)

_________________________________________ Prof. Kei May LAU

Thesis Examination Committee Member (Chairman)

_________________________________________ Prof. Mansun J. CHAN

Thesis Examination Committee Member

_________________________________________ Prof. Ross D. MURCH

Acting Head of the Department of Electrical and Electronic Engineering

SIGNATURE PAGE

Department of Electrical and Electronic Engineering

The Hong Kong University of Science and Technology

August 2004

iii

ACKNOWLEDGEMENTS

I must acknowledge first, my supervisor Prof. Kevin J. Chen, for his

encouragement, guidance and support throughout my study at HKUST. My work on

GaN transistors is also under the lead of Prof. Kei May Lau. I am grateful to her for

providing me discussion, suggestion and support. I appreciate Prof. Mansun Chan for

serving in my thesis examination committee and providing valuable insight into my

work.

I am indebted to my friend and long-term work partner, Dr. Yugang Zhou, who

has been standing with me and sharing with me his knowledge all the time since I

entered into the field of GaN research. Dr. Zhou spent countless hours growing GaN

samples for HEMT fabrication and none of this research would have been possible

without his expertise in material growth. Many thanks go to Mr. Kwok Wai Chan

and Mr. Kenneth Kin Pin Tsui, who shared with me their hands-on experience on

microwave measurement.

As an important part of my research work, the device fabrication was finished in

the microelectronic fabrication facility (MFF) at HKUST. A number of wonderful

people have made the experience in MFF enjoyable and rewarding. I want to express

my gratitude to Dr. Zhengdong Lu and Mr. Hu Liang, who helped me to get familiar

with basic micro-fabrication techniques at the beginning of my fabrication work. Mr.

Wai Hung Ho is gratefully acknowledged for his enthusiastic help and valuable

advice on device processing, especially on photolithography. It has been a pleasant

time to work with Mr. Jie Liu, Mr. Shuo Jia and Dr. Yong Cai during GaN transistor

fabrication and characterization, and to share with them my knowledge and

experience on GaN transistors.

iv

Magneto-transport measurement of AlGaN/GaN samples was conducted in

Prof. Jiannong Wang’s laboratory at physics department. I sincerely acknowledge

Dr. Wang and her group for their help and support during the measurement. I also

want to acknowledge Dr. Deliang Wang for his help in taking transmission electron

microscope (TEM) observation for our double-channel transistor sample. The TEM

observation results have been very critical in confirming the sample structure and

clarifying the confusion we had made.

My appreciation also extends to Prof. Youdou Zheng, who took me on as my

research advisor during my undergraduate time in Nanjing University. He brought

me into the fantastic world of semiconductor electronics, and has been always

inspiring me to strive for the highest level of professionalism. From Prof. Zheng, I

learned that a qualified researcher should keep open mind and pay persistent effort.

Without Prof. Zheng’s guidance and help, any success in my research career will

never be possible.

Lastly but not least, I want to thank my parents for their love, their endless

support, and all the sacrifices they have made over the years to provide me with the

opportunities to pursue my interest.

v

TABLE OF CONTENTS

Title Page ...................................................................................................................... i

Authorization ............................................................................................................... ii

Signature Page.............................................................................................................iii

Acknowledgements ..................................................................................................... iv

Table of Contents ........................................................................................................ vi

List of Figures ...........................................................................................................viii

List of Tables .............................................................................................................xii

Abstract ........................................................................................................................ 1

Chapter 1 Introduction ................................................................................................. 3

1.1 Principle of HEMTs ..................................................................................... 3

1.2 Development of AlGaN-GaN HEMTs....................................................... 10

1.3 Objective of Synopsis of This Thesis ........................................................ 14

Chapter 2 AlGaN-GaN Single-Channel HEMTs....................................................... 16

2.1 Electronic Properties of AlGaN/GaN Heterostructures ............................. 16

2.2 Device Processing Technologies ............................................................... 21

2.3 DC IV Characteristics ................................................................................ 25

2.4 RF Small-Signal Measurement and Analysis ............................................ 27

2.5 RF Large-Signal Power Measurement ....................................................... 30

Chapter 3 Trap States and Current Collapse in AlGaN/GaN HEMTs....................... 32

3.1 Overview of Trap States in AlGaN/GaN Heterostructures........................ 33

3.2 Characterization of Trap States in AlGaN/GaN Heterostructure............... 35

3.3 Analysis of Current Collapse in AlGaN/GaN HEMTs.............................. 41

Chapter 4 AlGaN-GaN Double-Channel HEMTs ..................................................... 48

vi

4.1 Motivation and Epilayer Design ................................................................ 48

4.2 DC IV Characteristics ................................................................................ 52

4.3 RF Small-Signal Measurement and Analysis ............................................ 54

4.3.1 AC Transconductance and Gate-to-Source Capacitance ................... 56

4.3.2 Output Impedance .............................................................................. 57

4.3.3 Current and Power Gain..................................................................... 58

4.4 Dynamic IV Characterization .................................................................... 62

4.5 RF Large-Signal Power and Linearity Measurement................................. 66

4.5.1 Single-Tone Power Performance ....................................................... 66

4.5.2 Two-Tone Intermodulation Distortion Profile ................................... 69

Chapter 5 Conclusion................................................................................................. 72

5.1 Summary .................................................................................................... 72

5.2 Suggestion of Future Work ........................................................................ 73

Bibliography and References ..................................................................................... 74

Appendix A: Process Flow for The Fabrication of GaN HEMTs.............................. 83

Appendix B: List of Publications during The Study in Master of Philosophy Program

.................................................................................................................................... 88

vii

LIST OF FIGURES

Fig. 1.1.1

Schematic of an AlGaAs/GaAs HEMT and the corresponding conduction band diagram. Location of the 2DEG is shown.

3

Fig. 1.1.2

Conduction band diagrams and 2DEG ppopulations of an AlGaAs/GaAs HEMT under different gate biases.

4

Fig. 1.1.3

Typical DC IV output (left side) and transfer (right side) characteristics of an AlGaAs/GaAs HEMT.

5

Fig. 1.1.4

Equivalent circuit model of the HEMT. Physical origin of each equivalent circuit element is shown.

5

Fig. 1.1.5

Schematic representation of a HEMT operating in Class A.

8

Fig. 1.2.1

Crystal structure of wurtzite GaN with Ga- and N-face polarity.

11

Fig. 1.2.2

Schematic representation of band profiles of AlGaN/GaN heterostructures with varing AlGaN thickness, which demonstrates how surface states participate in the screening of polarization field and contribute to the 2DEG formation.

12

Fig. 2.1.1

Transverse resistance of the AlGaN/GaN sample as a function of magnetic field measured at 1.4 K. The step-like plateaus are due to quantum Hall effect.

17

Fig. 2.1.2

Longitude resistance of the AlGaN/GaN sample as a function of magnetic field measured at 1.4 K. Oscillations in the resistance are due to SdH effect.

18

Fig. 2.1.3

Fast Fourier transformation of Rxx-1/B function. 2DEG densities are derived from the X-axis value of the peak position.

19

Fig. 2.2.1

Layout of a 2×50µm×1µm HEMT.

21

Fig. 2.2.2

Photograph showing the top view of a fabricated HEMT.

21

Fig. 2.2.3

Cross-sectional schematic of an AlGaN-GaN single-channel HEMT.

22

Fig. 2.2.4

TLM measurement results of an AlGaN-GaN single-channel HEMT sample after device processing. Contact resistance and sheet resistivity are derived.

22

Fig. 2.2.5

DC IV characteristics of a Schottky diode fabricated on the baseline AlGaN-GaN single-channel HEMT epilayer.

23

viii

Fig. 2.2.6

CV characteristics of an AlGaN/GaN Schottky diode measured at 3 MHz.

23

Fig. 2.2.7

Electron distribution profile extracted from the CV characteristics.

24

Fig. 2.3.1

DC IV output characteristics of an AlGaN-GaN single-channel HEMT.

25

Fig. 2.3.2

DC IV transfer characteristics of an AlGaN-GaN single-channel HEMT.

25

Fig. 2.3.3

DC IV transfer characteristics of an AlGaN-GaN single-channel HEMT in semilog scale.

26

Fig. 2.4.1

Schematic of the measurement setup for RF small-signal characterization.

27

Fig. 2.4.2

S-parameters of an AlGaN-GaN single-channel HEMT before (open circiles) and after (solid lines) performing open pad de-embedding.

28

Fig. 2.4.3

Frequency dependence of the current gain (|h21|2) and unilateral power gain (U). A comparison was made between results before and after de-embedding.

29

Fig. 2.4.4

fT and fmax of an AlGaN-GaN single–channel HEMT at different gate bias.

29

Fig. 2.5.1

Schematic of the measurement setup for RF large-signal characterization.

30

Fig. 2.5.2

Output power, power gain, and power added efficiency as a function of input power.

31

Fig. 3.1.1

Cross-sectional schematic of an AlGaN-GaN single-channel HEMT showing possible locations of trap states.

33

Fig. 3.2.1

(a) Capacitance and (b) conductance as a function of DC bias at different measurment frequencies for an AlGaN/GaN heterostructure sample with a 37 nm thick Al0.24Ga0.76N layer.

36

Fig. 3.2.2

Equivalent circuit model of an AlGaN/GaN Schottky diode, (a) without considering any trap states, (b) considering the AlGaN/GaN interface states, (c) considering both the interface and the surface trap states.

37

Fig. 3.2.3

Energy band diagrams of an AlGaN/GaN heterostructure in electron accumulation and depletion mode.

38

ix

Fig. 3.2.4

Extraction of interface trap state density (Dit) and time constant (τ) by fitting the normalized conductance (Git/2πf) vs. angular frequency (2πf) dependence at different gate biases.

40

Fig. 3.2.5

Interface state density as a function of the corresponding time constant for AlGaN/GaN samples with different AlGaN barrier thickness and Al content.

40

Fig. 3.3.1

Dynamic IV characteristics of an AlGaN-GaN single-channel HEMT at different quiescent bias points in comparison with DC output IV characteristics.

42 & 43

Fig. 3.3.2

Schematic drawing showing the occurrence of current collapse during the process of an off-to-on transient. Frequency response is delayed by trap states in the gate-to-drain spacing region.

44

Fig. 3.3.3

Schematic showing the post ICP treatment of the AlGaN/GaN HEMT.

45

Fig. 3.3.4

DC IV characteristics of a HEMT before and after ICP treatment.

46

Fig. 3.3.5

Dynamic IV characteristics of a HEMT before and after ICP treatment.

47

Fig. 4.1.1

Simulated conduction band diagrams (a), electron distribution profiles (b), and CV characteristics (c) of two hypothetical AlGaN/GaN/AlGaN/GaN multilayer structures. One is with polarization effect and without doping (solid lines); the other one is modulation n-doped but without polarization effect (dashed lines).

49

Fig. 4.1.2

Cross sectional structure of an AlGaN-GaN double-channel sample.

50

Fig. 4.1.3

Cross-sectional TEM image of a MOCVD grown AlGaN-GaN double-channel HEMT epilayer.

50

Fig. 4.1.4

Electron distribution profile of the double-channel HEMT extracted from measured CV characteristics.

51

Fig. 4.2.1

DC IV output characteristics of a double-channel HEMT.

52

Fig. 4.2.2

DC IV transfer characteristics of a double-channel HEMT.

53

Fig. 4.3.1

Current gain and unilateral power gain of a double-channel HEMT as a function of measurement frequency.

54

Fig. 4.3.2

Equivalent circuit model for parameter extraction of the AlGaN-GaN double-channel HEMT.

55

x

Fig. 4.3.3

Gate bias-dependent gate-to-source capacitance and transconductance of a double-channel HEMT.

56

Fig. 4.3.4

Gate bias-dependent output capacitance and conductance of a double-channel HEMT.

57

Fig. 4.3.5

Gate bias-dependence fT and fmax of a double-channel HEMT.

59

Fig. 4.3.6

Drain bias-dependence fT and fmax of a double-channel HEMT.

60

Fig. 4.3.7

Extraction of effective channel transit delay of the double-channel HEMT.

61

Fig. 4.4.1

Dynamic IV characteristics (circles) of an AlGaN-GaN double-channel HEMT in comparison with DC IV characteristics (black lines). VGS: -8 ~ 1 V.

62& 63

Fig. 4.4.2

Pulsed transfer characteristics of a double-channel HEMT. The pulse width is 1 ms, and the pulse separation is 100 ms.

65

Fig. 4.5.1

POUT, GT, and PAE of a double-channel HEMT measured at 2 GHz. The quiescent bias point is at 15 V VDS and varied VGS. Impedance matching is optimized for maximum POUT.

67

Fig. 4.5.2

Pout, Gt, and PAE of a double-channel HEMT measured at 2 GHz. The quiescent bias point is at -6 V Vgs and varied Vds. Impedance matching is optimized for maximum Pout.

68

Fig. 4.5.3

IM3 of a double-channel HEMT as the function of output power back-off at 20 V VDS and varied VGS. Impedance matching is optimized for maximum POUT.

69

Fig. 4.5.4

Schematic showing the input and output signal waveforms of the double channel HEMT in small- and large-signal operation. The quiescent VGS is at -4 V.

70

xi

LIST OF TABLES

Table I.

Material Properties of GaN in Comparison with Other Semiconductors

9

Table II.

Historical development of GaN HEMTs.

13

xii

AlGaN-GaN Single- and Double-Channel High Electron Mobility Transistors

by Rongming CHU

Department of Electrical and Electronic Engineering

The Hong Kong University of Science and Technology

ABSTRACT

Microwave power transistors made of conventional semiconductors have

already approached their performance limit. In order to meet the future needs of

wireless communication systems, research efforts are being putting on wide bandgap

semiconductors such as SiC and GaN. With combined merits of high power and high

speed, high electron mobility transistors (HEMTs) made of AlGaN-GaN materials

are the subject of this thesis.

Electronic properties of AlGaN/GaN epilayers were characterized to assess the

suitability for HEMT fabrication. Device processing technologies of the baseline

AlGaN-GaN single-channel HEMTs were established in HKUST. HEMTs fabricated

with those epilayers and processing technologies were subjected to extensive testing,

showing satisfactory DC and RF performance.

Though the AlGaN/GaN HEMTs possess superior performance in many aspects,

those devices are usually plagued with current collapse and instability problem,

which greatly limits the power performance. Possible reason leading to current

collapse is the large amount of defect-related trap states in the currently imperfect

AlGaN/GaN materials. Trap states in baseline AlGaN-GaN single-channel HEMTs

were characterized and analyzed. Correlation was found between the surface trap

states and the current collapse behaviors.

A novel AlGaN-GaN double-channel HEMT design was developed to enhance

the device performance and to study the operation mechanism of GaN-based HEMTs.

Benefiting from the polarization effect of nitride semiconductors, double-channel

HEMTs with optimized structure design exhibit favorable device performance. It was

found that the double-channel HEMTs have alleviated current collapse problem, and

additional degrees of freedom for linearity engineering.

2

CHAPTER 1

INTRODUCTION

1.1 Principle of HEMTs

The high electron mobility transistor (HEMT) is also called heterostructure

field-effect transistor (HFET), or modulation doped field-effect transistor

(MODFET). Development of HEMTs started in 1980 [1], immediately after the

successful experiments on modulation doped AlGaAs/GaAs heterostructures [2],

which revealed the formation of a two-dimensional electron gas (2DEG) with

enhanced electron mobility. The physics of 2DEGs in semiconductor

heterostructures has been covered in the author’s bachelor thesis [3] and references

therein.

Fig. 1.1.1 Schematic of an AlGaAs/GaAs HEMT and the corresponding conduction band diagram. Location of the 2DEG is shown. (After Heuken et al., Ref. 4)

Earlier HEMTs utilized the AlGaAs/GaAs system, which was the most widely

studied and the best understood heterojunction system at that time (see Ref. 5 and

3

references therein). The heart of a HEMT is the heterojunction between the channel

layer with lower energy conduction band and the barrier layer with higher energy

conduction band. At the interface between the channel and the barrier layers, a 2DEG

is created by modulation doping the barrier layer. The structure schematic and the

band diagram of an AlGaAs/GaAs HEMT are shown in Fig. 1.1.1. The most

important feature is that the 2DEG is separated from the ionized donors in the barrier

layer. High density and high mobility 2DEGs make HEMTs promising for high

frequency high power applications.

Fig. 1.1.2 Conduction band diagrams and 2DEG populations of an AlGaAs/GaAs HEMT under different gate biases. (After Heuken et al., Ref. 4)

In a typical HEMT, the drain current is controlled by gate modulation of 2DEG

density. With the gate bias at zero, there is 2DEG accumulated at the heterointerface,

and the channel is open. Drain current can be increased by applying a positive gate

voltage, which increases the 2DEG density and current density in the channel.

However, when the gate voltage is lower than the threshold (or pinch-off) voltage,

4

2DEG in the channel is depleted, and the drain current approaches zero regardless of

the drain bias. Fig.1.1.2 shows the mechanism of current control under gate bias. DC

IV characteristics of the HEMT are depicted in Fig. 1.1.3.

Fig. 1.1.3 Typical DC IV output (left side) and transfer (right side) characteristics of an AlGaAs/GaAs HEMT. (After Heuken et al., Ref. 4)

Fig. 1.1.4 Equivalent circuit model of the HEMT. Physical origin of each equivalent circuit element is shown. (After Heuken et al., Ref. 4)

Operation of the HEMT can be described by a small-signal equivalent circuit

model. Fig. 1.1.4 shows the HEMT equivalent circuit and the corresponding physical

5

origin of each circuit element. The gate-to-source capacitance, the transconductance,

the gate-to-drain capacitance, and the output resistance are intrinsic elements; while

the source resistance, drain resistance, and gate resistance are parasitic elements.

These elements predict AC operation behaviors of the HEMT, and can be extracted

from small-signal S-parameter measurements.

The device model can be used in conjunction with the characterization and

parameter extraction techniques to define performance characteristics of a HEMT.

Making preliminary judgments, however, about the ultimate performance potential of

devices or about which devices should be chosen for a particular application is often

desirable. The first-order calculation of several performance figures of merit (FOMs)

can be very useful for making preliminary judgments concerning active capabilities.

The starting point for estimation of many FOMs is either microwave characterization

data or determination of a complete equivalent circuit model for the device. FOMs of

particular interest include cutoff frequency (fT), maximum frequency of oscillation

(fmax), minimum noise figure (Fmin), output power density (Pout), power added

efficiency (PAE) etc. These technical FOMs are only first-order indicators of

ultimate performance limits. Data obtained in this fashion, however, can be used as a

basis of coarse comparisons of active devices.

The cutoff frequency of a HEMT is the frequency at which the short-circuit

current gain ( 21h ) of the device falls to unity. In the first order approximation, the

equivalent circuit in Fig. 1.1.4 gives the definition of fT:

gs

mT C

gf

π2=

The FOM fT does not represent the limiting frequency of microwave operation. This

FOM can, however, be used to compare the approximate operation speed limits of

6

different devices. In general, the device with a high fT value will function usefully at

higher frequency than a device with a much lower fT value. Considering the physical

mechanism of HEMT operation, fT can also be represented by the channel electron

drift velocity through the following equation.

g

satT L

vf

π2=

It is apparent that higher electron velocity and smaller gate length result in higher

cutoff frequency.

The maximum frequency of oscillation fmax is the highest frequency at which

power gain can be obtained from a device. This FOM, like fT, may be used as an

indicator of the ultimate frequency limits of a device. As with the fT value, a high

value of fmax is desirable if high frequency operation is of interest. For most

microwave applications, the frequency fmax appears to be a more useful FOM than

the frequency fT because microwave designers are typically concerned with power

gain into conjugately matched conditions. The maximum frequency of oscillation is

defined formally as the frequency at which the unilateral power gain (U) of a device

reaches unity. U can be written in terms of device y-parameters:

)]Re(*)Re()Re(*)[Re(4 21122211

21221

yyyyyy

U+

−=

fmax can be predicted from the device equivalent circuit. A first-order expression that

is often used to determine the fmax of device can be written as:

2/1max )(

2 gt

dsT

Rrff =

where rds is the device output resistance, Rgt is the sum of the gate resistance and

channel charging resistance, and fT is the cutoff frequency. Although neither fT nor

fmax is an ideal measure of the ultimate frequency capabilities of a device, when both

7

figures are considered in comparison, some insight is gained into the high frequency

performance of devices relative to one another.

Fig. 1.1.5 Schematic representation of a HEMT operating in Class A. (After Nguyen et al., Ref. 5)

The HEMT, with its high current density and operation frequency, is an ideal

candidate for microwave power amplifiers. While fT and fmax are small-signal FOMs,

large-signal FOMs, such as Pout and PAE, are needed to evaluate microwave power

performance of the device. For class A operation, which is the most important class

at microwave frequencies, the theoretical maximum output power of a HEMT is

given by:

))((81

minmaxmax. kdsout VBVIIP −−=

where Imax is the maximum channel current, Imin is the minimum drain current due to

gate-drain and/or source-drain leakage, BVds is the off-sate breakdown voltage, and

Vk is the knee voltage. This simple approximation is graphically presented in Fig.

8

1.1.5, in which the HEMT is assumed to operate along its ideal load line, with an

adequate large-signal gain.

In addition to the Pout, PAE is also an important parameter, which is related to

the device power gain for a class A power amplifier as follows:

)11(21)11(

aaDC

out

dc

inout

GGPP

PPP

PAE −=−=−

=

Thus in the lower frequency limit, in which 1/Ga approaches 0, the PAE approaches

1/2 under class A operation ( 4/π under class B operation).

Table I. Material Properties of GaN in Comparison with Other Semiconductors.

Material Bandgap (eV) Thermal Conductivity

(W/˚K-cm)

Breakdown

Field (V/cm) Mobility (cm2/V-s)

Si 1.1 1.5 3×105 1300

GaAs 1.4 0.54 4×105 5000

SiC 2.9 4 3.8×106 260

GaN 3.4 1.3 2×106 1500

9

1.2 Development of AlGaN-GaN HEMTs

Nitride semiconductors such as AlN, GaN, InN and their alloys are promising

materials for their potential application in electronic and optoelectronic devices [6].

These materials cover an energy band gap range of 0.8 eV to 6.2 eV, suitable for

light emission with colors ranging from red to ultraviolet. Furthermore, GaN’s large

bandgap, large field strength, high thermal conductivity, and good electron transport

properties (as listed in Table I) make GaN based electronic devices very promising in

high voltage, high power, and high frequency applications. As reported earlier [7],

one of the most unique properties of nitride semiconductors is the existence of strong

polarization field within the crystal, which has profound impact on electronic

properties of GaN-based heterostructures.

The polarization electric field in nitrides is two fold: spontaneous polarization

field and strain induced piezoelectric field. Being non-centro-symmetric, nitrides

exhibit large macroscopic polarization effects along the hexagonal c-axis in the

wurtzite lattice. The values of spontaneous polarization field in nitrides are quite

large, and of the same order of magnitude as in ferroelectric crystals. In addition,

nitrides lack inversion symmetry and exhibit piezoelectric effects when strained

along c-axis, the piezoelectric coefficients being an order of magnitude larger than

those in other traditional III-V semiconductors [8]. The direction of polarization field

in nitrides depends on the polarity of the crystal, namely whether the cation sites or

the anion sites of the crystal bi-layers are facing toward the sample surface [9]. In

cation-face samples, the polarization field points away from the surface to the

substrate. (Fig. 1.2.1) While in anion-face samples, the direction of polarization field

is inverted (Fig. 1.2.1). Almost all MOCVD grown nitrides are of cation-face. Nitride

10

alloys prepared by MBE are usually anion-face samples, yet one can invert the

polarity by depositing a thin AlN buffer layer prior to the growth of GaN. With a

strained Al0.3Ga0.7N layer coherently grown on a relaxed GaN substrate, polarization

charges with the density of in the order of 1013 electrons/cm2 can be generated at the

AlGaN/GaN heterointerface [10].

Fig. 1.2.1 Crystal structure of wurtzite GaN with Ga- and N-face polarity. (After Ambacher et al. Ref. 10)

In an AlGaN/GaN heterostructure, the formation of 2DEG at the heterointerface

is quite different from that in the AlGaAs/GaAs system. Due to the presence of a

strong polarization field across the AlGaN/GaN heterojunction, a 2DEG with the

density up to 1013 cm-2 can be achieved in the AlGaN/GaN heterostructure without

any doping [11]. There are several possible sources of electrons contributing to

2DEG accumulation at the AlGaN/GaN heterointerface: the GaN buffer layer, the

AlGaN barrier layer, and the AlGaN surface states. Charges in the GaN buffer layer

should be negative so that a potential well can be formed in the GaN side and the

2DEG can be confined. The transfer of electrons from the GaN buffer layer to the

11

AlGaN/GaN interface leaves behind positive charges and consequently potential

barriers, so by any means electrons in the 2DEG cannot come from the GaN buffer

layer. Similar to AlGaAs/GaAs, modulation doping in AlGaN barrier layer positively

contributes to the formation of 2DEG. However, in case the AlGaN barrier layer is

undoped, a 2DEG density of 1012 ~ 1013 cm-2 can still be achieved [11]. How comes

the 2DEG density so high without any doping? Ibbetson et al. theoretically and

experimentally studied the formation of 2DEGs in AlGaN/GaN heterostructures and

found that surface states act as source of the electrons in 2DEG [12]. The built-in

static electric field in the AlGaN layer induced by spontaneous and piezoelectric

polarization greatly alters the band diagram and the electron distribution of the

AlGaN/GaN heterostructure. Thus considerable amount of electrons transfer from

the surface states to the AlGaN/GaN heterointerface, leading to a 2DEG with the

density up to 1013 cm-2, as sketched in Fig. 1.2.2. Koley et al. detailed investigated

the surface potential of AlGaN/GaN heterostructures by using scanning Kelvin probe,

and confirmed the contribution of surface states to 2DEG formation [14].

Fig. 1.2.2 Schematic representation of band profiles of AlGaN/GaN heterostructures with varying AlGaN thicknesses, which demonstrates how surface states participate

in the screening of polarization field and contribute to the 2DEG formation. (After Morkoç et al., Ref. 13)

12

Making use of the high-density high-mobility AlGaN/GaN 2DEG, AlGaN/GaN

HEMTs can be fabricated. Since the first demonstration of an AlGaN/GaN HEMT by

Khan et al. in 1993 [15], tremendous progress has been made in the development of

AlGaN/GaN HEMTs for microwave power amplifier applications. Several groups

demonstrated power operation of AlGaN/GaN HEMTs at microwave frequencies,

including the record-breaking result of 30 W/mm at 8 GHz by Wu et al [16]. In Table

1.2.1, a historical view on the development of GaN HEMTs was shown. Further

development of GaN HEMTs relies on the improvement of material quality and the

optimization of device structure.

Table II. Historical development of GaN HEMTs.

Year Even Authors Ref.

1969 GaN by hydride vapor phase epitaxy Maruska and Tietjen 17

1971 GaN by MOCVD Manasevit et al. 18

1992 AlGaN/GaN two-dimensional electron gas Khan et al. 19

1993 AlGaN/GaN HEMT Khan et al. 15

1994 Microwave AlGaN/GaN HFET Khan et al. 20

1996 Microwave power AlGaN/GaN MODFET Wu et al. 21

1998 Reveal current compression in GaN MODFETs Kohn et al. 22

1999 6.9 W/mm @ 10 GHz GaN HEMT on SiC Sheppard et al. 23

2000 Surface passivated AlGaN/GaN HEMTs Green et al. 24

2004 30 W/mm @ 8 GHz GaN HEMT with field plate Wu et al. 16

13

1.3 Objective of Synopsis of This Thesis

The principal objective of this thesis is to establish a viable technology for the

fabrication of AlGaN-GaN HEMTs, to understand the operation mechanism of

AlGaN-GaN HEMTs with a focus on main issues limiting device performance, and

to develop novel HEMT structures with better performance and/or more functions.

The organization of this thesis is as the follows.

Chapter 2 gives an overall description on the development of baseline AlGaN-

GaN single-channel HEMTs. The AlGaN/GaN 2DEG was characterized by studying

low-temperature magneto-transport properties. Layout design and device processing

technologies for HEMT fabrication are described. DC IV, RF small-signal, and RF

large-signal measurement methods and results are presented.

Chapter 3 focuses on the analysis of trap states in AlGaN/GaN HEMTs. Trap

states were characterized by measuring frequency- and bias-dependent admittance of

an AlGaN/GaN Schottky diode. Interface state density and the corresponding time

constant were extracted. Dynamic IV characterization was carried out for HEMTs

subjected to different surface treatments. Correlation was found between surface trap

states and current collapse, which is the bottleneck limiting large-signal performance

of AlGaN-GaN HEMTs. Mechanism and consequence of the current collapse are

discussed.

Chapter 4 contains the major portion of the thesis author’s original research

contribution during the study in master program. This chapter introduces the AlGaN-

GaN double-channel HEMT, which exhibit enhanced performance compared with

the baseline AlGaN-GaN single-channel HEMT. Structure design and device

characterizations including DC IV, RF small-signal and large-signal measurements

14

are treated in details. Unique features resulting from the novel double-channel design

are emphasized.

Finally, this thesis is summarized in chapter 5. Suggestions on future work are

provided.

15

CHAPTER 2

ALGAN-GAN SINGLE-CHANNEL HEMTS

2.1 Electronic Properties of AlGaN/GaN Heterostructures

Operation of the AlGaN/GaN HEMT relies on the two-dimensional electron gas

(2DEG) in the AlGaN/GaN interface. Assessment on the electronic properties of

2DEGs is crucial for evaluating the suitability of AlGaN/GaN epilayers toward

HEMT fabrications. Among the 2DEG properties, electron density and mobility are

two most important concerns.

AlGaN/GaN heterostructure epilayers for material characterization and device

fabrication were grown by MOCVD method in HKUST. The epilayers are grown on

sapphire substrates, typically consisting of a 2.5-µm-thick GaN buffer layer and a

selectively doped Al0.3Ga0.7N layer with the thickness in the range of 20~30 nm. Van

der Pauw Hall measurement is the conventional method to determine the carrier

density and mobility of a given sample [25]. An AlGaN-GaN single-channel HEMT

epilayer with the AlGaN thickness of 30 nm and the doping level of 5×1018 cm-3

(denoted as sample A) has a Hall density of 2×1013 cm-2 and a Hall mobility of 853

cm2/V-s at room temperature. Another sample with similar structure but 24 nm thick

AlGaN and 2×1018 cm-3 doping (denoted as sample B) has a Hall density of 1.5×1013

cm-2 and a Hall mobility of 980 cm2/V-s. We can see that thicker AlGaN and/or

heavier doping result in larger carrier density but lower mobility.

Van der Pauw Hall measurements are usually under single-carrier channel

approximation. Accuracy will be affected when the sample has more than one carrier

channel, e.g. an additional parallel conduction path. Another source of measurement

16

inaccuracy is that the results derived from Van der Pauw method somewhat depends

on sample geometry and ohmic contact resistance. In order to obtain more accurate

information about the 2DEG properties, magneto-transport studies were carried out

under very low temperature (1.4 K) and very high magnetic field (up to 12 Tesla).

7 8 9 10 11 12400

500

600

700

(a)

R

xy (Ω

)

B (Tesla)

7 8 9 10 11 12400

500

600

700

(b)

Rxy

(Ω)

B (Tesla)

Fig. 2.1.1 Transverse resistance of the AlGaN/GaN sample as a function of magnetic field measured at 1.4 K. The step-like plateaus are due to quantum Hall effect.

At low temperature and high magnetic field, 2DEGs show Shubnikov-de Haas

(SdH) oscillations in the longitude resistance and quantum Hall effect plateaus in the

transverse resistance. Details about the 2DEG magneto-transport properties can be

found in Ref. 26 and 27. 2DEG characteristics such as electron density and mobility

17

can be extracted from the dependence of longitude/transverse resistance on magnetic

field. Background electrons generated by impurity doping are usually frozen at low

temperature, thus the low-temperature magneto-transport measurement is a more

direct characterization of the 2DEGs.

0 2 4 6 8 10 123500

3600

3700

3800

3900

4000

4100

(a)

Rxx

(Ω)

B (Tesla)

0 2 4 6 8 10 125000

5100

5200

5300

5400

5500

5600

(b)

Rxx

(Ω)

B (Tesla)

Fig. 2.1.2 Longitude resistance of the AlGaN/GaN sample as a function of magnetic field measured at 1.4 K. Oscillations in the resistance are due to SdH effect.

Fig. 2.1.1 (a) and (b) shows magnetic field dependence of transverse resistance

of the aforementioned two AlGaN/GaN samples. Both samples show quantum Hall

effect plateaus, indicating quantum confinement of the 2DEGs. Magnetic field

dependence of longitude resistance was shown in Fig. 2.1.2 (a) and (b). SdH

oscillations appear for both samples. With increasing magnetic field, sample A

18

shows a trend of rising up in the longitude resistance, suggesting the existence of a

parallel conduction in the heavily doped AlGaN layer [28]. As to sample B, the

longitude resistance tends to drop down as the magnetic field increases. The

parabolic-like decreasing trend of longitude resistance was related to electron-

electron interaction of the high-density 2DEG [29]. From the Fourier transformation

of Rxx-1/B function, 2DEG density can be calculated [30]. As shown in Fig. 2.1.3 (a)

and (b), sample A and B have a 2DEG density of 1.1×1013 cm-2 and 1.2×1013 cm-2

respectively; only the ground quantum state was occupied.

0 200 400 600 800 1000

(a)

Ns = 1.2E13 cm-2

Frequency (1/B) (Tesla)

FFT

(a. u

.)

0 200 400 600 800 1000

(b)Ns = 1.1E13 cm-2

Frequency (1/B) (Tesla)

FFT

(a. u

.)

Fig. 2.1.3 Fast Fourier transformation of Rxx-1/B function. 2DEG densities are derived from the X-axis value of the peak position.

19

Without observing the trend of rising up in longitude resistance, Sample B has

minimal parallel conduction in the AlGaN barrier layer. Discrepancies between

electron densities obtained from Hall measurement and SdH measurement are

attributed to background electrons in the GaN buffer layer. Assuming a uniform

distribution of background electrons in the 2-µm-thick GaN buffer layer, a

background electron concentration of 2×1016 cm-3 was derived. Origin of the

unintentional n-doping could be nitrogen vacancies and/or oxygen impurities. Hall

carrier density of sample A is 8×1012 cm-2 higher than that extracted from SdH

measurement. After subtracting a background-related carrier density of 4×1012 cm-2,

we can deduce that there is a parallel carrier channel with the density 4×1012 cm-2 of

in the heavily doped AlGaN layer.

From above measurement and calculation results, we can draw following

conclusions. Background electron concentration of the GaN buffer is around 2×1016

cm-3. AlGaN/GaN epilayers grown in our group have a 2DEG density about 1×1013

cm-2. Thicker AlGaN and/or heavier doping slightly increase the 2DEG density, and

introduce significant parallel conduction in the AlGaN barrier layer. In the remainder

of this chapter, all results are derived from HEMTs with epilayer structure the same

as that of sample B.

20

2.2 Device Processing Technologies

A basic HEMT process flow includes mesa isolation, source/drain metallization,

gate metallization, and an optional passivation step. Fig. 2.2.1 shows the layout of a

typical HEMT with 1-µm-long and 2×50-µm-wide gate. The photograph of a HEMT

after device processing is demonstrated in Fig. 2.2.2. Cross sectional schematic of

the AlGaN-GaN single-channel HEMT is shown in Fig.2.2.3.

Fig. 2.2.1 Layout of a 2×50µm×1µm HEMT.

Fig. 2.2.2 Photograph showing the top view of a fabricated HEMT.

21

Substrate (SiC, Sapphire)

GaN

Gate Drain SourcAlGaN

Fig. 2.2.3 Cross-sectional schematic of an AlGaN-GaN single-channel HEMT.

Chlorine gas-based Inductively-Coupled-Plasma (ICP) etching was used for

mesa isolation. Etching was performed in a STS ICP system. With the RF power of

135 W, the Cl2 gas flow of 15 sccm, and the He gas flow of 10 sccm, 40 seconds

etching results in an etched depth of 300 nm for a baseline AlGaN-GaN single-

channel HEMT sample. Note that the etching rate is not a linear function of time, and

AlGaN layers with higher Al composition have slower etching rate.

0 5 10 15 20 25 30 350

30

60

90

120

150

Measurement Data Linear Fit

Res

ista

nce

(Ω)

Spacing (µm)

RC = ~ 1.1 Ω-mmρS= 354 Ω/square

Fig. 2.2.4 TLM measurement results of an AlGaN-GaN single-channel HEMT sample after device processing. Contact resistance and sheet resistivity are derived.

For source/drain ohmic metallization, Ti/Al/Ni/Au (20nm/150nm/50nm/80nm)

multilayer was deposited in an e-beam evaporation system and annealed at 850 ºC in

22

N2 ambient for 30 seconds. With this method, contact resistance of 1 Ω-mm can be

achieved on a reproducible basis. Fig. 2.2.4 shows transfer-length-measurement

(TLM) results of metal contacts on baseline AlGaN-GaN single-channel HEMT. A

sheet resistivity of and a contact resistance of are derived.

-12 -9 -6 -3 0 3-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

I (

A/m

m2 )

Bias (V)

Fig. 2.2.5 DC IV characteristics of a Schottky diode fabricated on the baseline AlGaN-GaN single-channel HEMT epilayer.

-10 -8 -6 -4 -2 00

200

400

600

800

1000

Cap

acita

nce

(pf/c

m2 )

Bias (V)

3 MHz

Fig. 2.2.6 CV characteristics of an AlGaN/GaN Schottky diode measured at 3 MHz.

23

Owing to the large work function and the good adhesion with semiconductor

materials, e-beam evaporated Ni/Au (20 nm/300 nm) bi-layer was chosen as the

Schottky metal. Fig. 2.2.5 shows DC IV characteristics of an AlGaN/GaN Schottky

diode. Capacitance-voltage (CV) characteristics of the AlGaN/GaN Schottky diode

can be measured, as shown in Fig. 2.2.6. And the carrier distribution profile can be

extracted using the method described in Ref. 31, as shown in Fig. 2.2.7. Results are

in good agreement with transport measurements described in Section 2.1.

0 50 100 150 200 250

1016

1017

1018

1019

1020

Ele

ctro

n C

once

ntra

tion

(cm

-3)

Depth (nm)

Fig. 2.2.7 Electron distribution profile extracted from the CV characteristics.

24

2.3 DC IV Characteristics

0 2 4 6 8 10 12 140

200

400

600

800

1000

1200

I D (m

A/m

m)

VDS (V)

VG Start: 1 V, Step: -1 V

Fig. 2.3.1 DC IV output characteristics of an AlGaN-GaN single-channel HEMT.

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 10

200

400

600

800

1000

1200

0

40

80

120

160

200

240

I D (m

A/m

m)

VGS (V)

VDS = 10 VG

m (m

S/m

m)

Fig. 2.3.2 DC IV transfer characteristics of an AlGaN-GaN single-channel HEMT.

DC characteristics of HEMTs with 10-µm-wide gate were measured by using an

HP 4156 semiconductor parameter analyzer. Output and transfer IV characteristics

are shown in Fig. 2.3.1 and Fig. 2.3.2 respectively. The maximum drain current is

25

930 mA/mm at +1 V gate bias. The peak transconductance is 185 mS/mm. And the

pinch-off voltage is -4.5 V. The off-sate drain breakdown voltage is around 60 V.

There is a notable hysteresis between the transfer curves obtained from upward and

downward sweeping directions. The upward trace shows lower drain current as well

as smaller transconductance. This behavior is attributed to trap-related delay, or

namely the “current collapse”, of AlGaN/GaN HEMTs. A detailed discussion on the

current collapse will appear in Chapter 3. Fig. 2.3.3 shows the drain current and the

gate current as a function of gate bias in semilog scale. Both the drain leakage and

the gate leakage are in the order of 0.1 mA/mm at VDS = 10 V

-8 -6 -4 -2 0 210-5

10-4

10-3

10-2

10-1

100

101

102

103

ID IG

VDS = 10 V

I D, I

G (m

A/m

m)

VGS (V)

Fig. 2.3.3 DC IV transfer characteristics of an AlGaN-GaN single-channel HEMT in semilog scale.

26

2.4 RF Small-Signal Measurement and Analysis

Small-signal characteristics of the HEMTs can be obtained by measuring S-

parameters at different DC biases. Fig. 2.4.1 depicts the small-signal measurement

setup. An Agilent 8277 vector network analyzer (VNA) was used for s-parameter

measurement in 50 MHz ~ 40 GHz range. An Agilent 4142 source/monitor unit was

used for DC biasing. RF probe station with co-planar microwave probes and co-axial

cables were used to connect the device to the VNA.

Fig. 2.4.1 Schematic of the measurement setup for RF small-signal characterization.

From the measured S-parameters, we can derive RF FOMs such as fT and fmax.

As shown in Fig. 2.4.2, a baseline AlGaN-GaN single-channel HEMT with 1 µm

gate length has a fT of 13 GHz and a fmax of 35 GHz. Nominally undoped GaN films

grown by the present MOCVD techniques usually show slight n-type, with a

background electron concentration in the order of 1016 cm-3, as discussed in section

2.1 of this chapter. Consequently, there are parasitic capacitance and conductance

components between the contact pads and the non-insulating GaN buffer layer,

27

which contribute to RF measurement results and may mask some intrinsic electrical

characteristics of the HEMTs. Open-pad de-embedding with the S-parameters of a

dummy pad can effectively strip pad-related parasitics. The de-embedding process is

performed in this way: (1) convert S-parameters of the HEMT (Smeas) into Y-

parameters (Ymeas); (2) convert S-parameters of the dummy pad (Spad) into Y-

parameters (Ypad); (3) subtract Ypad from Ymeas, yielding de-embedded Y-parameters

(Y); (4) convert the de-embeded Y into S-parameters (S). A comparison between S-

parameters before and after de-embedding is presented in Fig. 2.4.2. After de-

embedding, both the current gain (|h21|2) and the unilateral power gain (U) show

better 20 dB per decade roll-off, as shown in Fig. 2.4.3. The de-embedded fT and fmax

are slightly higher than the as measured results.

frequency (100000000.000 to 39100000000.000)

me

as_

s[0

,30

,::]

s

Fig. 2.4.2 S-parameters of an AlGaN-GaN single-channel HEMT before (open circiles) and after (solid lines) performing open pad de-embedding.

28

100M 1G 10G 100G0

10

20

30

40

50

after de-embedding |h21|

2

U

as measured

VGS=-3.5V, VDS=10V

Gai

n (d

B)

Frequency (Hz)

Fig. 2.4.3 Frequency dependence of the current gain (|h21|2) and unilateral power gain (U). A comparison was made between results before and after de-embedding.

Possibly due to the degradation of electron velocity induced by AlGaN/GaN

interface scattering, the AlGaN-GaN single-channel HEMTs usually show reduction

of fT and fmax at high current levels (Fig.2.4.4). The degradation of RF performance

at high current level limits RF power performance of the HEMTs. This problem is to

be solved to make the GaN HEMTs suitable for power application at high frequency.

-5 -4 -3 -2 -1 0 10

10

20

30

40 VDS = 10 V

De-embedded

As Meausured

fT fmax

Freq

uenc

y (G

Hz)

VGS (V)

Fig. 2.4.4 fT and fmax of an AlGaN-GaN single–channel HEMT at different gate bias.

29

2.5 RF Large-Signal Power Measurement

SiN passivation can alleviate the current collapse problem and increase the

breakdown voltage [24], though it is not always effective. RF power performance of

SiN passivated AlGaN-GaN single-channel HEMTs was measured by using a Maury

load-pull system. A signal generator gives the RF input signal. A DC source/monitor

unit provides DC bias through the bias tee. The power meter in conjunction with the

power sensor measures the RF output signal. Spectrum analyzer reads power levels

at different frequency bands. Impedances at the input and the output side can be

tuned to optimize the output power. Measurement is controlled by a computer. Fig.

2.5.1 depicts the setup of the load-pull system.

Fig. 2.5.1 Schematic of the measurement setup for RF large-signal characterization.

At a frequency of 2 GHz, a maximum output power density of 1 W/mm and a

peak PAE of 17% were achieved from an AlGaN-GaN single-channel HEMT with 1

µm gate length and 100 µm gate width. This power performance is far below the

theoretical value expected from DC IV characteristics. Trap-related current collapse

and power loss are possible reasons leading to the inferior large-signal performance.

30

Poor thermal conductivity of the sapphire substrate could result in inefficient heat

dissipation and limit the power performance.

-10 -5 0 5 10 150

5

10

15

20

25VGS= -3V, VDS = 25V

Pout Gain PAE

P out (

dBm

), G

ain

(dB)

PAE(

%)

Pin (dBm)

Fig. 2.5.2 Output power, power gain, and power added efficiency as a function of input power.

31

CHAPTER 3

TRAP STATES AND CURRENT COLLAPSE IN ALGAN/GAN HEMTS

Trap states are inevitable in GaN-based HEMTs. The trap states refer to deep

level states in the forbidden gap, which may delay frequency response and induce

power loss. Most performance degradation and device instability problems, such as

the current collapse, are caused by trap states. In this chapter, an investigation is

given on the trap states and the current collapse behaviors of AlGaN/GaN HEMTs.

First, an overview on the location and origin of the trap states in AlGaN/GaN

heterostructures is provided. After that, those trap states are studied by frequency-

and bias-dependent admittance measurement and analysis. In the last section, we

introduce the dynamic IV characterization, a method to evaluate current collapse

behaviors of the AlGaN/GaN HEMTs. Mechanism and consequence of the current

collapse were analyzed. By comparing dynamic IV characteristics of AlGaN/GaN

HEMTs subjected to different surface treatments, correlation was found between

current collapse and trap states on device surface.

32

3.1 Overview of Trap States in AlGaN/GaN Heterostructures

Development of AlGaN/GaN HEMT microwave power amplifiers are largely

hindered by the limiting effect of the trap states in AlGaN/GaN heterostructures [32].

These traps may appear on the AlGaN surface, in the AlGaN barrier layer, at the

AlGaN/GaN heterointerface, or in the GaN buffer layer, as the schematic shown in

Fig. 3.1.1. Presence of trap states in AlGaN/GaN HEMTs can cause a voltage delay

in device operation through trapping and de-trapping process, thereby degrading the

power handling capability at high frequency.

Interface Traps Surface Traps

Buffer Traps

Gate Drain Source

Substrate (SiC or Sapphire)

AlGaN

GaN

Fig. 3.1.1 Cross-sectional schematic of an AlGaN-GaN single-channel HEMT showing possible locations of trap states.

The surface of a crystal interrupts the perfect periodicity of the crystal lattice.

The layer of atoms at the surface has un-terminated or “dangling” bonds. It is hence

easy to imagine that at the surface of the crystal, the band structure can be modified,

and there can now be states in the otherwise forbidden energy gap. These states are

33

localized and exist only at the crystal surface. There are two kinds of surface trap

states: intrinsic surface states and defect related extrinsic surface states. The term

“intrinsic” refers to the fact these states would exist in an ideally perfect surface.

They correspond to solution of Schrödinger equation with energy levels within the

forbidden gap. The extrinsic surface states are caused by defects or impurities at the

surface, forming during crystal growth or in subsequent device fabrication processes.

Similar to the case of surface states, interruption of the periodicity of the

crystallattice at the heterointerface forms interface states. Interface states can also be

induced by interface roughness and compositional non-uniformities.

In lack of a suitable substrate, GaN and GaN-based alloys are usually grown on

sapphire or SiC with large lattice mismatch. Consequently, AlGaN/GaN epilayers

grown with presently available technology are imperfect crystals with dislocations,

impurities, and defects in the material. These defects may cause the formation of

deep level trap states within the GaN and AlGaN layer.

34

3.2 Characterization of Trap States in AlGaN/GaN Heterostructure

With its capacitance nature and the charging resistance associated with the

capacitance, trap states usually cause a frequency dispersion of admittance. Bias- and

frequency-dependent admittance measurements were carried out to characterize trap

states in AlGaN/GaN heterostructures. An equivalent circuit model was deployed to

analyze the location, distribution, and electronic properties of trap states.

Modulation doped AlGaN/GaN heterostructure samples with different AlGaN

thicknesses and Al compositions were grown by metal organic chemical vapor

deposition (MOCVD). Based on high-resolution X-ray diffraction measurements, the

AlGaN film thickness and Al composition of three samples in our study were

determined to be 37 nm of Al0.24Ga0.76N, 37 nm of Al0.19Ga0.81N, and 29 nm of

Al0.19Ga0.81N, respectively. Room-temperature Hall mobility and carrier density

measured by the Van der Pauw technique are around 1400 cm2/V-s and 7×1012 cm-2,

respectively. In order to facilitate the frequency- and bias-dependent admittance

measurements, circular Schottky diodes were fabricated using Ti/Al/Pt/Au as Ohmic

metal, and Pt/Au as Schottky metal, as depicted in the inset of Fig. 3.2.1.

Capacitance and conductance of the AlGaN/GaN Schottky diodes were

measured using an HP 4284 Precision LCR Meter. The amplitude of the AC signal

was kept at 20 mV so that small signal conditions were maintained. The DC bias

voltage was swept from -6.0 to +1.0 V and the measurement frequency was varied

from 10 KHz to 1 MHz. High-frequency probes and cables were used to connect the

Schottky diodes to the LCR meter, and calibration was done for each measurement

frequency. As an example, the measured results of one AlGaN/GaN sample with a 37

nm Al0.24Ga0.76N layer are shown in Fig. 3.2.1.

35

Fig. 3.2.1 (a) Capacitance and (b) conductance as a function of DC bias at different measurment frequencies for an AlGaN/GaN heterostructure sample with a 37 nm

thick Al0.24Ga0.76N layer.

A modulation doped AlGaN/GaN heterostructure can be treated as a metal-

insulator-semiconductor capacitor with the AlGaN layer acting like an insulator [33].

As shown in Fig. 3.2.2 (a), the gate-to-channel capacitance of an ideal AlGaN/GaN

Schottky diode contains two components: the capacitance of the fully depleted

AlGaN layer (CAlGaN) and the capacitance of the GaN depletion region (CGaN). As a

result of lattice mismatch between AlGaN and GaN, and possibly compositional non-

uniformity caused by alloy clustering, there could be considerable amount of trap

states at the AlGaN/GaN interface. Adopting an analytical model of interface trap

states in metal-oxide-silicon (MOS) system [34], the electrical behavior of the

interface trap states can be modeled as a capacitive (Cit) and a conductive (Git)

component in parallel connection with the GaN depletion region capacitor (CGaN).

Taking into account the effect of interface trap states, the equivalent circuit of an

AlGaN/GaN Schottky diode is modeled as shown in Fig. 3.2.2 (b).

36

Fig. 3.2.2 Equivalent circuit model of an AlGaN/GaN Schottky diode, (a) without considering any trap states, (b) considering the AlGaN/GaN interface states, (c)

considering both the interface and the surface trap states.

Similar to the interface trap states, surface states are present at any metal-

semiconductor interface. For a typical Schottky diode fabricated in ambient

conditions, there exists an insulating interfacial layer between the metal and

semiconductor surface [35]. The insulating interfacial layer is only a few monolayers

thick so that the metal can easily communicate electrons with the trap states at the

semiconductor surface. This trapping and de-trapping process can be modeled as a

serial combination of the surface-traps-related resistance (Rsurf) and capacitance

(Csurf) in parallel connection with the interfacial layer capacitor (Ci). Considering

both the interface and the surface trap states, the equivalent circuit representation of

an AlGaN/GaN schottky diode is as shown in Fig. 3.2.2 (C). It should be noted that

in addition to the interface and surface trap states, there might be traps within the

bulk GaN and AlGaN related to crystal defects and imperfections. Nevertheless,

since these traps states are usually deep below the conduction band edge and have

time constants as large as milliseconds [36], their effects are negligible as we are

37

investigating the trap-induced device phenomena in the 10 KHz to 1MHz frequency

range.

Fig. 3.2.3 Energy band diagrams of an AlGaN/GaN heterostructure in electron accumulation and depletion mode.

As shown in Fig. 3.2.1, frequency dispersion of the admittance strongly depends

on the external bias, and becomes significant in the vicinity of threshold voltage,

indicating interface trap states are the dominant trapping mechanism in the 10 KHz ~

1MHz frequency regime. The trapping/de-trapping of AlGaN/GaN interface states

does not take place at zero or very small reverse bias, where the AlGaN/GaN

heterojunction is in electron accumulation mode (Fig. 3.2.3, left side). As the reverse

bias voltage increases, electrons in AlGaN/GaN interface are gradually depleted, the

Fermi level at the interface sweeps downward into the GaN bandgap, and

AlGaN/GaN interface trap states start to respond to external voltage signals (Fig.

3.2.3, right side). From Fig. 3.2.3, the relative position between the surface Fermi

level and the energy levels of AlGaN surface states remains roughly constant at

different biases. As a result, the response of the surface states to an external AC

signal remains the same whether the 2DEG channel is under accumulation (zero or

very small bias) or depletion (large reverse bias). From this point of view, the

surface trap states related component can be eliminated and the effect of the interface

38

trap states can be extracted by comparing the admittance measured at electron

accumulation and depletion.

Based on the admittance measurement results, the following extraction was

carried out: (1) convert the measured admittance data to impedance form. At any bias

voltage, the combined impedance of the GaN depletion region and interface traps

related component could be obtained by subtracting the total impedance with the

impedance measured at zero bias (where CGaN, Cit and Git can be neglected, Fig. 3.2.2

(c)); (2) convert the combined impedance of GaN depletion region and interface

traps related component back to admittance form; (3) considering a continuous

interface trap state distribution, the trap state density (Dit) and the corresponding time

constant (τ) can be extracted based on the frequency dependence of the parallel

conductance (Git). The detailed extraction routine can be found in Ref [33] and [34].

The experimental and fitted conductance-frequency curves of sample A are

shown in Fig. 3.2.4. The extracted Dit and τ are in the order of 1013 ~ 1014 cm-2eV-1

and 0.1~1 µs, respectively. With increase of the reverse bias and the GaN layer

changing from accumulation to depletion, the Fermi level at the AlGaN/GaN

heterointerface gets swept through the interface trap energy levels that are located

within the GaN bandgap. Among the interface trap states, those with higher energy

level exhibits shorter time constant for the trapping/de-trapping process. As a result,

the time constant is a good indicator of the trap state energy level. In order to gain a

qualitative understanding on the distribution of interface trap states within the GaN

bandgap, we plotted the trap state density against the corresponding time constant, as

illustrated in Fig. 3.2.5. Measurement of different samples indicates that an increase

of AlGaN layer thickness and/or an increase of the Al content lead to higher interface

39

trap state density. This can be explained by greater dislocation density and interface

roughness induced by lattice mismatch between AlGaN and GaN.

0.1 1 100.0

0.5

1.0

1.5

2.0

2.5Scattered dots: experimetal dataLines: fitted curves

V = -3.5 V

V = -0.5 V

Git/

2πf (

nf)

2πf (MHz)

Fig. 3.2.4 Extraction of interface trap state density (Dit) and time constant (τ) by fitting the normalized conductance (Git/2πf) vs. angular frequency (2πf) dependence

at different gate biases.

0.1 1 101

2

3

4

5

6

7

8

A 37nm Al0.24Ga0.76N B 37nm Al0.19Ga0.81N C 29nm Al0.19Ga0.81N

Dit

(1013

cm

-2eV

-1)

τ (µs)

Fig. 3.2.5 Interface state density as a function of the corresponding time constant for AlGaN/GaN samples with different AlGaN barrier thickness and Al content.

40

3.3 Analysis of Current Collapse in AlGaN/GaN HEMTs

HEMTs made of AlGaN/GaN heterostructures usually exhibit high drain current

density at DC and excellent RF characteristics in small-signal condition. However,

the AlGaN/GaN HEMTs are usually plagued with current collapse [37, 38], which is

also referred to as current slump, current compression, current instability, or RF

dispersion. Under high-frequency large-signal input drive, the output current swing

gets compressed drastically, resulting in reduced output power density (Pout) and

power added efficiency (PAE). Trapping/de-trapping of surface states in the gate-to-

drain region is likely responsible of the current collapse [39, 40]. There are also other

alternative explanations such as trapping within the AlGaN layer [41], virtual back

gate effect of the GaN buffer [42], gate bias-induced nonuniform strain in the AlGaN

barrier layer [43], and source resistance modulation due to space-charge suppression

of the electric field in the source-to-gate region [44]. Introduce of surface passivation

layer such as Si3N4 sometimes alleviates the current collapse [24, 45]. However, the

effect of Si3N4 passivation is very sensitive to Si3N4 film quality and device surface

condition before Si3N4 deposition. Reproducibility of the Si3N4 passivation is low.

One is motivated to gain better understanding on the mechanism of current collapse,

and to find hints for reducing the current collapse on a reproducible basis.

Dynamic IV measurement was adopted to characterize current collapse of the

AlGaN/GaN HEMTs. The dynamic IV measurement was performed in this way:

biasing the device to a quiescent point, drain current at each bias point of the IV

plane are recorded immediately after pulsing the gate (VGS) and drain bias (VDS)

synchronously from the quiescent point to the bias under testing. This method can

simulate RF large-signal behaviors and provides a powerful tool for the analysis of

41

RF current collapse in AlGaN/GaN HEMTs. Dynamic IV characteristics of an un-

passivated AlGaN-GaN single-channel HEMT were measured using Accent’s DIVA

D225 system and shown in Fig. 3.3.1. The device under testing has a gate length of 1

µm and a gate width of 2×50 µm. The pulse width is 1 µs and the pulse separation is

1 ms. During the measurement, 6 different quiescent points were chosen. DC output

IV characteristics of the same HEMT were given for comparison.

0 2 4 6 8 100

20

40

60

80

100

120(a)

DC Quiescent Point VGS = 1 V, VDS = 0 V Quiescent Point VGS = 1 V, VDS = 10 V

VGS: -5 ~ 1 V, in 1 V step

I DS (

mA/

mm

)

VDS

0 2 4 6 8 100

20

40

60

80

100

120(b)

DC Quiescent Point V

GS = -2 V, V

DS = 0 V

Quiescent Point VGS

= -2 V, VDS

= 10 VVGS: -5 ~ 1 V, in 1 V step

I DS (

mA/

mm

)

VDS

42

0 2 4 6 8 100

20

40

60

80

100

120(c)

DC Quiescent Point VGS = -5 V, VDS = 0 V Quiescent Point VGS = -5 V, VDS = 10 V

VGS: -5 ~ 1 V, in 1 V step

I DS (m

A/m

m)

VDS

Fig. 3.3.1 Dynamic IV characteristics of an AlGaN-GaN single-channel HEMT at different quiescent bias points in comparison with DC output IV characteristics.

When the quiescent point is at VGS=1 V and VDS= 0 V, the pulsed drain current

does not show any collapse and is higher than the DC current due to the alleviation

of self-heating effect (Fig. 3.1.1 (a)). As the quiescent VGS goes to higher drain bias,

e.g. 10 V, current collapse starts to occur (Fig. 3.1.1 (a)). Similar behaviors are

observed when the VGS is at –2 V, which is the gate bias for class A operation (Fig.

3.1.1 (b)). We found that drain current pulsed from VGS = 1 V and VDS = 10 V is

slightly smaller than that pulsed from VGS = -2 V and VDS = 10 V, which is possibly

induced by more severe self-heating when biased at higher current level. However,

as the VGS moves to the pinch-off voltage –5 V, current collapse becomes very

severe (Fig. 3.1.1 (C)). In view of this trend, we conclude that current collapse tends

to occur in the process of pulsing the drain-side edge of the gate from off-state (or

depletion mode, usually at negative VGS or high VDS) to on-state (or accumulation

mode). This observation is in agreement with the model that trapping/de-trapping in

43

the gate-to-drain spacing region induce current collapse, as sketched in Fig. 3.3.2.

Fig. 3.3.2 Schematic drawing showing the occurrence of current collapse during the process of an off-to-on transient. Frequency response is delayed by trap states in the

gate-to-drain spacing region.

Under negative gate bias and/or high drain bias, the drain side of the gate edge

is depleted and highly resistive. When the working point moves to higher current

level, the drain side of the gate edge will immediately get populated with electrons

and become conductive in the ideal case. However, the electron population process is

44

delayed by trap states (e.g. surface states) in the gate-to-drain spacing region. As a

result, the drain side of the gate edge remains highly resistive in the transient state

and the RF current is largely compressed comparing with the DC current at steady

state. It is noteworthy that when the device is biased at higher VDS, the pulsed drain

current shows more collapse. This is due to the fact that there is an effective off-to-

on pulse between the gate and drain terminals when VDS is pulsed from a high

voltage to smaller voltages. All of these results suggest that during large signal

operation of AlGaN/GaN HEMTs, the drain current will collapse when the output

AC signal is swinging from the quiescent bias point toward lower VDS and/or higher

VGS, as the arrow pointing in Fig. 3.3.2. In another word, during large-signal

operation of AlGaN/GaN HEMTs, upside waveform of the output signal more likely

suffers from current collapse, while the downside waveform of the output signal

remains un-collapsed.

ICP Damage

Gate Drain Source

Substrate (SiC, Sapphire)

AlGaN

GaN

Fig. 3.3.3 Schematic showing the post ICP treatment of the AlGaN/GaN HEMT.

45

In order to find out correlations between current collapse and surface trap states,

one will be interested in making comparisons between an AlGaN/GaN HEMT with

less surface states and another one with more surface states. With the present device

technologies, it is hard to find a way to reduce the surface states of AlGaN/GaN

HEMTs on a steady and controllable basis. We chose to intentionally increase the

surface trap states of some HEMTs for comparison with those baseline HEMTs. It is

well known that ICP treatment of the AlGaN/GaN samples causes harsh surface

damage and induces additional surface trap states [46]. Current collapse behaviors of

AlGaN/GaN HEMTs with and without ICP post-treatment were compared. After

device fabrication, AlGaN/GaN HEMTs intended for surface damage were loaded

into ICP chamber for plasma treatment. The plasma power is 60 W; the gas flow is

25 sccm He, and the chamber pressure is 5 mTorr. With gate and source/drain metals

acting as etching masks, only the spacing region between gate and source/drain metal

was subjected to ICP treatment, as the schematic shown in Fig. 3.3.3

0 2 4 6 8 100

20

40

60

80

100 Before ICP After ICP

I D (m

A)

VDS (V)

DC

Fig. 3.3.4 DC IV characteristics of a HEMT before and after ICP treatment.

46

0 2 4 6 8 100

20

40

60

80

100 Before ICP After ICP

I D (m

A)

VDS (V)

Bias point: VDS=10V, VGS=-5VPulse Width: 1 µsPulse Seperation: 1 ms

Fig. 3.3.5 Dynamic IV characteristics of a HEMT before and after ICP treatment.

Fig. 3.3.4 shows DC IV characteristics of an AlGaN/GaN HEMT before and

after He gas ICP treatment. We can see that after 15 seconds ICP treatment, DC

characteristics of the device remain unchanged, indicating there is no etching of the

AlGaN/GaN epilayer. However, it is obvious in Fig. 3.3.5 that dynamic IV

characteristics were drastically changed by the ICP treatment. Device after the ICP

treatment has more surface traps and shows more severe current collapse, indicating

that surface traps in the gate-to-drain region are possibly the dominant trap states

responsible for the current collapse.

47

CHAPTER 4

ALGAN-GAN DOUBLE-CHANNEL HEMTS

4.1 Motivation and Epilayer Design

Motivated by the enhancement of drain current density and the additional

freedom of modulating gain linearity [47-49], there were research efforts in

developing double- or multi-channel HEMTs with GaAs- [48] and InP-based

materials [49]. Device epilayers made of these materials usually require intentional

doping in the channel or the barrier layer to form carrier channels, leading to low

electron mobility and large buffer leakage. Owing to the novel polarization effect of

nitride semiconductors, high-density and high-mobility 2DEG can be achieved at the

AlGaN/GaN interface without any intentional doping [11]. This unique feature

enables the design of AlGaN-GaN double- or multi-channel HEMTs with excellent

electron transport and hard pinch-off.

By applying Poisson equation and Fermi-Dirac statistics, we calculated the band

profile and the electron distribution of an AlGaN/GaN/AlGaN/GaN multilayer

structure. Parameters used for calculation are the same as those listed in Ref. 51. For

comparison, we also did calculation for a hypothetical AlGaN/GaN/AlGaN/GaN

multilayer structure, where polarization effect is not present but the AlGaN barrier

layers are modulation n-doped. As shown in Fig. 4.1.1 (a) and (b), polarization-

induced 2DEG channels are well confined at the AlGaN/GaN interfaces without

forming parasitic conduction channel in the AlGaN barrier layers. When the

polarization effect is in absence, n-type doping of the AlGaN layers not only leads to

accumulation of 2DEG channels, but also results in parallel conduction paths within

48

the doped AlGaN layers. Capacitance-voltage (CV) characteristics of the two

AlGaN/GaN/AlGaN/GaN multilayer structures were simulated and plotted in Fig.

4.1.1 (c), showing that polarization effect results in well-defined two electron

channels with high electron densities (1.0×1013 cm-2 in the upper channel and

0.3×1013 cm-2 in the lower channel).

0 30 60 90 121015

1017

1019

0

(b)

(a)

Con

cent

ratio

n (c

m-3)

Depth (nm)

-0.5

0.0

0.5

1.0 with polarization with n-doping

AlGaN GaNGaN

Ener

gy (e

V)

AlGaN

-10 -8 -6 -4 -2 0 20

200

400

600

800

(c)

Cap

acita

nce

(nf/c

m2 )

Bias (V)

with polarizationupper channel: Ns=1.0E13 cm-2

lower channel: Ns=0.3E13 cm-2

with n-dopingupper channel: Ns=0.7E13 cm-2

Lower channel: Ns=0.2E13 cm-2

Fig.4.1.1 Simulated conduction band diagrams (a), electron distribution profiles (b), and CV characteristics (c) of two hypothetical AlGaN/GaN/AlGaN/GaN multilayer structures. One is with polarization effect and without doping (solid lines); the other

one is modulation n-doped but without polarization effect (dashed lines).

49

Fig. 4.1.2 Cross sectional structure of an AlGaN-GaN double-channel sample.

3 nm undoped Al0.3Ga0.7N

18 nm Si-doped Al0.3Ga0.7N

3 nm undoped Al0.3Ga0.7N

14 nm undoped GaN

2.5 µm GaN buffer

Sapphire

21 nm undoped AlxG1-xN

(x graded from 3% to 6%)

Fig. 4.1.3 Cross-sectional TEM image of a MOCVD grown AlGaN-GaN double-

channel HEMT epilayer.

Epilayers for the double-channel HEMT fabrication were grown in an Axitron

2000HT metal organic chemical vapor deposition (MOCVD) reactor on sapphire

substrates. As depicted in Fig. 4.1.2, the layered structure consists of a 2.5-µm-thick

undoped GaN buffer layer, a 21-nm-thick AlGaN bottom barrier layer with the Al

composition graded from 3% at the lower interface to 6% at the upper interface, a

14-nm-thick GaN channel layer, and a 24-nm-thick Al0.3Ga0.7N top barrier layer. In

50

order to enhance the channel electron density and facilitate good ohmic contacts, the

top AlGaN barrier layer was selectively doped with Si. Grading profile of Al

composition of the lower AlGaN layer was designed to allow efficient access to the

lower electron channel [51]. Cross-sectional structure of the MOCVD grown

AlGaN/GaN/AlGaN/GaN multilayer was confirmed by a transmission electron

microscope (TEM) image shown in Fig. 4.1.3.

0 50 100 150 200 250 3001015

1016

1017

1018

1019

1020

Ele

ctro

n C

once

ntra

tion

(cm

-3)

Distance from The Surface (nm)

Fig. 4.1.4 Electron distribution profile of the double-channel HEMT extracted from measured CV characteristics.

Under single carrier channel approximation, the measured Hall mobility and

sheet electron density are 1050 cm2/V-s and 1.6×1013 cm-2 at room temperature, 4030

cm2/V-s and 1.5×1013 cm-2 at liquid nitrogen temperature. CV measurements were

carried out to profile the carrier distribution, showing that there are two electron

channels with a 35 nm peak-to-peak separation (Fig. 4.1.4). The two electron

channels are located at the upper and lower AlGaN/GaN interfaces (i.e., 24 nm and

59 nm below sample surface) of the AlGaN/GaN/AlGaN/GaN multilayer structure

respectively. These characterization results suggested successful implementation of a

double-channel HEMT epilayer with good electron transport properties.

51

4.2 DC IV Characteristics

Double-channel HEMTs were fabricated using the AlGaN/GaN/AlGaN/GaN

epilayer described above. Mesa isolation was formed by etching a depth of 300 nm

with Cl2-based inductively coupled plasma (ICP). Source and drain ohmic contacts

were made of Ti/Al/Ni/Au multilayer annealed at 850 oC for 30 seconds, yielding

contact resistance typically around 1.0 Ω-mm. Finally, Ni/Au Schottky gate was

defined by optical lithography with 1 µm gate length. Devices were not passivated

prior to DC and RF small-signal testing.

0 3 6 9 12 150

200

400

600

800

1000

1200

I D (m

A/m

m)

VDS (V)

VGS start: 1 V step: -1 V

Fig. 4.2.1 DC IV output characteristics of a double-channel HEMT.

10-µm-wide devices typically show a maximum drain current density (ID)

around 1100 mA/mm and a peak transconductance (GM) of 180 mS/mm. DC-IV

output and transfer characteristics of a 100-wide device were measured with an HP

4142B DC source/monitor unit, as plotted in Fig. 4.2.1 and Fig. 4.2.2. Limited by the

self-heating and the poor thermal conductivity of sapphire substrate, large gate width

devices have a reduction of ID together with the appearance of negative differential

52

resistance in the saturation region of the output IV curves, as shown in Fig. 4.2.1.

From Fig. 4.2.2, we can clearly see a double-hump feature in the GM versus gate bias

(VGS) curve, which corresponds to the effective gate modulation of the upper and the

lower 2DEG channel respectively. The double-channel HEMT shows hard pinch-off,

with subthreshold ID in the order of 0.1 mA/mm at 10 V drain bias (VDS). It is

noteworthy that the device shows small on-resistance without any kink in the linear

region of the output IV curves, indicating favorable access to the lower carrier

channel.

-12 -9 -6 -3 0 3

200

400

600

800

1000

0

30

60

90

120

150

I D (m

A/m

m)

VGS (V)

VDS = 10 VG

M (m

S/m

m)

Fig. 4.2.2 DC IV transfer characteristics of a double-channel HEMT.

53

4.3 RF Small-Signal Measurement and Analysis

100M 1G 10G 100G0

10

20

30

40

50

after de-embedding |h21|

2

U

as measured

VGS=-6.5V, VDS=10V

Gai

n (d

B)

Frequency (Hz)

Fig. 4.3.1 Current gain and unilateral power gain of a double-channel HEMT as a function of measurement frequency.

RF small-signal characterization was performed on wafer by measuring S-

parameters at a variety of DC bias points. An Agilent 8722E network analyzer was

used for S-parameter measurement in 50 MHz ~ 40 GHz frequency range; an HP

4142B DC source/monitor unit was used to set the DC bias; microwave probes and

cables were used to connect the device-under-testing to measurement ports. At the

optimum bias point, a current gain cutoff frequency (fT) of 12 GHz and a maximum

oscillation frequency (fmax) of 30 GHz were derived, which are comparable to

baseline AlGaN/GaN HEMTs fabricated in the same processing run. Nominally

undoped GaN films grown with the present MOCVD techniques usually show slight

n-type, with a background electron concentration in the order of 1016 cm-3.

Consequently, there are parasitic capacitance and conductance components between

54

the contact pads and the non-insulating GaN buffer layer, which contribute to RF

measurement results and may mask some intrinsic electrical characteristics of the

HEMTs. Open-pad de-embedding with the S-parameters of a dummy pad can

effectively strip pad-related parasitics. After de-embedding, both the current gain

(|h21|2) and the unilateral power gain (U) show better 20 dB per decade roll-off, as

shown in Fig. 4.3.1. The de-embedded fT and fmax are usually slightly higher than the

measured results.

Small-signal characteristics of the HEMT can be considered with an equivalent

circuit model shown in Fig. 4.3.2. Employing Golio’s parameter extraction method

[52], some equivalent circuit elements, such as the gate-to-source capacitance (CGS),

the transconductance (GM), the drain-to-source capacitance (CDS), and the drain-to-

source conductance (GDS), can be extracted from the measured S-parameters. In the

remainder of this section, detailed discussions on small-signal characteristics of the

AlGaN-GaN double-channel HEMT will be carried out. All of these results are

deduced from S-parameters after de-embedding.

Fig. 4.3.2 Equivalent circuit model for parameter extraction of the AlGaN-GaN double-channel HEMT.

55

4.3.1 AC Transconductance and Gate-to-Source Capacitance

One of the most distinct features of the double-channel HEMT is the double-

hump shape in the GM-VGS curve derived from DC-IV measurements. At RF, gate

modulation of the two carrier channels can be examined by showing CGS and GM as a

function of VGS. With the aforementioned equivalent circuit model, we performed

parameter extraction at 10 V VDS and 2.1 GHz measurement frequency. The AC GM

and CGS were extracted and plotted in Fig. 4.3.3. Similar to what observed in DC

transfer characteristics, there appears a double-hump feature in the GM-VGS curve at

2.1 GHz frequency. In the CGS-VGS curve, there are two plateaus, corresponding to

effective gate modulation of the upper and the lower carrier channel respectively.

These results indicate the AlGaN-GaN double-channel HEMT manifests well-

defined double-channel behaviors up to RF.

-9 -6 -3 00.0

0.1

0.2

0.3

0.4

0.5

0

30

60

90

120

150

CG

S(pF

)

VGS (V)

@ 2.1 GHzVDS = 10 V

GM (m

S/m

m)

Fig. 4.3.3 Gate bias-dependent gate-to-source capacitance and transconductance of a double-channel HEMT.

56

Located further away from the gate, the lower carrier channel has relatively low

GM. Nevertheless, charging capacitance (CGS) of the lower channel is smaller than

the upper channel. Since fT is roughly proportional to GM and inversely proportional

CGS, the lower channel still gives a high fT in spite of the low GM.

4.3.2 Output Impedance

-9 -6 -3 00.00

0.02

0.04

0.06

0.08

0.10

0

10

20

30

40

50

CD

S (pF

)

VGS (V)

@ 2.1 GHzVDS = 10 V

GD

S (m

S/m

m)

Fig. 4.3.4 Gate bias-dependent output capacitance and conductance of a double-channel HEMT.

In spite of the existence of a second carrier channel relatively far away from the

gate, the double channel HEMT shows a hard pinch-off with the subthreshold drain

current in the order of 0.1 mA/mm at 10 V VDS. It is of interest to see if the two

carrier channels behave as well-controlled current sources at RF. Using Golio’s

parameter extraction method, the output impedance was extracted at 10 V VDS and

2.1 GHz frequency in form of output capacitance (CDS) and output conductance

(GDS), as shown in Fig. 4.3.4. At gate biases below -4 V where the lower carrier

57

channel is under effective gate modulation, output impedance of the double-channel

HEMT are as large as those obtained from baseline AlGaN/GaN HEMTs fabricated

in the same processing run, suggesting good microwave power performance. In the

first order approximation, fmax is related to fT in the following equation [53].

2/1max )(21

CHG

DS

T RRR

ff

+=

fT is determined by the channel carrier velocity and the gate length; RG is the

gate resistance; and RCH is the channel charging resistance. A larger value of output

resistance RDS gives higher fmax. The extracted large output resistance of the double-

channel HEMT is consistent with the fact that the device has an fmax to fT ratio as

high as 2.5 in the operation of the lower channel.

4.3.3 Current and Power Gain

Large-signal gain and linearity characteristics of transistors can be predicted by

bias-dependent fT and fmax within the output IV plane. Fig. 4.3.5 shows de-embedded

fT and fmax of the double-channel HEMT as a function of the VGS at 10 V VDS. Once

again, double-channel behaviors can be observed. There are two major factors

determining the fT value. One is the effective electron drift velocity through the

channel; and the other is the channel charging delay associated with channel

resistance. As VGS increases from pinch-off voltage, the number of mobile electrons

increases, the channel resistance decreases, and the fT value increases. After reaching

the peak fT at -6.5 V VGS, further increase of VGS results in more electrons close to

the lower AlGaN/GaN interface, and make the electrons suffering more from

interface scattering. Consequently, the overall electron drift velocity decreases, and

the fT value drops down. When the VGS becomes larger than -4 V, electrons begin to

58

populate the upper channel, and the fT value rises again due to the increasing number

of high-velocity electrons in the upper channel and the decrease of the associated

channel charging resistance. Similar to what happened in the lower channel, further

increase of VGS results in a reduction of overall electron drift velocity in the upper

channel and degrades the fT value. To the first order approximation, GM can be

expressed by CGS and fT as G GSTM Cfπ2= . Since the fT-VGS curve has a double-

hump shape (Fig. 4.3.5 (a)) and the CGS monotonically depends on VGS (Fig. 4.3.3),

there has to be a double-hump structure in the GM-VGS curve, as shown in Fig. 4.2.2

and Fig. 4.3.3.

-9 -6 -3 00

10

20

30

40

fT fmax

VDS = 10 V

Freq

uenc

y (G

Hz)

VGS(V)

Fig. 4.3.5 Gate bias-dependence fT and fmax of a double-channel HEMT.

It is noteworthy that currently all of the AlGaN/GaN HEMTs exhibit gain

compression (e.g. GM, fT, and fMAX reduction) at high current level. This problem

hinders the AlGaN/GaN HEMTs for high power applications at high frequency.

There are models attempting to explain the gain compression behavior, such as the

hot phonon effect [54], and the nonlinear source resistance [55]. However, none of

those models can explain the fT-VGS profile of our double-channel HEMTs. Electron

59

velocity degradation induced by interface scattering is likely a plausible explanation.

Fixing VGS at -6.5 V, we also examined the dependence of fT and fmax on VDS.

From Fig. 4.3.6, one can see that with the increase of VDS, fT and fmax saturate

quickly and remain constant within the whole saturation region. The ability to

maintain a high fT at high VDS indicates excellent high-field electron drift velocity

and minimal extension of gate depletion region toward the drain side. The flat fT &

fmax versus VDS curves mean more linear operation can be achieved at high voltage

operations, where the load line covers a larger VDS range and the linearity is more

VDS dependent [56].

0 2 4 6 8 10 120

10

20

30

40

VGS = -6.5 V fT fmax

Freq

uenc

y (G

Hz)

VDS (V)

Fig. 4.3.6 Drain bias-dependence fT and fmax of a double-channel HEMT.

Electron transport properties of the lower channel can be evaluated through the

bias dependent fT near pinch-off voltage. Following the method of Moll et al. [57],

the channel transit time delay (or the effective electron drift velocity) of the lower

channel can be extracted by plotting the total transistor delay (τ=1/2πfT) against the

inverse of drain current (1/ID), as shown in Fig. 4.3.7. At low current level when only

60

the lower channel is turned on, the delay time τ decreases linearly as the 1/ID

decreases. The linear decay term is the channel charging delay, which approaches

zero at infinite ID. The extrapolated intersect at 1/ID =0 then corresponds to the

channel transit time delay of the lower channel. From Fig. 4.3.7, the extrapolated

channel transit time is 8.8 ps (equivalent to an effective electron drift velocity of

1.14×107 cm/s, which is comparable to the value of 1.2×107 cm/s reported by Akita

et al. [58]), indicating excellent transport properties of the lower 2DEG. At higher

current levels when both channels are turned on, the delay time once again decreases

as a function of decreasing 1/ID. An extrapolation of this region will give the channel

transit time delay of the upper channel, which reflects electron transport properties of

the upper 2DEG. However, there are not enough data points available for accurate

extrapolation.

0 50 100 150 200 25005

10152025303540

Feature related toupper channel

τ = 8.8 ps (lower channel)

τ (p

s)

1/IDS(A-1)

Fig. 4.3.7 Extraction of effective channel transit delay of the double-channel HEMT.

61

4.4 Dynamic IV Characterization

0 2 4 6 8 100

300

600

900

1200

1500(a)Quiescent point: VDS=0V, VGS=0V

I D (m

A/m

m)

VDS (V)

0 2 4 6 8 100

300

600

900

1200

1500

I D (m

A/m

m)

VDS (V)

(b)Quiescent point: VDS=0V, VGS=-8V

0 2 4 6 8 100

300

600

900

1200

1500

I D (m

A/m

m)

VDS (V)

(c)Quiescent point: VDS=10V, VGS=0V

62

0 2 4 6 8 100

300

600

900

1200

1500

I D (m

A/m

m)

VDS (V)

(d)Quiescent point: VDS=10V, VGS=-8V

Fig. 4.4.1 Dynamic IV characteristics (circles) of an AlGaN-GaN double-channel HEMT in comparison with DC IV characteristics (black lines). VGS: -8 ~ 1 V.

Using the measurement method described in Section 3.3, we conducted

qualitative analysis of current collapse in the AlGaN-GaN double-channel HEMT.

Dynamic IV characteristics of an unpassivated AlGaN-GaN double-channel HEMT

were measured and shown in Fig. 4.4.1. The measured device has a gate length of 1

µm and a gate width of 2×50 µm. The pulse width is 1 µS and the pulse separation is

1 ms. 4 different quiescent bias points were chosen. Meanwhile, DC output IV

characteristics of the same HEMT were given for comparison.

Similar to what observed from the AlGaN-GaN single-channel HEMT, when

the quiescent point is at VGS=0 V and VDS= 0 V, the pulsed drain current does not

show any collapse and becomes higher than the DC current due to the alleviation of

self-heating effect (Fig. 4.4.1 (a)). As the quiescent VGS moves to pinch-off voltage

(Fig. 4.4.1 (b)), current collapse starts to occur. The current collapse becomes more

severe when the quiescent VDS moves to higher voltage (Fig. 4.4.1 (d)). It is

noteworthy that when pulsed from VDS=10 V and VGS= 0 V, the drain current shows

considerable amount of collapse (Fig. 4.4.1 (c)). This is due to the fact that there is

63

an effective off-to-on pulse between the gate and drain terminals when VDS is pulsed

from 10 V to smaller voltages. From Fig. 4.4.1., one can observe that unlike single-

channel HEMTs, the double-channel HEMT does not have any current collapse at

low current levels (e.g. VGS <-4 V), where the drain current is contributed by the

lower carrier channel. This observation suggests that the lower channel is immune to

the current collapse problem.

In order to highlight current collapse behaviors of the two individual channels in

the AlGaN-GaN double-channel HEMT, we did pulsed IV measurement in the

transfer curve fashion. VDS is fixed at 10 V, VGS is pulsed from on-state (0 V) or off-

state (-8 V) to any point in the curve. In order to obtain smooth curves with enough

data points for transconductance (GM) calculation, we chose relatively long pulses (1

ms pulse width and 100 ms pulse separation) using an HP 4142B DC source/monitor

unit. The pulsed transfer characteristics are shown in Fig. 4.4.2. It is obvious that

when pulsed from pinch-off, the upper carrier channel heavily suffers from current

collapse with a drastic reduction of both ID and GM. Meanwhile, the lower carrier

channel has minimal collapse. The current-collapse feature can be maintained to a

current level of 25 mA/mm, which is about 30% of the saturation drain current.

Alleviation of current collapse in the lower channel is in agreement with the surface

trapping/de-trapping model described in Section 3.3. In the gate-to-drain spacing

region, the upper channel has a large number of mobile electrons and effectively

screens the lower carrier channel from potential fluctuations on device surface.

Consequently, surface trapping/de-trapping has little chance to reduce the current or

delay the frequency response of the lower channel. This novel feature indicates that

current collapse has a chance to be alleviated on a reproducible basis by developing

double- or multiple-channel HEMTs with properly managed carrier density in each

64

channel.

-10 -8 -6 -4 -2 0 20

200

400

600

800

1000

0

30

60

90

120

150 From VGS = 0 V From VGS = -8 V

VDS = 10 V

I D (m

A/m

m)

VGS (V)

GM (m

S/m

m)

Fig. 4.4.2 Pulsed transfer characteristics of a double-channel HEMT. The pulse width is 1 ms, and the pulse separation is 100 ms.

65

4.5 RF Large-Signal Power and Linearity Measurement

In order to increase the breakdown voltage and reduce the current collapse of

the upper channel [24], we passivated the surface of AlGaN/GaN/AlGaN/GaN

double-channel HEMTs with a 200-nm-thick Si3N4 layer grown by plasma-enhanced

chemical vapor deposition (PECVD). After passivation, continuous wave (CW) RF

large-signal characteristics, such as power and linearity, of a 1×100 µm2 device were

measured with a Maury MT986 automated load-pull system at 2 GHz frequency.

During measurement, the input and output impedance were optimized to achieve

maximum output power; no cooling system was used.

4.5.1 Single-Tone Power Performance

Under single-tone input signal drive, the output power (POUT), the transducer

power gain (GT), and the power added efficiency (PAE) were measured as a function

of the input power (PIN) at different quiescent DC biases. From Fig. 4.5.1 (a), we can

see that under small-signal drive (PIN less than 5 dBm), the power gain has the largest

value at -6 V VGS, which is consistent with the result that fmax peaks around -6 V (Fig.

4.2.2.). Whereas in large-signal operation (PIN larger than 8 dBm), both POUT and GT

decrease monotonously, as the quiescent VGS approaches pinch-off voltage. This is

due to the fact that the AC current swing tends to become more severely collapsed at

more negative quiescent VGS (Fig. 4.4.1 (c) and (d)). Theoretically, PAE increases

during the transition from Class A mode to class B mode (i.e., quiescent VGS moves

from -3 V to -8V for this double-channel HEMT under study). However, aggravation

of current collapse at too negative VGS (class B operation mode) makes the PAE

66

reduced for class B operation and the highest for class AB operation, as shown in Fig.

4.5.1 (b).

-10 -5 0 5 10 150

5

10

15

20

25

30 VGS = -4 V VGS = -6 V VGS = -8 V

GT

P OU

T (dB

m),

GT (

dB)

PIN (dBm)

POUT

(a)

-10 -5 0 5 10 150

5

10

15

20

25

30(b) VGS = -4 V

VGS = -6 V VGS = -8 V

PAE

(%)

PIN (dBm)

Fig. 4.5.1 POUT, GT, and PAE of a double-channel HEMT measured at 2 GHz. The quiescent bias point is at 15 V VDS and varied VGS. Impedance matching is optimized

for maximum POUT.

Fixing the quiescent VGS at -5 V, the power measurement was conducted at a

series of quiescent VDS (Fig. 4.5.2). With the increase of quiescent VDS, POUT shows

very slight increase, with the highest out power density of 1.74 W/mm at 30 V VDS

67

bias, whereas PAE degrades steadily. This result is in agreement with the argument

made in section II that current collapse aggravates at higher quiescent VDS (Fig. 4.4.1

(b) and (d). From this point of view, the current collapse problem greatly hinders the

power performance of AlGaN/GaN HEMTs, especially at high voltage operation.

-10 -5 0 5 10 150

5

10

15

20

25

30

VDS = 15V VDS = 20V VDS = 25V VDS = 30V

Pou

t (dB

m),

GT (

dB)

Pin (dBm)

1.74w/mm

-10 -5 0 5 10 150

5

10

15

20

25

30

VDS = 15V VDS = 20V VDS = 25V VDS = 30V

PA

E (%

)

Pin (dBm)

Fig. 4.5.2 POUT, GT, and PAE of a double-channel HEMT measured at 2 GHz. The quiescent bias point is at -5 V VGS and varied VDS. Impedance matching is optimized

for maximum POUT.

68

4.5.2 Two-Tone Intermodulation Distortion Profile

Linearity of AlGaN/GaN HEMTs is a crucial consideration in RF power

amplifier applications [56, 59]. One conventional method of characterizing amplifier

linearity is to measure the two-tone 3rd order intermodulation (IM3) distortion profile.

In our measurement, frequencies of the two-tone drive were 2 GHz and 2.001 GHz

with 1 MHz frequency spacing. Fig. 4.5.3 is the IM3 as a function of back-off power

from the saturated POUT at different quiescent VGS.

20 15 10 5 0

-50

-40

-30

-20

-10 VGS = -3 V VGS = -4 V VGS = -5 V VGS = -6 V VGS = -7 V

IM3

(dBc

)

Back-off (dB)

Fig. 4.5.3 IM3 of a double-channel HEMT as the function of output power back-off at 20 V VDS and varied VGS. Impedance matching is optimized for maximum POUT.

Under small-signal drive (i.e. large back-off), IM3 is low at -3 V, -6V and -7 V

VGS. From the DC transfer (Fig. 4.2.2) and AC transconductance characteristics (Fig.

4.3.3), we can find that at those bias points, small-signal operation takes place within

effective operation of each individual channel in the double-channel HEMT. Under

these conditions, a symmetric output signal waveform can be expected, yielding little

intermodulation distortion. While -4 V and -5 V quiescent VGS points are located

around the boundary between effective operations of the two channels, the output

69

signal waveform has to be asymmetric even under small-signal drive, due to different

electrical characteristics (e.g. higher GM of the upper channel) of the two channels.

The asymmetric output signal waveform resulted in large intermodulation distortion

and poor linearity.

Fig. 4.5.4 Schematic showing the input and output signal waveforms of the double channel HEMT in small- and large-signal operation. The quiescent VGS is at -4 V.

However, under large-signal drive (i.e. small back-off), sweet spot was formed

and the lowest IM3 was achieved at -4 V quiescent VGS. At this quiescent bias point,

effective operation of the upper channel forms upside waveform of the output signal;

and effective operation of the lower channel forms downside waveform. Good

linearity is achieved through a balance of two opposite effects: higher power deliver

capability and more current collapse during the effective operation of the upper

channel (i.e. the upside waveform). There are two factors leading to more current

collapse of the upper channel: on one side, the upper channel is closer to surface and

more sensitive to surface trapping/de-trapping; on another side, effective operation of

the upper channel forms the upside waveform of the output signal which is more

likely to collapse, as discussed in section II. During large-signal operation of the

70

upper channel, high power deliver capability is compensated by severe current

collapse, as sketched in Fig. 4.5.4. Consequently, at -4 V quiescent VGS the output

signal waveform becomes more symmetric under large-signal drive, yielding a sweet

pot in the linearity characteristics.

71

CHAPTER 5

CONCLUSION

5.1 Summary

In conclusion, we performed systematic study on the AlGaN-GaN single- and

double-channel HEMTs. Baseline HEMT processing technologies were established.

The fabricated AlGaN-GaN single-channel HEMTs have drain current density larger

than 900 mA/mm, transconductance around 180 mS/mm, fT of 13 GHz and fmax of 35

GHz for 1 µm gate length device.

Trap states in AlGaN/GaN single-heterostructures were characterized by bias-

and frequency-dependent admittance measurements. AlGaN/GaN interface trap state

density and time constant were extracted. Dynamic IV characterization of AlGaN-

GaN single-channel HEMTs suggest that trap/de-trapping in the device gate-to-drain

region is mainly responsible for current collapse. Correlation was found between

surface trap states and the current collapse.

AlGaN-GaN double-channel HEMTs were proposed and developed. This

design increases the 2DEG/current density, reduces the current collapse, and

provides more degrees of freedom for linearity engineering. What is more important,

the double-channel HEMT provides a ideal platform for the in-depth investigation of

several bottleneck problems of GaN-based HEMTs, such as current collapse and gain

compression at high current levels.

72

5.2 Suggestion of Future Work

As mentioned in 5.1, there are two major problems limiting the GaN HEMT

performance. One is the current collapse; the other is the gain compression at high

current level. Further work is suggested to overcome these two problems.

In light of the fact that trapping/de-trapping leading to current collapse manily

occurs in the gate-to-drain spacing region, one is motivated to minimize this spacing

region. Gate self-aligned HEMTs could be an ideal solution to the current collapse

problem. However, self-aligned implantation need very high activation temperature,

where as self-aligned gate recess etching causes harsh surface damage. Progress in

the self-aligned technology will likely lead to a big breakthrough in the device

performance, and drive GaN HEMTs into commercialization.

Gain compression at high current levels largely limits the power performance of

GaN HEMTs, especially at high frequency bands (such as millimeter wave

operation). In addition, the gain compression degrades device linearity, which is an

important concern in wireless communications. This thesis’s primary results

indicated that interface scattering could be the main source of current collapse.

Optimization of AlGaN/GaN interface quality may alleviate this problem. Epilayer

structure design, such as quantum well channel HEMT, may also helps.

73

BIBLIOGRAPHY AND REFERENCES

[1] T. Mimura, S. Hiyamizu, T. Fujii, and K. Nanbu, “A new field-effect transistor

with selectively doped GaAs/n-AlGaAs heterojunctions,” Jpn. J. Appl. Phys.,

vol. 19, no. 5, pp. L225-L227, May 1980.

[2] R. Dingle, H. L. Störmer, A. C. Gossard, and W. Wiegmann, “Electron

mobilities in modulation-doped semiconductor heterojunction supperlattices,”

Appl. Phys. Lett., vol. 33, no. 7, pp. 665-667, Oct. 1978.

[3] R. M. Chu, “Electronic properties of Gallium Nitride-based heterostructures,”

B. S. thesis, Dept. Phys., Nanjing Univ., Nanjing, China, June 2002.

[4] M. Heuken, “High electron mobility transistor – basic principles, current

status, and future trends,” Lecture Notes, Hong Kong Univ. of Sci. & Tech.,

Hong Kong, China, April 2004.

[5] L. D. Nguyen, L. E. Larson, and U. K. Mishra, “Ultra-high-speed modulation-

doped field-effect transistors: a tutorial review,” Proc. IEEE, vol. 80, no. 4, pp.

494-518, April 1992.

[6] J. I. Pankove and T. D. Moustakas, Gallium Nitride (I & II). San Diego:

Academic Press, 1998 & 1999.

[7] F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and

piezoelectric constants of III-V nitrides,” Phys. Rev. B, vol. 56, no. 16, pp.

10024-10027, Oct. 1997.

[8] V. Fiorentini, F. Bernardini, and O. Ambacher, “Evidence for nonlinear

macroscopic polarization in III-V nitride alloy heterostructures,” Appl. Phys.

Lett., vol. 80, no. 7, pp. 1204-1206, Feb. 2002.

74

[9] M. Stutzmann, O. Ambacher, M. Eickhoff, U. Karrer, A. L. Pimenta, R.

Neuberger, J. Schalwig, R. Dimitrov, P. J Schuck, and R. D. Grober, “Playing

with polarity,” Phys. Stat. Sol. B, vol. 228, no. 2, pp. 505-512, Nov. 2001.

[10] O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, W.

J. Schaff, L. F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger,

and J. Hilsenbeck, “Two-dimensional electron gases induced by spontaneous

and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN

heterostructures,” J. Appl. Phys., vol. 85, no. 6, pp. 3222-3233, March 1999.

[11] O. Ambacher, B. Foutz, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M.

Murphy, A. J. Sierakowski, W. J. Schaff, L. F. Eastman, R. Dimitrov, A.

Mitchell, and M. Stutzmann, “Two dimensional electron gases induced by

spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN

heterostructures,” J. Appl. Phys., vol. 87, no. 1, pp. 334-344, Jan. 2000.

[12] J. P. Ibbetson, P. T. Fini, K. D. Ness, S. P. DenBaars, J. S. Speck, and U. K.

Mishra, “Polarization effects, surface states, and the source of electrons in

AlGaN/GaN heterostructure field effect transistors,” Appl. Phys. Lett., vol. 77,

no. 2, pp. 250-252, July 2000.

[13] H. Morkoç, A. Di Carlo, and R. Cingolani, “GaN-based modulation doped

FETs and UV detectors,” Solid-State Electronics, vol. 46, no. 2, pp. 157-202,

Feb. 2000.

[14] G. Koley and M. G. Spencer, “Surface potential measurements on GaN and

AlGaN/GaN heterostructures by scanning Kelvin probe microscopy,” J. Appl.

Phys., vol. 90, no. 1, pp. 337-344, Jul. 2001.

75

[15] M. A. Khan, A. Bhattarai, J. N. Kuznia, and D. T. Olson, “High-electron-

mobility transistor based on a GaN-AlxGa1-xN heterojunction,” Appl. Phys.

Lett., vol. 63, no. 9, pp. 1214-1215, Aug. 1993.

[16] Y. F. Wu, A. Saxler, M. Moore, R. P. Smith, S. Sheppard, P. M. Chavarkar, T.

Wisleder, U. K. Mishra, and P. Parikh, “30-W/mm GaN HEMTs by field plate

optimization,” IEEE Electron Device Lett., vol. 25, no. 3, pp. 117-119, March

2004.

[17] H. P. Maruska and J. J. Tietjen, “The preparation and properties of vapor-

deposited single-crystalline GaN,” Appl. Phys. Lett., vol. 15, no. 10, pp. 327-

329, Nov. 1969.

[18] H. M. Manasevit, F. M. Erdmann, and W. I. Simpson, “The use of

metalorganics in the preparation of semiconductor materials—IV: the nitrides

of aluminum and gallium,” J. Electronchem. Soc. vol. 118, no. 11, pp. 1864-

1868, 1971.

[19] M. A. Khan, J. N. Kuznia, J. M. Vanhove, N. Pan, and J. Carter, “Observation

of a two-dimensional electron-gas in low-pressure metalorganic chemical

vapor-deposited GaN-AlxGa1-xN heterojunctions,” Appl. Phys. Lett., vol. 60,

no. 24, pp. 3027-3029, June 1992.

[20] M. A. Khan, J. N. Kuznia, D. T. Olson, W. J. Schaff, J. W. Burm, and M. S.

Shur, “Microwave performance of a 0.25 µm gate AlGaN/GaN heterostructure

field-effect transistor,” Appl. Phys. Lett., vol. 65, no. 9, 1121-1123, Aug. 1994.

[21] Y. F. Wu, B. P. Keller, S. Keller, D. Kapolnek, S. P. DenBaars, and U. K.

Mishra, “Measured microwave power performance of AlGaN/GaN

MODFET,” IEEE Electron Device Lett., vol. 17, no. 9, 455-457, Sep. 1996.

76

[22] E. Kohn, I. Daumiller, P. Schmid, N. X. Nguyen, and C. N. Nguyen, “Large

signal frequency dispersion of AlGaN/GaN heterostructure field effect

transistors,” Electron. Lett., vol. 35, no. 12, pp. 1022-1024, June 1999.

[23] S. T. Sheppard, K. Doverspike, W. L. Pribble, S. T. Allen, J. W. Palmour, L.

T. Kehias, and T. J. Jenkins, “High-power microwave GaN/AlGaN HEMT’s

on semi-insulating silicon carbide substrates,” IEEE Electron Device Lett., vol.

20, no. 4, pp. 161-163, April 1999.

[24] B. M. Green, K. K. Chu, E. M. Chumbes, J. A. Smart, J. R. Shealy, and L. F.

Eastman, “The effect of surface passivation on the microwave characteristics

of undoped AlGaN/GaN HEMT’s,” IEEE Electron Device Lett., vol. 21, no. 6,

pp. 268-270, June 2000.

[25] D. K. Schroder, Semiconductor Material and Device Characterization. New

York: Wiley, 1990, pp. 195-216.

[26] T. Ando, A. B. Fowler, and F. Stern, “Electronic properties of two-

dimensional systems,” Rev. Mod. Phys., vol. 54, no. 2, pp. 437-672, April

1982.

[27] C. Weisbuch, and B. Vinter, Quantum Semiconductor Structures. Boston:

Academic Press, 1991.

[28] S. Contreras, W. Knap, E. Frayssinet, M. L. Sadowski, M. Goiran, and M.

Shur, “High magnetic field studies of two-dimensional electron gas in a

GaN/AlGaN heterostructure: mechanism of parallel conduction,” J. Appl.

Phys., vol. 89, no. 2, pp. 1251-1255, Jan. 2001.

[29] M. A. Paalanen, D. C. Tsui, and J. C. M. Hwang, “Parabolic

magnetoresistance from the interaction effect in a two-dimensional electron

gas,” Phys. Rev. Lett., vol. 51, no. 24, pp. 2226-2229, Dec. 1983.

77

[30] E. Skuras, R. Kumar, R. L. Wiiliams, R. A. Strading, J. E. Dmochowski, E. A.

Johnson, A. Mackinnon, J. J. Harris, R. B. Beall, C. Skierbeszewski, J.

Singleton, P. J. van der Wel, and P. Wisniewski, “Subband dependent

mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs

in the high density limit,” Semicond. Sci. Technol., no. 6, pp. 535-546, June

1991.

[31] D. K. Schroder, Semiconductor Material and Device Characterization. New

York: Wiley, 1990, pp. 41-58.

[32] S. C. Binari, K. Ikossi, J. A. Roussos, W. Kruppa, D. Park, H. B. Dietrich, D.

D. Koleske, A. E. Wickenden, and R. L. Henry, “Trapping effects and

microwave power performance in AlGaN/GaN HEMTs,” IEEE Trans.

Electron Devices, vol. 48, no. 3, pp. 465-471, March 2001.

[33] E. J. Miller, X. Z. Dang, H. H. Wieder, P. M. Asbeck, E. T. Yu, G. J. Sullivan,

and J. M. Redwing, “Trap characterization by gate-drain conductance and

capacitance dispersion studies of an AlGaN/GaN heterostructure field-effect

transistor,” J. Appl. Phys., vol. 87, no. 11, pp. 8070-8073, June 2000.

[34] E. H. Nicollian and A. Goetzberger, “The Si-SiO2 interface—electrical

properties as determined by the metal-insulator-silicon conductance

technique,” Bell. Syst. Tech. J., vol. 46, no. 6, pp. 1055-1133, Aug. 1967.

[35] H. Rohdin, N. Moll, A. M. Bratkovsky, and C. Y. Su, “Dispersion and

tunneling analysis of the interfacial gate resistance in Schottky barriers,” Phys.

Rev. B, vol. 59, no. 20, 13102-13113, May 1999.

[36] A. Hierro, S. A. Ringel, M. Hansen, J. S. Speck, U. K. Mishra, and S. P.

DenBaars, “Hydrogen passivation of deep levels in n-GaN,” Appl. Phys. Lett.,

vol. 77, no. 10, 1499-1501, Sep. 2000.

78

[37] C. Nguyen, N. X. Nguyen, and D. E. Grider, “Drain current compression in

GaN MODFETs under large-signal modulation at microwave frequencies,”

Electron. Lett., vol. 35, no. 16, pp. 1380-1382, Aug. 1999.

[38] I. Daumiller, D. Theron, C. Gaquière, A. Vescan, R. Dietrich, A. Wieszt, H.

Leier, R. Ventury, U. K. Mishra, I. P. Smorchkova, S. Keller, N. X. Nguyen, C

Nguyen, and E. Kohn, “Current instabilities in GaN-based devices,” IEEE

Electron Device Lett., vol. 22, no. 2, pp. 62-64, Feb. 2001.

[39] R. Vetury, N. Q. Zhang, S. Keller, and U. K. Mishra, “The impact of surface

states on the DC and RF characteristics of AlGaN/GaN HFETs,” IEEE Trans.

Electron Devices, vol. 48, no. 3, pp. 560-566, March 2001.

[40] G. Verzellesi, R. Pierobon, F. Rampazzo, G. Meneghesso, A. Chini, U. K.

Mishra, C. Canali, and E. Zanoni, “Experimental/numerical investigation on

current collapse in AlGaN/GaN HEMT’s,” in IEDM Tech. Digest, pp. 689-692,

Dec. 2002.

[41] A. Tarakji, G. Simin, N. Ilinskaya, X. Hu, A. Kumar, A. Koudymov, J. Yang,

M. A. Khan, M. S. Shur, R. Gaska, “Mechanism of radio-frequency current

collapse in GaN-AlGaN field-effect transistors,” Appl. Phys. Lett., vol. 78, no.

15, pp. 2169-2171, April 2001.

[42] K. Joshin, T. Kikkawa, H. Hayashi, S. Yokogawa, M. Yokoyama, N. Adachi,

and M. Takikawa, “A 174 W high-efficiency GaN HEMT power amplifier for

W-CDMA base station applications,” in IEDM Tech. Digest, pp. 983-985, Dec.

2003.

[43] G. Simin, A. Koudymov, A. Tarakji, X. Hu, J. Yang, M. A. Khan, M. S. Shur,

R. Gaska, “Induced strain mechanism of current collapse in AlGaN/GaN

79

heterostructure field-effect transistors,” Appl. Phys. Lett., vol. 79, no. 16, pp.

2651-2653, Oct. 2001.

[44] R. J. Trew, “SiC and GaN transistors – is there one winner for microwave

power applications?” Proc. IEEE, vol. 90, no. 6, pp. 1032–1047, June 2002.

[45] J. S. Lee, A. Vescan, A. Wieszt, R. Dietrich, H. Leier, and Y. S. Kwon,

“Characteristics of AlGaN/GaN HEMT devices with SiN passivation,” in

IEDM Tech. Digest, pp. 381-384, Dec. 2000.

[46] K. J. Choi, H. W. Jang, and J. L. Lee, “Observation of inductively coupled-

plasma-induced damage on n-type GaN using deep-level transient

spectroscopy,” Appl. Phys. Lett., vol. 82, no. 8, pp. 1233-1235, Feb. 2003.

[47] R. Allam, C. Kolanowski, D. Theron, and Y. Crosnier, “Subharmonic gate

mixer on a multichannel HEMT,” IEEE Microwave and Guided Wave Lett.,

vol. 5, no. 4, pp. 122-123, April 1995.

[48] N. H. Sheng, C. P. Lee, R. T. Chen, D. L. Miller, and S. J. Lee, “Multiple-

channel GaAs/AlGaAs high electron mobility transistors,” IEEE Electron

Device Lett., vol. 6, no. 6, pp 307-310, June 1985.

[49] D. C. Dumka, G. Cueva, H. Hier, O. A. Aina, and I. Adesida, “DC and RF

characteristics of doped multichannel AlAsSb/InGaAs field effect transistors

with variable gate-lengths,” IEEE Electron Device Lett., vol. 22, no. 1, Jan.

2001.

[50] R. M. Chu, Y. D. Zheng, Y. G. Zhou, S. L. Gu, B. Shen, P. Han, R. Zhang, R.

L. Jiang, and Y. Shi, "Strong quantum confinement and high carrier

concentration in AlGaN/InGaN/GaN heterostructure field-effect transistors,"

Appl. Phys. A, vol. 77, pp. 669-671, Oct. 2003.

80

[51] S. Heikman, S. Keller, D. S. Green, S. P. DenBaars, and U. K. Mishra, “High

conductivity modulation doped AlGaN/GaN multiple channel

heterostructures,” J. Appl. Phys., vol. 94, no. 8, pp. 5321-5325, Oct. 2003.

[52] J. M. Golio, Microwave MESFETs & HEMTs. Boston: Artech House, 1991,

pp. 73-79, pp. 216-236.

[53] J. M. Golio, Microwave MESFETs & HEMTs. Boston: Artech House, 1991,

pp. 299.

[54] A. Matulionis, “Comparative analysis of hot phonon effects in nitride and

arsenide channels for HEMTs,” in DRC Conference Digest, pp. 145-146, Jun.

2004.

[55] T. Palacios, S. Rajan, L. Shen, A. Chakraborty, S. Heikman, S. Keller, S. P.

DenBarrs, and U. K. Mishra, “Influence of the access resistance in the RF

performance of mm-wave AlGaN/GaN HEMTs,” in DRC Conference Digest,

pp. 75-76, Jun. 2004.

[56] M. Nagahara, T. Kikkawa, N. Adachi, Y. Tateno, S. Kato, M. Yokoyana, T.

Kimura, Y. Yamaguchi, N. Hara, and K. Joshin, “Improved intermodulation

distortion profile of AlGaN/GaN HEMT at high drain bias voltage,” in IEDM

Tech. Digest, pp. 693-696, Dec. 2002.

[57] N. Moll, M. R. Hueschen, and A. Fischer-colbrie, “Pulse-doped

AlGaAs/InGaAs pseudomorphic MODFETs,” IEEE Trans. Electron Devices,

vol. 35, no. 7, pp. 879-886, July 1988.

[58] M. Akita, S. Kishimoto, K. Maezawa, and T. Mizutani, “Evaluation of

effective electron velocity in AlGaN/GaN HEMTs,” Electron. Lett., vol. 36, no.

20, pp. 1736-1737, Sep. 2000.

81

[59] Y. F. Wu, P. M. Chavarkar, M. Moore, P. Parikh, and U. K. Mishra, “Linearity

and gain characteristics of AlGaN/GaN HEMTs,” in IEDM Tech. Digest, pp.

697-699, Dec. 2002.

82

APPENDIX A:

PROCESS FLOW FOR THE FABRICATION OF GAN HEMTS

1 Mesa Isolation

1.1 Solvent Cleaning

1) Check the resistivity of DI water. It should be > 17 MΩ 2) ACE 3 minutes, ultrasonic 1 minute. 3) ISO 3 minutes, ultrasonic 1 minute. 4) DI water spray rinse, 4 cycles. 5) Blow dry with N2 gun. 6) Dehydration bake, 120 oC, 10 minutes in oven.

1.2 Photoresist Application

1) Cool down after dehydration, 5 minutes. 2) Put wafer on spinner chuck with vacuum on, blow with N2. 3) Coat AZ 703 photoresist. 4) Spin at 4000 rpm for 30 seconds, ~1 µm thick. 5) Soft bake, 90 oC, 1 minute, on hotplate.

1.3 Photoresist Exposure and Development

1) Exposure for 3.8 seconds, Karl Suss MA6 Aligner, hard contact mode. 2) Post-exposure bake, 110 oC, 1 minute, on hotplate. 3) Develop in FHD-5 for 60 seconds. 4) DI water spray rinse, 4 cycles. 5) Check under microscopy.

1.4 Oxygen Plasma Descum of Photoresist (Optional)

1) Chamber pressure = 300 mT of O2. 2) Temperature: 70 oC. 3) Run for 0.7 minute.

1.5 ICP Mesa Etch

83

1) Cl2 flow rate = 15.0 sccm 2) He flow rate = 10.0 sccm 3) Chamber pressure = 5.0 mTorr. 4) Power = 135 W. 5) 40 seconds etch, 300 nm.

2 Mesa Isolation

2.1 Solvent Cleaning

7) Check the resistivity of DI water. It should be > 17 MΩ 8) ACE 3 minutes, ultrasonic 1 minute. 9) ISO 3 minutes, ultrasonic 1 minute. 10) DI water spray rinse, 4 cycles. 11) Blow dry with N2 gun. 12) Dehydration bake, 120 oC, 10 minutes in oven.

2.2 Photoresist Application

6) Cool down after dehydration, 5 minutes. 7) Put wafer on spinner chuck with vacuum on, blow with N2. 8) Coat AZ 703 photoresist. 9) Spin at 4000 rpm for 30 seconds, ~1 µm thick. 10) Soft bake, 90 oC, 1 minute, on hotplate.

2.3 Photoresist Exposure and Development

6) Exposure for 3.8 seconds, Karl Suss MA6 Aligner, hard contact mode. 7) Post-exposure bake, 110 oC, 1 minute, on hotplate. 8) Develop in FHD-5 for 60 seconds. 9) DI water spray rinse, 4 cycles. 10) Check under microscopy.

2.4 Oxygen Plasma Descum of Photoresist (Optional)

4) Chamber pressure = 300 mT of O2. 5) Temperature: 70 oC. 6) Run for 0.7 minute.

2.5 Surface Preparation

84

1) Mix a dilute solution of HCl : H2O :: 1 : 10. 2) Dip in dilute HCl for 15 seconds. 3) DI water spray rinse, 4 cycles. 4) Blow dry with N2 gun.

2.6 Evaporation

1) Mount wafer into e-beam chamber. 2) Pump down to below 1E-6 torr. 3) Deposit material:

Material Thickness Deposition rate

Ti 200 A 2.0 A/sec

Al 1500 A 3.0 A/Sec

Ni 500 A 2.0 A/Sec

Au 800 A 3.0 A/Sec

2.7 Liftoff

1) Soak wafer in ACE untill metal becomes loose. 2) Rinse with ISO. 3) DI water spray rinse, 4 cycles. 4) Blow dry with N2 gun. 5) Check under microscopy, and then measure the metal thickness.

2.8 Annealing

1) Run the RTA 2~3 times with a dummy wafer to check the temperature stability.

2) Load wafer into the chamber slowly. 3) Wait several minutes before heating up, anneal at 850 oC for 30 seconds. 4) Unload wafer after RTA cools down. 5) Check Ohmic contacts with I-V measurement.

85

3 Mesa Isolation

3.1 Solvent Cleaning

13) Check the resistivity of DI water. It should be > 17 MΩ 14) ACE 3 minutes, ultrasonic 1 minute. 15) ISO 3 minutes, ultrasonic 1 minute. 16) DI water spray rinse, 4 cycles. 17) Blow dry with N2 gun. 18) Dehydration bake, 120 oC, 10 minutes in oven.

3.2 Photoresist Application

11) Cool down after dehydration, 5 minutes. 12) Put wafer on spinner chuck with vacuum on, blow with N2. 13) Coat AZ 703 photoresist. 14) Spin at 4000 rpm for 30 seconds, ~1 µm thick. 15) Soft bake, 90 oC, 1 minute, on hotplate.

3.3 Photoresist Exposure and Development

11) Exposure for 3.8 seconds, Karl Suss MA6 Aligner, hard contact mode. 12) Post-exposure bake, 11 oC, 1 minute, on hotplate. 13) Develop in FHD-5 for 60 seconds. 14) DI water spray rinse, 4 cycles. 15) Check under microscopy.

3.4 Oxygen Plasma Descum of Photoresist (Optional)

7) Chamber pressure = 300 mT of O2. 8) Temperature: 70 oC. 9) Run for 0.7 minute.

3.5 Surface Preparation

5) Mix a dilute solution of HCl : H2O :: 1 : 10. 6) Dip in dilute HCl for 15 seconds. 7) DI water spray rinse, 4 cycles. 8) Blow dry with N2 gun.

86

3.6 Evaporation

4) Mount wafer into e-beam chamber. 5) Pump down to below 1E-6 torr. 6) Deposit material:

Material Thickness Deposition rate

Ni 200 A 2.0 A/Sec

Au 3000 A 3.0 A/Sec

3.7 Liftoff

6) Soak wafer in ACE until metal becomes loose. 7) Rinse with ISO. 8) DI water spray rinse, 4 cycles. 9) Blow dry with N2 gun. 10) Check under microscopy, and then measure the metal thickness.

87

APPENDIX B:

LIST OF PUBLICATIONS DURING THE STUDY IN MASTER OF PHILOSOPHY PROGRAM

Referred Journal Papers

R. M. Chu, Y. G. Zhou, K. J. Chen, and K. M. Lau, "Admittance characterization and analysis of trap states in AlGaN/GaN heterostructures," Physica Status Solidi C, Vol. 7, pp. 2400, 2003. T. Suligoj, H. T. Liu, J. K. O. Sin, K. Tsui, R. M. Chu, K. J. Chen, P. Biljanovic, K. L. Wang, “A low-cost horizontal current bipolar transistor (HCBT) technology for the BiCMOS integration with FinFETs,” Accepted by Solid-State Electronics, 2004. Y. G. Zhou, D. Wang, R. M. Chu, C. W. Tang, Y. D. Qi, Z. D. Lu, K. J. Chen, K. M. Lau, "Correlation of in-situ reflectance spectra and resistivity of GaN/Al2O3 interfacial layer in metalorganic chemical vapor deposition," Accepted by Journal of Electronic Materials, 2004. R. M. Chu, Y. G. Zhou, J. Liu, K. J. Chen, and K. M. Lau, "AlGaN-GaN double-channel HEMTs," Submitted to IEEE Transaction on Electron Devices, 2004.

Conference Presentations (Abstract Only)

R. M. Chu, Y. G. Zhou, K. J. Chen, and K. M. Lau, "Admittance Characterization and analysis of trap states in AlGaN/GaN heterostructures," Poster Presentation, Digest of The 5th International Conference on Nitride Semiconductors, pp. 275, Nara, Japan, May 25-30, 2003 R. M. Chu, Y. G. Zhou, K. J. Chen, and K. M. Lau, "Trap states induced frequency dispersion of AlGaN/GaN heterostructure field-effect transistors," Oral presentation, Digest of The 45th Electronic Material Conference, pp. 85, Salt Lake City, USA, June 25-27, 2003 Y. G. Zhou, R. M. Chu, K. J. Chen, and K. M. Lau, "AlGaN/GaN/Graded-AlGaN double heterostructure HEMT," Oral Presentation, Digest of The 2003 International Conference on Solid State Devices and Materials, pp. 918, Tokyo, Japan, September 16-18, 2003 R. M. Chu, Y. G. Zhou, J. Liu, K. J. Chen, K. M. Lau, "Reduction of current collapse in an Un-passivated AlGaN-GaN double-channel HEMT," Oral presentation, To appear in Digest of The 46th Electronic Material Conference, Notre Dame, USA, June 23-25, 2004 Y. G. Zhou, R. M. Chu, J. Liu, K. J. Chen and K. M. Lau, "Gate leakage in AlGaN/GaN HEMTs and its suppression by optimization of MOCVD growth," To appear in Digest of The 2004 International Workshop on Nitride Semiconductors, Pittsburgh, USA, July 19-23, 2004

88