i

COMPARATIVE STUDY OF FIELD PLATED

AlGaN/GaN AND InAlN/GaN HEMT FOR

MICROWAVE ELECTRONIC APPLICATIONS

A Project report submitted in partial fulfillment of the requirements for

the award of the degree of

BACHELOR OF ENGINEERING

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

Submitted by

R.Lahari(316126512181) P. Anusha(316126512159)

P.Asha Srikar(316126512158) M.Divya Sri(316126512153)

Under the guidance of

Dr.P.Murugapandiyan,M.E,Ph.D

Associate Professor,ECE

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING

ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY AND SCIENCES

(Permanently Affiliated to AU, Approved by AICTE and Accredited by NBA & NAAC with ‘A’ Grade)

Sangivalasa, bheemili mandal, visakhapatnam dist.(A.P)

2019-2020

ii

ACKNOWLEDGEMENT

We would like to express our deep gratitude to our project guide

Dr.P.Murugapandiyan Associate Professor, Department of Electronics and

Communication Engineering, ANITS, for his guidance with unsurpassed knowledge

and immense encouragement. We are grateful to Dr. V. Rajyalakshmi, Head of the

Department, Electronics and Communication Engineering, for providing us with the

required facilities for the completion of the project work.

We are very much thankful to the Principal and Management, ANITS, Sangivalasa,

for their encouragement and cooperation to carry out this work.

We express our thanks to all teaching faculty of Department of ECE, whose

suggestions during reviews helped us in accomplishment of our project. We would like

to thank all non-teaching staff of the Department of ECE, ANITS for providing great

assistance in accomplishment of our project.

We would like to thank our parents, friends, and classmates for their encouragement

throughout our project period. At last but not the least, we thank everyone for

supporting us directly or indirectly in completing this project successfully.

PROJECT STUDENTS

R.Lahari(316126512181),

P.Anusha(316126512159),

P.Asha Srikar(316126512158),

M.Divya Sri(316126512153)

iii

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING

ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY AND SCIENCES

(Permanently Affiliated to AU, Approved by AICTE and Accredited by NBA &

NAAC with ‘A’ Grade)

Sangivalasa, bheemili mandal, visakhapatnam dist.(A.P)

CERTIFICATE

This is to certify that the project report entitled “Comparative Study of Field Plated

AlGaN/GaN and InAlN/GaN HEMTs for Microwave Electronic Applications”

submitted by R.Lahari(316126512181), P.Anusha(316126512159),

P.AshaSrikar(316126512158),M.Divya Sri(316126512153) in partial fulfillment of

the requirements for the award of the degree of Bachelor of Engineering in

Electronics & Communication Engineering of Andhra University, Visakhapatnam is

a record of bonafide work carried out under my guidance and supervision.

Project Guide Head of the Department

Dr.P.Murugapandiyan Dr.V.Rajyalakshmi

Associate Professor Department of E.C.E

Department of E.C.E ANITS

ANITS

iv

CONTENTS

ABSTRACT vi

LIST OF SYMBOLS vii

LIST OF FIGURES viii

LIST OF TABLES ix

LIST OF ABBREVATIONS x

CHAPTER 1 INTRODUCTION 1

1.1 Nitride Based Semiconductors 1

1.1.1 GaN Based Transistor 3

1.1.2 Basic Properties of III-Nitride Semiconductors 3

1.1.3 Band Stricture and Lattice Constant of III-N Semiconductors 4

1.1.4 Spontaneous and Piezoelectric Polarization 5

1.1.5 Advantages of III-N Semiconductors 7

1.2 Project Objective 11

1.2.1 Scope of the project 12

1.3 Silvaco TCAD Tool 13

1.3.1 Atlas Overview 13

1.3.1.1 Atlas inputs and outputs 13

1.3.2 Deckbuild Overview 15

1.3.3 Dev-Edit Overiew 15

1.3.4 Device Basic Construction Layout 15

CHAPTER 2 AlGaN/GaN High Electron Mobility Transistors(HEMTs) 16

2.1 Basic Hemt Structure 17

2.1.1 Field plated HEMT 17

2.1.2 Working Principle 21

2.2 Two Dimensional Electron Gas(2-DEG) in HEMT 23

2.3 Results and Discussions 24

2.3.1 Drain Current Characteristics 24

2.3.2 Transfer Characteristics 26

v

2.3.3 Log Id vs Vg Characteristics 27

2.3.4 Gate lekage current Characteristics 29

2.3.5 Breakdown Characteristics 31

2.3.6 Transconductance 33

2.3.7 Cutoff Frequency 34

2.3.8 Maximum Oscillation Frequency 36

CHAPTER 3 InAlN/GaN High Electron Mobility Transistors(HEMTs) 38

3.1 Basic Structure 38

3.2 Results and Discussions 41

3.2.1 Drain Current Characteristics 41

3.2.2 Transfer Characteristics 42

3.2.3 Log Id vs Vg Characteristics 43

3.2.4 Gate lekage current Characteristics 44

3.2.5 Breakdown Characteristics 45

3.2.6 Transconductance 46

3.2.7 Cutoff Frequency 47

3.2.8 Maximum Oscillation Frequency 48

CONCLUSION 49

FUTURE WORK 51

REFERENCES 52

vi

ABSTRACT

Gallium Nitride (GaN) has recently attracted widespread attention for promising

applications in the RF domain. Due to wider bandgap, GaN based devices show

exceptional performance at higher temperatures. Additionally, they permit device

operation at higher voltages and enable high frequency operation because of higher

bandgap energy (i.e. 3.4 eV). Higher sheet carrier densities, higher thermal

conductivity, higher electron mobility, and higher breakdown field are the major factors

which provide better performance in case of GaN based heterostructures. Additionally,

GaN based heterostructures have unintentionally doped two-dimensional electron gas

(2DEG). Owing to their high sheet carrier density in 2-D Electron Gas (2DEG), GaN

based High Electron Mobility Transistors (HEMTs) have been observed to provide

excellent performance in high–power switching applications and large breakdown field

strength. The device output characteristics and small signal characteristics of the

HEMTs degrades as temperature increases. Moreover, the breakdown characteristics of

the AlGaN/GaN and InAlN/GaN HEMTs show less dependence on temperature

variations.

The power–switching performance is however limited by the conduction losses in the

ON–state resistance RON and the OFF–state breakdown voltage VBR. It is therefore a

necessity to enhance the OFF-state breakdown while keeping RON as low as possible.

The objective of this project is to design Various device configurations in order to

improve the device performance characteristics such as the cut–off frequency ( fT),

power density, power added efficiency, breakdown voltage, etc.

vii

LIST OF SYMBOLS

E Longitudinal Electric Field

EC Conduction energy band

EV Valence energy band

μ

ɳs

Mobility of electron

Charge Concentration

n(x) Sheet Charge Density

v(x) Channel Potential

Lg Gate Length

Ldg Drain to Gate length

Lsg Source to Gate length

PSP Spontaneous Polarization

PPZ Piezoelectric polarization

Ids Drain saturated current

Ig f Gate Current

Ron On-resistance

Vgs Gate to Source Voltage

Vds Drain to Source Voltage

Vth Threshold voltage

gm Transconductance

fT Cutoff Frequency

fmax Maximum Oscillation Frequency

viii

LIST OF FIGURES

Fig. 1.1 Potential applications of GaN 2

Fig. 1.2 Wurtzite hexagonal unit cell. 6

Fig. 1.3(a) Total polarization for III-nitride materials grown pseudomorpjically

on Ga-polar c-plane GaN

7

Fig. 1.3(b) Corresponding polarization charge density 7

Fig. 1.4(a) Dots: specific ON-resistance vs breakdown voltage for SiC 9

Fig. 1.4(b) Best reported output power density at various microwave frequencies

for AlGaN/GaN HEMTs

9

Fig. 1.5 Simulation flow 14

Fig. 1.6 Atlas Command Groups with the primary statements in each group 14

Fig. 2.1(a) Structure of AlGaN/GaN hemt with fieldplate 20

Fig. 2.1(b) Simulated structure of AlGaN/GaN hemt with fieldplate 20

Fig. 2.2(a) Generalized band diagram of HEMT 22

Fig. 2.2(b) Simulated band diagram of AlGaN/GaN HEMT 22

Fig. 2.3 Drain characteristics 24

Fig. 2.4 Transfer characteristics 26

Fig. 2.5 Log Id vs Vg characteristics 28

Fig. 2.6 Log Ig vs Vg characteristics 30

Fig. 2.7 Breakdown field distribution with and without fieldplate. 31

Fig. 2.8 Breakdown curve for different lengths of field-plate 31

Fig. 2.9 𝑔𝑚 vs Vgs characteristics 33

Fig. 2.10(a) HEMT Structure 35

Fig. 2.10(b) Small signal equivalent circuits of HEMT 35

Fig.2.11 Current gain vs cutoff frequency 36

Fig.2.12 Power gain vs cutoff frequency 37

Fig. 3.1 InAlN/GaN HEMT structure with field plated 39

Fig. 3.2 Simulated result of InAlN/GaN HEMT with field plate length 2.25μm 40

ix

Fig. 3.3 Band diagram of InAlN/GaN HEMT 40

Fig. 3.4 Drain current charcteristics 41

Fig. 3.5 Transfer characteristics 42

Fig. 3.6 Log Id vs Vg characteristics 43

Fig. 3.7 Log Ig vs Vg Characteristics 44

Fig. 3.8 Breakdown characteristics of this device for different field plated

lengths 45

Fig. 3.9 gm vs Vgs characteristics 46

Fig. 3.10 Current gain vs Cutoff frequency 47

Fig. 3.11 Power gain vs frequency characteristics 48

LIST OF TABLES

Table 1.1 Semiconductor materials properties 2

Table 1.2 Band gap Eg and lattice constant a and c of binary nitride and

together with bowing parameter b of ternary nitride alloys 4

Table 3.1 DC and microwave performance of GaN based hemts 50

x

LIST OF ABBREVATIONS

FET Field Effect Transistor

HEMT High Electron Mobility Transistor

MESFET Metal Semiconductor Field Effect Transistor

MODFET Modulation Doped Field Effect Transistor

GaN Gallium Nitride

GaAs Gallium Arsenic

AlGaN Aluminium Gallium Nitride

InAlN Indium Aluminium Nitride

2-DEG Two Dimensional Electron Gas

FP Fieldplate

I-V Current-Voltage

xi

1

CHAPTER 1

INTRODUCTION

The unique properties of GaN materials in comparison with other

semiconductor materials are fully investigated in this chapter. In this way the questions

should be answered, why the nitride material system has been in the focus of intense

research for more than a decade and still is today.

1.1 NITRIDE BASED SEMICONDUCTORS

In recent 20-30 years, a nitride-based semiconductor technology has been enormously

developed since essential problems preventative its progress, like high quality growth

and p-type doping, are solved through in substantial research work[16,17]. The

excellent material properties of the nitride alloys have encouraged many different

fields, which expands many possible application areas as shown in Figure 1.1.

In optoelectronics, the complete solution for emission and detection from infrared to

ultraviolet are achieved by the nitride based devices thanks to its wide range of potential

energy band gaps from 0.7 eV (InN) to 6.2 eV (AlN).

In 1971, the first GaN electroluminescent diode was demonstrated [18]. In continuation

with that GaN-based light emitting diodes (LED) were developed remarkably [17,19]

and in 1993 the very first commercial LEDs were launched in Nichia Chemical

Industries. Then its application extended to optoelectronics applications[20,21]. In

electronics and telecommunication industry GaN-based devices playing a vital role for

both high speed and high power applications. The key electrical properties of all

semiconductor materials are displayed in Table 1.1. GaN provides a high electric

breakdown field among all other maerials such as Si and GaAs. Moreover, the peak

saturation velocity with higher mobility makes the GaN-based electron devices is the

suitable candidates for high frequency application for handling high power.

2

Figure 1.1 Potential applications of GaN

Table 1.1 Semiconductor materials properties.

Even though, SiC electrical properties is similar to GaN, due to critical issues in

heterojunction formation and extreme process condition bounds the applications of SiC.

On the other hand, GaN material can easily form the heterojunctions with other nitride

alloys and this enabling it to design high electron mobility transistors (HEMTs) which

has been used in various fields.

Material Properties Si GaAs SiC GaN Diomand

Band gap 𝐸𝑔 (𝑒𝑉) 1.1 1.4 3.3 3.4 5.5

Permitivity 𝜀𝑟 11.8 13.1 10 9.0 5.5

Electron mobility 𝜇𝑛 (𝑐𝑚 2 𝑉 −1 𝑠−1 ) 1350 8500 700 1200 1900

Saturation velocity 𝑣𝑠𝑎𝑡 (107𝑐𝑚/𝑠) 1.0 1.0 2.0 2.5 2.7

Critical electric field 𝐸𝑐𝑟 (𝑀𝑉/𝑐𝑚) 0.3 0.4 3.0 3.3 5.6

Thermal conductivity 𝜃 (𝑊𝐾 −1 𝑚−1 ) 150 43 330 130 2000

JFoM/JFoMSi 1.0 1.3 20.0 27.5 50.4

BFoM/BFoMSi 1.0 14.4 11.9 20.0 82.0

3

Moreover, the availability of polarization induced charge carriers (2DEG) at the

heterojuction interface enables the device fabrication simple, because without the need

of external doping high sheet charge density available in the 2DEG. Due to the

aforementioned key features, GaN material widely used in the electronics in the past

three decades.

1.1.1 GaN based transistor

Since the first GaN-based MESFET demonstrated in 1993[22,23], several substantial

research have been carried out for verifying the existence of 2DEG with higher mobility

by using AlGaN/GaN HEMT structure[24-26]. And hence the HEMT structure thus

becomes the main structure of GaN-based transistors [22,23]. The unique properties of

both high electric breakdown field and high mobility of GaN-based transistor, it has

been captivated for high power switching and high power RF applications.

For high power switching applications, the operating frequency range is varying from

kHZ to MHz, where the breakdown voltage is in the range of 10 to 1000 V. Another

important application of GaN-based transistors is high power RF applications where

the device giving maximum output power while operating at high frequency. The high

power handling capability and high frequency operation of the device is related with

breakdown voltage of the device and saturation velocity of electron in 2DEG. Therefore

GaN-based semiconductor devices are the future forerunner of high power microwave

applications.

1.1.2 Basic properties of III-nitride semiconductors

The III-N compound semiconductors are majorly classified in to 1.Binay (AlN,GaN

and InN) compounds material 2.Ternary compound materials (AlxGa(1-x)N, InxAl(1-x)N

and InxGa(1-x)N) and 3. Quaternary alloys (InxAl(1- x)Ga(1-x-y)N). The GaN is the

interesting material among all in the nitride family as it can be grown with high quality

on sapphire, SiC and Si substrate. This is the major reason why GaN material used as

building blocks of a wide range of nitride-based devices such as, RF power transistor,

high frequency MMICs, laser diode light emitting diodes, MEMS and power

4

conversion. However, GaN alone is not creating the devices. It includes the AlN,

AlGaN, InGaN or InAlGaN layers for creating heterostructures for functional

electronics and optoelectronics. In this section we revisit the III-N semiconductors, in

particular more relevant to electronic devices.

1.1.3 Band Structure and Lattice constant of III-N semiconductors

As a consequence of the strength of the metal-nitrogen bonds, the III- nitrides are

characterized, except for InAlN, by larger band gaps and shorter lattice constants

compared to other III-V compound semiconductors families, like arsenides,

phosphides, antimonides and their alloys. The band gap and lattice constant of binary

nitride materials are listed in Table.1.2 band gaps and lattice constant of Nitride

semiconductor alloys are shown in Figure.1.along with other III-V semiconductor

materials. The energy band gaps of AlxGa(1- x)N,InxAl(1-x)N and InxGa(1-x)N are follows

the quadratic equation:

𝐸𝑔,𝐴𝑥𝐵1−𝑥𝑁 = 𝑥𝐸𝑔,𝐴𝑁 +( 1−𝑥) 𝐸𝑔,𝐵𝑁 −𝑏𝐴𝐵𝑁𝑥(1−𝑥) (1.1)

Where A and B stand for Al,Ga or In depending on the compound desired and b is the

bowing parameter. The III-N family bowing parameter is listed in Table 1.2.

Table.1.2. Band gap Eg and lattice constant a and c of binary nitride and together with

bowing parameter b of ternary nitride alloys [27,28]

Material Eg (eV) a (Aº) c(Aº) Alloy b(eV)

AlN 6.14 3.112 4.982 AlGaN 0.7

GaN 3.42 3.189 5.185 InGaN 1.4

InN 0.64 3.545 5.703 InAlN 5.36

5

1.1.4 Spontaneous and Piezoelectric Polarization

The basic wurtzite cell structure of III-Nitrides is displayed in Figure1.2. The main

advantage of wurtzite structure, which is not isotropic, is that III-nitrides are strongly

polar materials. The relatively low symmetry of the wurtzite structure give rise to a

spontaneous polarization PSP, directed along c-axis. The magnitude of the spontaneous

polarization of whole nitride family has been obtained from the following expression

[29] given in c/m2:

𝑃𝐴𝑙𝑥𝐺𝑎(1−𝑥)𝑁𝑆𝑃 = [−0.090𝑥 − 0.034(1 − 𝑥) + 0.021𝑥(1 − 𝑥) (1.2)

𝑃𝐼𝑛𝑥𝐺𝑎(1−𝑥)𝑁𝑆𝑃 = [−0.042𝑥 − 0.034(1 − 𝑥)] (1.3)

𝑃𝐼𝑛𝑥𝐴𝑙(1−𝑥)𝑁𝑆𝑃 = [−0.042𝑥 − 0.090(1 − 𝑥) + 0.070𝑥(1 − 𝑥)] (1.4)

The [0001] direction is commonly referred to as the Ga-polar direction, while the [000Ī]

one is normally called the N-polar direction. When considering the pseudomorphic

heterostructures where one layer is under tensile or compressive stress, a piezoelectr ic

component PPZ has to be added to the polarization. This term comes from the fact that

bond lengths are modified by the applied stress, and this in turn affects the polarization.

The piezoelectric polarization term can take complicated forms depending on which

kind of stress is applied to a layer. However, in this research we will consider

exclusively the relatively simple case of heterostructure grown pseudomorphically on

relaxed GaN along the c- axis with GaN-polarity. In this case, GaN has no piezoelectric

polarization, while for the other nitrides PPZ is given by (in C/m2)[29].

𝑃𝐴𝑙𝑥𝐺𝑎(1−𝑥)𝑁𝑃𝑍 = −0.525𝑥 − 0.0282𝑥(1 − 𝑥) + 0.0938(1 − 𝑥) (1.5)

𝑃𝐼𝑛𝑥𝐴𝑙(1−𝑥)𝑁𝑃𝑍 = 0.148𝑥 − 0.0525(1 − 𝑥) + 0.0938(1. 𝑥) (1.6)

𝑃𝐼𝑛𝑥𝐺𝑎(1−𝑥)𝑁𝑃𝑍 = 0.148𝑥 − 0.0424(1 − 𝑥) (1.7)

The total polarization

𝑃 = 𝑃𝑆𝑃 + 𝑃𝑃𝑍 (1.8)

The total polarization 𝑃 = 𝑃𝑆𝑃 + 𝑃𝑃𝑍 for nitrides pseudomorphic to GaN is summerized in

Figure 1.2.

6

Figure 1.2 Wurtzite hexagonal unit cell.

The „a‟ and „c‟ lattice constants are indicated, together with the c-axis direction and the

basal c-plane.According to Maxwell‟s laws, a spatial variation of the polarization P is

associated to a fixed charge density ρ, given by the gradient of P:

𝜌 = −𝛻. 𝑃 (1.9)

In heterostructures, the polarization undergoes abrupt variation at interfaces. As a

consequence, interfaces between III-nitride materials are associated with a sheet density

of fixed interface charges σ. For the heterostructures grown along the Ga -polar

direction, the magnitude of σ is simply given by the polarization difference ∆P between

the two materials:

𝒆𝝈 = −∆. 𝑷 (1.10)

The (2DEG) sheet charge density for III-nitride heterojunctions interface is

summarized in Figure 1.3.

7

Figure 1.3 (a) Total polarization for III-nitride materials grown pseudomorpjically on

Ga-polar c-plane GaN. (b) Corresponding polarization charge density[29].

1.1.5 Advantages of III-N Semiconductors

The nitrides have several features among the large semiconductor materials, which

makes the interest for various electronics applications where compact devices are

required to efficiently handle high power levels. The wide band gap of GaN and III-

nitride based alloys makes the breakdown voltage of III-nitride based devices

generally higher than analogous devices of the same size fabricated with Si or

classical III-Vs. At the same time, with respect to other wide band gap materials like

SiC, GaN is characterized by a higher saturation velocity, which means that parasitic

resistances can be reduced, making the devices more energetically efficient. A final

strength of III-nitride semiconductors is that they easily allow for heterostructure

based devices. Therefore it is possible to play with band engineering for improving

device performance, as for classical III-Vs, for example through the introduction of

quantum well (QW) or 2DEGs, while this is not possible for SiC or diamond based

devices. As a consequence of their properties, GaN-based electron devices have

8

𝑉

found applications mainly in two fields: power electronics and microwave transistors.

Table 1.1 Johnson‟s figure of merit and Baliga‟s figure of merit of semiconductor

materials [29]. The figure of merits normalized to si.

In the field of power electronics, GaN has allowed the realization of devices with

high breakdown voltage (VBR) and very low specific on-resistance (Ron). While a high

breakdown voltage (VBR) is required to the device handling high power densities, a

low Ron is necessary to minimize the power dissipation in the ON-state. The figure

of merit that best measures the potentials of a semiconductor for power electronics

applications is the Baliga‟s figure of merit (BFOM)[30]:

𝐵𝐹𝑂𝑀 = 𝜖𝜇𝐸𝐵𝑅3 (1.11)

Where ε is the dielectric constant of semiconductor, µ is the electron mobility in the

drift region and EBR the electric field for breakdown. As BFOM describes the best

possible trade-off between VBR and Ron, an equivalent and suitable expression, more

suitable for comparing real devices, is the following one:

𝐵𝐹𝑂𝑀 = 𝑉𝐵𝑅

2

𝑅𝑜𝑛 (1.12)

The theoretical 𝐵𝐹𝑂𝑀 for the principal semiconductors are listed in Table 1.1 and the

summery of VBR and Ron data for published devices is presented in Figure 1.5(a) From

this it can be seen that both theoretically and practically, GaN based devices offer

reduced Ron and thus lower dissipation. At present, high performance GaN based P-N

diode have been demonstrated with 3.7 kV breakdown voltages[31], and GaN based

vertical transistors with low specific on-resistance and breakdown voltage as high

1.5 kV have been reported[31]. This indicates that GaN holds great potential for

low/medium voltage power circuits and is being considered seriously for improving the

efficiency of DC-DC converters[32]s, LED[33] and motor drivers[34],etc.

9

Figure 1.4 (a) Dots: specific ON-resistance vs breakdown voltage for

SiC[35][36][37][38] and GaN devices[31,33,39], lines: theoretical limits for Si, SiC

and GaN. (b) Best reported output power density at various microwave frequencies for

AlGaN/GaN HEMTs[40-44].

For microwave transistors, which are the objectives of this research work, GaN is an

interesting material as it combines a high breakdown voltage and associated with high

saturation electron velocity. Analogously to power devices, a high breakdown voltage

is fundamental for increasing the power density a transistor can handle. A high

saturation velocity is instead advantageous to reduce the electron transit time under the

gate electrode or across the base region, and therefore the switching time.

10

Combining these two features, GaN based microwave transistors and amplifiers hold

the promise of delivering very high output power levels and at the same time highcut-

off frequencies. The suitability of a material for power microwave applications can be

measured by means of Johnson‟s figure of merit (JFOM)[45]:

JFOM= 𝐸𝐵𝑅 𝑣𝑠𝑎𝑡

2𝜋 (1.13)

where 𝑣𝑠𝑎𝑡 is the saturated velocity.

JFOM quantities the trade-off between VBR and device speed, which is quantified by

the cut-off frequency fT. Thus, an equivalent definition suitable for comparing device

is the following one:

JFOM=𝑉𝐵𝑅𝑓𝑇 (1.14)

The 𝐽𝐹𝑂𝑀 for the semiconductors are listed in Table 1.1. It can be seen that GaN JFOM

is ten times that of GaAs and is surpassed only by diomond. However, if we consider

that diamond is not easy to dope and does not easily allow for heterostructures, GaN

emerges as the ultimate semiconductor for RF applications. GaN-based heterostructure

has been used for the realization of 2DEG based devices, in particular HEMTs, which

have demonstrated in the last 15 years breakdown voltages much higher than III-V

based HEMTs or bipolar transistors[46]. As a direct consequence, nitride based HEMTs

have reached record power outputs and efficiencies, and are currently available on the

market for RF systems requiring high power levels, like cellular base stations. Figure

1.4 (b) summarizes the highest power levels reached by AlGaN/GaN HEMTs, which

have been the workhorse for the RF community in the past years.

11

1.2 Project objective

To reinforce the next generation high power mixed signal monolithic microwave

integrated circuits (MMIC), the requirement of high power microwave transistors are

more demands. In recent years, GaN-based High Electron Mobility Transistors

(HEMTs) are attracted enormous attention for future deployment of high power RF

applications, which offers high output power density than other materials systems such

as GaAs or InP at high- frequency operation. The unique combination of larger band

gap (3.44eV), breakdown voltage (800V) of GaN-based HEMTs are used in high power

RF applications, such as next-generation wireless communications, development of

satellite telecommunication, high power amplifiers for radar and space research,

microwave image sensing and low noise wide bandwidth amplifiers design.GaN-based

high electron mobility transistors (HEMTs) have successfully proven their potential as

high power microwave amplifiers in wireless communication market.With the upsurge

of the wireless communication, the requirement of high speed and high power

microwave transistors are more demands.

The goal of this project is to optimize the GaN-based HEMTs structure for future high

power and high frequency applications. In order to improve the DC and microwave

characteristics of the HEMT, the following design techniques are used.

Addition of fieldplate to the device structure to attain high breakdown

voltage.

The heavily doped GaN source and region with ohmic contacts effectively

minimize the contact resistances.

Passivation layer for drastic reduction in gate access resistance and

parasitic capacitances to improve the frequency reesponse.

Lattice matched Wide bandgap barrier material to GaN for large sheet

charge density, low gate leakage current, high transconductance and

resistance to short channel effects.

12

1.2.1 Scope of the project

This project accords the DC and microwave qualities of the GaN-based HEMTs by

using Silvaco TCAD tool. These transistors to be a optimistic applicant for future high

power millimetre wave applications including wireless communication, satellite

communication and TV broadcasting, high speed data transmission, military

applications, radio astronomy and low noise amplifiers etc.

In this work, the eminent physical properties of InAlN,AlGaN barrier and GaN

channel materials are completely exploited in the sub 400 nm gate length HEMTs with

different field-plate lengths are reported. From the simulation results GaN-based

HEMTs are expected to be the best microwave transistors for future high power

millimetre electronics.

Several design optimization techniques adopted in this work to uplift the

microwave performance of the GaN-based HEMTs. Scaling the gate length in nm

regime enables the low gate capacitance (Cg) while maintaining high transconductance,

ultrathin wide bandgap barrier material (InAlN) provides large sheet charge density and

low gate leakage current, heavily doped GaN soure/drain region minimizes the parasitic

resistance and capacitive effects for enhancing the cut-off frequency, presence of field

plate contributes for high breakdown voltage.

A comparison table of the behavior for different barriers(InAlN,AlGaN) in

properties is prepared and mentioned at the end of the report.

13

1.3 Silvaco TCAD tool

Silvaco TCAD tool is a widely used software package for the simulation of various type

of Si devices.

• There are many sub-packages in Silvaco in order to implement the various

required steps for the Si device simulation:

1. ATHENA (for fabrication process simulation),

2. DevEdit(for defining the structure geometry and mesh editing),

3. DeckBuild (which shows the run time output of the simulations)

4. ATLAS (which is a main device simulator tool in Silvaco)

5. TonyPlot (which can be used to visualize the simulation results)

1.3.1 Atlas overview

Atlas provides general capabilities for physically-based two (2D) and three-

dimensional (3D) simulation of semiconductor devices. It predicts the electrical

behavior of specified semiconductor structures and provides insight into the internal

physical mechanisms associated with device operation.

1.3.1.1 Atlas inputs and outputs

Figure 1.4 shows the types of information that flow in and out of Atlas. Most Atlas

simulations use two input files. The first input file is a text file that contains commands

for Atlas to execute. The second input file is a structure file that defines the structure

that will be simulated. Atlas produces three types of output files. The first type of output

file is the run-time output, which gives you the progress and the error and warning

messages as the simulation proceeds.

The second type of output file is the log file, which stores all terminal voltages and

currents from the device analysis. The third type of output file is the solution file,

which stores 2D and 3D data relating to the values of solution variables within the

device at a given bias point.

14

Figure1.5 Simulation flow

THE ORDER OF ATLAS COMMANDS

Figure1. 6 Atlas Command Groups with the Primary Statements in each Group

15

1.3.2 Deckbuild overview

DECKBUILD is an interactive, graphic runtime environment for developing process

and device simulation input decks. It consists of a window for input deck creation and

editing, a window for simulator output and control, and a set of popups for each

simulator that provide full language and run-time support.

DeckBuild also contains many other convenience features:

• A built-in tool palette allows interactive plotting of the current structure.

• Full interactive control of the simulator, including a history function that

allows you to back up in the deck and try again.

• The ability to define an arbitrary number of stop points where the

simulator is halted automatically.

• An indication in the input deck of the currently executing line.

1.3.3 Dev-edit overview

DEVEDIT is a device structure editor. It can be used to generate a new mesh on an

existing structure and can be used to create or modify a device. These devices can then

be used by 2D 2D and 3D simulators. DEVEDIT can be used as a simulator under

DECKBUILD or through a Graphical User Interface (GUI).

1.3.4 Device basic construction layout

Mesh construction

Regions and electrodes

Doping

Material

Models

Solution method

Solution specification

Data extraction and Plotting

16

CHAPTER 2

AlGaN/GaN HIGH ELECTRON MOBILITY

TRANSISTORS(HEMTs)

The basic concept of the HEMT was introduced by Dingle et al. In their concept Dingle

et al. describe how the electrons transferred from wide band gap semiconductor to

narrow band gap material and forms a heterojunction with a triangular well at its

interface, which confines the electrons in 2DEG. Because the 2DEG is separated from

the ionized donors this technique significantly reduces ionized impurity scattering and

thus results in a high electron mobility and saturation velocity. Because the technique

of Dingle et al. is also referred to as modulation doping the HEMT is also called

Modulation Doped Field Effect Transistor (MODFET).

Focusing on the 2DEG the designation Two-Dimensional-Electron-Gas Field Effect

Transistor (TEGFET) is also convenient. Other labelling are Separately-Doped-Field

Effect Transistor (SEDFET) or Selectively Doped Heterojunction Transistor (SDHT)

or just Heterojunction Field Effect Transistor (HFET). If the barrier layer, e.g. made of

AlGaAs, is undoped the resulting structure is comparable to a MOS structure and the

resulting FET may be referred to as Heterostructure Insulated Gate Field Effect

Transistor (HIGFET)[47].

HEMTs are field effect devices based on 2DEG as current channel. According to the

general field effect transistor construction, they are constituted by two ohmic contacts

to the 2DEG, called the source and drain contacts and a schottky contact, called the gate

contact, placed in drain-source spacing. A vertical cross section of HEMT is shown in

Figure 1.9. The role of the electrode is to control the current flowing between the source

and drain electrodes through the 2DEG.

17

2.1 Basic HEMT structure

A high-electron-mobility transistor (HEMT), also known as heterostructure

FET (HFET) or modulation-doped FET (MODFET), is a field-effect

transistor incorporating a junction between two materials with different band gaps (i.e.

a heterojunction) as the channel instead of a doped region (as is generally the case for

a MOSFET). A commonly used material combination is GaAs with AlGaAs, though

there is wide variation, dependent on the application of the device. Devices

incorporating more indium generally show better high-frequency performance, while

in recent years, gallium nitride HEMTs have attracted attention due to their high-power

performance. Like other FETs, HEMTs are used in integrated circuits as digital on-off

switches. FETs can also be used as amplifiers for large amounts of current using a small

voltage as a control signal. Both of these uses are made possible by the FET’s

unique current–voltage characteristics. HEMT transistors are able to operate at higher

frequencies than ordinary transistors, up to millimeter wave frequencies, and are used

in high-frequency products such as cell phones, satellite television receivers, voltage

converters, and radar equipment. They are widely used in satellite receivers, in low

power amplifiers and in the defense industry.

The dimensions of this structure are:

GaN layer is of length 0.18 micro meter with 1*105/cm3 concentration

Al0.295Ga0.705N layer is of length 0.02 micrometer with 1*1016/cm3

concentration

The 2-DEG charge concentration is 1*1013/cm3

The gate length is 400nm,Ldg=510nm,Lsg=500nm

2.1.1 Feildplated HEMT

In recent years, high electron mobility transistors (HEMTs) based on GaN have

demonstrated a power density up to 9.2 W/mm at 8 GHz and 30 V bias , which is well

beyond the capability of GaAs FETs. Thus, AlGaN/GaN HEMTs have established

themselves as microwave frequency power devices. To explore the full range of

18

applications of the AlGaN/GaN HEMT, it is also worth investigating its potential as a

high- voltage power switch. This is because, GaN has a breakdown field approximately

( ~3MV/cm),which is~3 times of that in Si or GaAs. The Baliga figure of merit gives

the ideal t h e specific on-resistance for a high voltage lateral FET as given

𝑅𝑜𝑛 =𝑉𝑏𝑟

2

𝑞𝜇𝑛𝑠𝐸𝑐2 (2.1)The

mobility AlGaN/GaN HEMTs is an order of magnitude higher than in Si or GaAs FETs,

since polarization effects induce a 2-DEG concentration of 1013/cm at the

heterointerface.

Thus, for a given Vbr, a GaN device has 10–100 times lower Ron than a device made

in other material. Devices with several hundred volts Vbr and low Ron are of paramount

importance in power switching applications such as factory automation,

telecommunications and motor control etc.

Recently,significant improvement in Vbr of an AlGaN/GaN HEMT was experimentally

demonstrated, by employing a field plate (FP) connected to the gate and placed in the

gate to drain separation,Ldg over a uniform insulator. However, a comprehensive

theoretical study of the effects of all the critical geometrical and material variables of the

FP-HEMT on its Vb r and frequency response is lacking. In this paper, all these effects

are identified, their physical basis is explained, and therefrom, a systematic procedure

for device optimization is developed. Also, the parameters of an optimized device are

calculated using a numerical simulation of the internal 2-D po- tential distribution and

the terminal Id-Vd (drain current–drain voltage) characteristics.

The application of the FP technique of Vbr improvement to HEMTs follows its

application to other lateral FETs such as LD-MOSFETs and MESFETs, after its original

proposal in the context of high voltage planar p-n junctions. The structure of the

AlGaN/GaN hemt with field plate is shown in figure 2.1(a) and simulated result of structure

field-plate lengths 2.25μm in figure 2.1(b).Thus, extensive literature on this technique is

available. Still, the present work is necessary for several reasons. First, the FP work on

MESFETs and LD-MOSFETs was restricted to specific effects. A systematic and

quantitative account of all the various FP effects on Vbr is available for planar junc-

19

tions only, and this account based on numerical simulations did not discuss the physical

basis of all the effects . Second, this account of “vertical” planar junctions cannot be

transferred to lateral FETs, since the structural and potential pictures of the crucial

semiconductor region below the FP in the two types of devices are different. Third,

although these pictures of the various lateral FETs are broadly similar, the charge

concentration and the material (i.e., Si, GaAs, or GaN, etc.) of the crucial region can

differ significantly. This calls for a separate study for each of these types of devices.

For example, unlike in MOSFETs and MESFETs, in AlGaN/GaN HEMTs, the

polarization and interface charges rather than doping determine the channel charge and

need to be modeled properly. Further, the Vbr values of a few tens of volts in GaAs

MESFETs are much lower than the several hundred volts target of the GaN HEMTs in

the present study. Finally, in contrast to the practice in earlier FP studies, of inferring

the breakdown condition indirectly from the simulated peak electric field value, the

present work extracts the Vbr directly from the simulated Id-Vd characteristics.

The main purpose of the field plate is to reshape the electric field distribution in the

channel and to reduce its peak value on the drain side of the gate edge. Introducing a

field plate in AlGaN/GaN HEMTs reduces the current collapse which in turn increases

the power performance. The benefit is an increase of the breakdown voltage and a

reduced high-field trapping effect. Overall the power density is increased. Various field

plate methods like gate field plate, a floating field plate, multiple field plate, etc have

been reported so far. It has been found that by increasing the gate to drain distance and

by reducing the field plate length, breakdown voltage increases for a particular

structure.

In The field plate technique, an additional lithography is done in order to place a metal

plate covering the gate and extending to the access region on the drain side. This metal

plate is electrically connected to the gate. It tracks the potential of the gate electrode,

acting as a quasi-electric field plate.The field distribution at the drain of the gate region

splits and reshape the distribution of the electric field, hence the peak electric field

reduces and increases the device breakdown. The field-plate length dictates the size of

the field-reshaping region. This not only increases device breakdown voltage but also

reduces the high-field trapping effect, hence enhancing current-voltage swings at high

20

frequencies. The trade-off of the field plate structure includes addition of the gate-drain

capacitance at low voltages and extension of the gate-drain depletion length at high

voltages, which reduces gain as well as frequency of the device.

Figure2.1(a):Structure of AlGaN/GaN hemt with fieldplate

Figure2.1(b):Simulated structure of AlGaN/GaN hemt with fieldplate

21

2.1.2 WORKING PRINCIPLE

HEMTs are essentially heterojunctions formed by semiconductors having dissimilar

band‐ gaps. When a heterojunction is formed, the conduction band and valence band

throughout the material must bend to form a continuous level. The wide band element

has excess elec‐ trons in the conduction band as it is doped with donor atoms (or due to

polarization charge in GaN‐based HEMTs). The narrow band material has conduction

band states with lower energy. Therefore, electrons will diffuse from wide bandgap

material to the adjacent lower bandgap material as it has states with lower energy. Thus,

a change in potential will occur due to movement of electrons and an electric field will

be induced between the materials. The induced electric field will drift electrons back to

the conduction band of the wide bandgap element. The drift and diffusion processes

continue until they balance each other, creating a junction at equilibrium like a p‐n

junction. Note that the undoped narrow bandgap material now has excess majority

charge carriers, which yield high switching speed. An interesting fact is that the low

bandgap undoped semiconductor has no donor atoms to cause scattering and thus

ensures high mobility.

Another interesting aspect of HEMTs is that the band discontinuities across the

conduction and valence bands can be engineered to control the type of carriers in and

out of the device. This diffusion of carriers leads to the accumulation of electrons along

the boundary of the two regions inside the narrow bandgap material. The accumulation

of electrons can lead to a very high current in these devices. The accumulated electrons

are also known as 2DEG. Figure 2.2(a) shows the generalized band diagram formed at

the heterojunction for typical HEMTs and 2.2(b) shows the simulated bandgiagram for

400nm gate length AlGaN/GaN HEMT with 2.25micro meter length field-plate .Both

the conduction band (Ec ) and valence band (Ev ) bend with respect to the Fermi level

(EF ) resulting in a quantum well filled with 2DEG and eventually, a conducting channel

is formed.

22

Figure:2.2(a)Generalized band diagram of HEMT

Figure 2.2(b):Simulated band diagram of AlGaN/GaN HEMT

23

2.2 Two Dimensional Electron Gas (2DEG) in HEMT

A schematic band diagram of a modulation doped heterostructure is shown in Figure 2.2(a).

It consists of a wide gap semiconductor (AlGaN) and a semiconductor with narrower gap

(GaN). At the interface a triangular quantum well is formed in the undoped narrow gap

material. Electrons from the wider band gap material fall into this potential well and are

confined within the well. Because of such quantum mechanical confinement in a very

narrow dimension, they form a high density of electron gas in two dimensions. Electrons

can move freely within the plane of the heterointerface, while the motion in the direction

perpendicular to the heterointerface is restricted to a well-defined space region by energy,

momentum, and wave function quantization, thus forms the so-called 2DEG. As the narrow

gap material is undoped and these electrons are away from the interface, the electron

mobility can be simultaneously increased with high concentration of carriers. Within the

AlGaN layer there are two polarization vectors Ppz, AlGaN and Psp, AlGaN for the

piezoelectric and spontaneous polarizations respectively. The polarization in the AlGaN

causes dipole charges to form at the borders of the material with a negative sheet charge at

the surface and an equal positive sheet charge at the AlGaN/GaN junction. The polarization

in the GaN layer causes a negative sheet charge at the AlGaN/GaN junction and an equal

positive sheet charge on the bottom surface. Since the total polarization in the AlGaN is

larger, the overall result is a net positive sheet charge at the AlGaN/GaN interface. Notice

that the bottom GaN and AlGaN/GaN interfaces are both positive while the charge density

at the top AlGaN interface is negative. The critical point to remember is that the interface

sheet charges here are not free carriers. They are induced charges as in a typically polarised

dielectric. However, it is the presence of these charges and the polarization induced electric

field in the AlGaN which allows for 2DEG formation without barrier doping.

24

2.3 Results and Discussions

2.3.1 Drain current characteristics

When a dc voltage applied to the drain electrode with the source electrode grounded, a

current Id, whose magnitude depends on the total resistivity of the 2DEG channel, flows

between two electrodes. The gate electrode, by acting on the 2DEG density below it,

modulates the channel resistivity and thus Ids. The V-I characteristics, also known as the

output characteristics, can be obtained by imposing the constant current condition along

the channel at a given gate voltage Vgs.

The Id vs Vds characteristics of this device is simulated and shown in figure 2.3.At constant

Vgs ,Vds is swept from 0V to 20V.The device remained in off-state(i.e Ids=0 A)

when Vgs=-3V.The drain current when Vgs=0V is 1.8A/mm.

Figure 2.3:Drain characteristics

Vgs=-3V

Vgs=-2V

Vgs=-1V

Vgs=0V

Vgs=1V

V

Vgs=2V

25

The drain current Ids depends on the mobility of electron (µ), longitudinal electric field (E)

and sheet charge density ɳs. Therefore the expression for Ids as follows:

𝐼𝑑𝑠 = 𝑤𝑒𝑛(𝑥)𝐸(𝑥)𝜇 (2.2)

W is the gate length of the device and the sheet charge density n(x) in the gate region as a

function of channel potential V(x):

𝑛(𝑥) = 1

𝑒𝐶𝑏(𝑉𝑔𝑠−𝑉(𝑥)−𝑉𝑜𝑓𝑓) (2.3)

Considering 𝐹(𝑥) =𝑑𝑉(𝑥)

𝑑𝑥, Equation (2.2) can be written as:

𝐼𝑑𝑠 = 𝑊𝜇𝐶𝑏(𝑉g𝑠 −𝑉( 𝑥) −𝑉𝑜𝑓𝑓)𝑑𝑉(𝑥)

𝑑𝑥 (2.4)

Integrating this equation over the gate length, we finally obtain a relation between 𝐼𝑑𝑠 and

𝑉𝑑𝑠:

𝐼𝑑𝑠 = 𝑤𝜇𝐶𝑏2𝐿𝑔

[(𝑉𝑔𝑠−𝑉𝑜𝑓𝑓)2−(𝑉𝑔𝑠−𝑉𝑜𝑓𝑓−𝑉𝑑𝑠)2] (2.5)

This expression is valid only for 𝑉𝑑𝑠<𝑉𝑔𝑠−𝑉𝑜𝑓𝑓 and for 𝑉𝑔𝑠> 𝑉𝑜𝑓𝑓, i.e in the so called linear

regime. At 𝑉𝑑𝑠=𝑉𝑔𝑠−𝑉𝑜𝑓𝑓 the channel at the drain edge is fully depleted. In this case, the

current does not increase further with 𝑉𝑑𝑠 and remains approximately constant. This is the

saturation regime. Finally, for 𝑉g𝑠< 𝑉𝑜𝑓𝑓the channel is completely depleted whatever 𝑉𝑑𝑠

and the device is in the pinch-off state.

26

2.3.2 Transfer characteristics

The Ids vs Vgs characteristics of this device is simulated and shown in figure 2.4 which

describes the Ids-Vgs dependence at a fixed 𝑉𝑑𝑠, usually in saturation regime. At constant

Vds(the value where current is saturated),Vgs is swept from -10V to 4V.The minimum gate-

to-source voltage VGS (th) that is needed to create a conducting path between the source and

drain terminals is called threshold voltage(Vth).

The Vth for this device is determined as -2.989V.

Figure 2.4:Transfer characteristics

Vds=15V

Vds=17V

27

2.3.3 Log Id vs Vg Characteristics

The log Id vs Vg characteristics of this device is simulated and shown in figure 2.5.

The slope of this curve is called subthreshold slopeand detrmined as 26.7594mV/dec.

Short channel effects can be also important causes of non-ideal behaviour. If the gate

does not have a very strong electrostatic control over the channel, the drain voltage can

still strongly influence the drain-source current even at the saturation regime. This

happens particularly when the gate to channel distance,i.e. the barrier thickness, is not

negligible with respect to Lg. short channel effects have a strong effect in the pinch-off

region. At high Vds, the gate can no longer block the current flow and a non negligible

Ids can appear. Short channel effects can be therefore assimilated to a Vds dependent Voff

and are measured through the drain induced barrier lowering (DIBL) parameters.

Drain induced barrier lowering(DIBL):

Drain induced barrier lowering is a short channel effect in HEMTs which refers to the

reduction of the threshold voltage of the transistor at higher drain voltages. The origin of

the threshold decrease can be understood as a consequence of charge neutrality: the Yau

charge-sharing model. The combined charge in the depletion region of the device and that

in the channel of the device is balanced by three electrode charges: the gate, the source and

the drain. As the drain voltage is increased,the depletion region of the p-n junction between

the drain and body increases in size and extends under the gate, so the drain assumes a

greater portion of the burden of balancing depletion region charge, leaving a smaller burden

for the gate. As a result, the charge present on the gate retains charge balance by attracting

more carriers into the channel, an effect equivalent to lowering the threshold voltage of the

device. In effect, the channel becomes more attractive for electrons. In other words, the

potential energy barrier for electrons in the channel is lowered. Hence the term "barrier

lowering" is used to describe these phenomena.

28

Figure 2.5 Log Id vs Vg characteristics

Unfortunately, it is not easy to come up with accurate analytical results using the barrier

lowering concept. Barrier lowering increases as channel length is reduced, even at zero

applied drain bias, because the source and drain form pn junctions with the body, and so

have associated built-in depletion layers associated with them that become significant

partners in charge balance at short channel lengths, even with no reverse bias applied to

increase depletion widths. As channel length is reduced, the effects of DIBL in

the subthreshold region (weak inversion) show up initially as a simple translation of the

subthreshold current vs. gate bias curve with change in drain-voltage, which can be

modeled as a simple change in threshold voltage with drain bias. However, at shorter

lengths the slope of the current vs. gate bias curve is reduced, that is, it requires a larger

change in gate bias to effect the same change in drain current. At extremely short lengths,

the gate entirely fails to turn the device off.

𝐷𝐼𝐵𝐿 =𝜕𝑉𝑜𝑓𝑓

𝜕𝑉𝑑𝑠 (2.6)

DIBL of this device is determined as 0.0985

In order to keep the SCEs to acceptable level, Lg should be atleast 5 times of the barrier

thickness or even more [48]. This imposes a limit of scaling of a transistor dimensions.

29

Ion/Ioff

For a device working as a switch it conducts an on current Ion when it is on and off current

Ioff when it is switched off. For an ideal switch the off current is ideally zero and the on

current is limited by the external circuit.

In case of solid sate switches as the HEMTs. the off current is not zero but it is a very small

current. It is the current passing in the transistor when its gate to source voltage equals zero

and Vds= Vdd. It is observes as the transistor is scaled down, the off current increases

appreciably because of the short channel effects with the most effective one is the induced

barrier lowering.

The on current is limited by the W/L ratio, and the excess voltage of the transistor Vgs-Vth

in addition to the mobility. Therefore, because of this trend one has to maximize the Ion/Ioff

ratio for static transistor losses.

Ion/Ioff of this device is 1014

2.3.Gate lekage current characteristics

The gate lekage current charcteristics of this device is simulated and shown in figure 2.6.

Gate lekage current

As stated previously, AlGaN/GaN HEMT devices are well suited to high-power high-

frequency applications such as high power amplifiers and applications for wireless base

stations. For such cases there is a general requirement for a low input gate current and a

high reliability figure for the device.The role played by the degradation with time in the

gate leakage current is important in the understanding of the reliability issue for the

device.From a physical point of view the degradation, and hence changes observed with

the device, arise from defects under the gate region. These become more evident at a critical

point in the value of the electric field. Trap formation in the device at either the

semiconductor surface or within the bulk is also a performance-limiting issue. The gate

leakage current surges as a consequence of surface processing and passivation issues.

30

Figure 2.6 Log Ig vs Vg characteristics

In Field Effect devices quantum mechanical tunnelling is an important effect to be

accounted . For example, electrons tunnelling from the gate can create a gate-to-drain

leakage current by hopping from trap to trap. Alternatively, the electrons can accumulate

on the surface next to the gate or move through the AlGaN layer to the conducting

channel.As the number of electrons increases at the gate edge as a function of time the gate

leakage current reduces due to the feedback. In addition, the increased electron density on

the AlGaN surface decreases the number of 2DEG electrons and this causes the gate current

to decrease.

We found a strong correlation between the gate leakage current and the transistor

subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and

the steeper the subthreshold slope. In order to reduce off-state power consumption, it is

necessary to reduce leakage current.

The gate leakage current of this device is determined as 10-17 A/mm.

31

2.3.5 Breakdown characteristics

The breakdown voltage is the drain-source voltage, which causes the transistor to enter the

breakdown region.It is the region where the device receives too much voltage across its

drain-source terminal, which causes the drain-source terminal to break down, which makes

the grain current drastically increase.Figure 2.7 shows a HEMT without a FP, the breakdown

field distribution along the 2-DEG over , is confined over a small distance from the gate

edge. So, a high field is reached even for small values of and is low.

Figure 2.7:Breakdown field distribution with and without fieldplate.

Inclusion of a FP reduces and spreads the field along the 2-DEG.The breakdown

characteristics of this device for different field-plate lengths 1,1.25,1,5,1.75,2,1.25(μm) is

simulated at 300ok and shown in figure2.8.

Figure 2.8 Breakdown curve for different lengths of field-plate

32

Breakdown is an important problem for high power HEMT devices. In order to tackle

breakdown issue of HEMT many investigations are carried out to understand its physical

origin. Some of the relevant breakdown mechanism in HEMT devices are,

Source-drain breakdown due to short channel effect

Hetero structure fabrication defects and trap related problem.

Impact ionisation mechanism, sudden increase in drain current due to carrier

collapse.

High gate electric field, due to the schottky contact leakage current or through

surface related conduction.

There are different techniques available to improve the Breakdown voltage of a transistor,

some of the common techniques and its review is given.

Schottky source/drain contact HEMT

Field plated HEMT

Passivation layer in HEMT

The breakdown down charcteristics is also simulated for 400o k and 5000k for 2.5 micro

meter fieldplte length and the breakdown volatage values are determined as:

Vbr=819.443V (300ok)

Vbr=813.961V (400ok)

Vbr=810.282V (500ok)

The breakdown voltage decreases with increase in temperature and increase in field-plate

length beyond 2.25μm length.

33

2.3.6 Transconductance(gm)

Transconductance is the ratio of the change in drain current to the change in gate voltage

over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.

𝑔𝑚 =𝜕𝐼𝑑𝑠

𝜕𝑉𝑔𝑠 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉𝑑𝑠 (2.6)

As for the output characteristics, what is usually represented is the Vgs dependence of gm

in the saturation regime. The transconductunce vs Vgs characteeristics for this device is

simulated and shown in figure 2.9.

In an ideal HEMT, both Ids and gm increase indefinitely with Vgs. In real devices,

however, several non linearities contribute to modify the output and transfer

characteristics.

Figure 2.9:𝑔𝑚 vs Vgs characteristics

A first effect comes from the fact that µ depends on the longitudinal electric field, in

particular it decreases as the field increases. This reduces the current the transistor can

handle in the saturation regime. A second important effect comes from the presence of an

access and output region on the source and drain side of the gate, respectively. The

resistance of the two regions, which is composed of the contact resistance and the

34

resistance of the 2DEG portion connecting the source or drain electrode to the gate region,

increases the on resistance of the device. As a consequence, the saturation regime is

reached for Vds values higher than Vgs-Voff. Source and drain resistances induce

furthermore a significant reduction of the saturation current, the effect becoming stronger

with higher resistances.

A second important phenomenon in GaN based HEMTs is an access and output resistances

with increasing current levels[49][50]. This is due to nonlinearity velocity field

characteristics of GaN (1.7), and its major consequence is a sublinear at high Vgs (Figure

1.10(b)). Vgs is finally limited by the onset of forward conduction in the schottky contact,

which usually take place for Vgs>1V.

The transconductance of this device is maximum at Vgs=0V with and determined

as 0.0525 in value.

2.3.7 Cutoff frequency(fT)

The simplest way for understanding the evolution of the transistor properties with

frequency is to start with its behaviour under small signal conditions. The small signal

equivalent circuit of HEMT is shown in Figure 2.10. The Rg and Lg are the resistance and

inductance of source resistance and access region. Rd, Rg, Ld and Lg are the resistances and

inductances of the drain and gate electrodes. Cgs, Cgd and Cds are, respectively, the gate-

source, gate-drain and drain source capacitances. Rds is the drain-source resistance and is

related to short channel effects and buffer conductivity.Rgs has a less evident physical

origin. Among all these components, the most important ones are by far 𝑔𝑚 and Cgs.The

other components are less important and can be considered as parasitic components. If

these parasitic components are negligible, the current gain G I can be easily worked out:

𝐺𝐼 =𝑔𝑚

2𝜋𝑓𝐶𝑔𝑠 (2.7)

Where f is the frequency. From this equation, we can see that the current gain decreases

with frequency.

35

Figure 2.10 (a) HEMT structure, (b) Small signal equivalent circuits of HEMT

If expressed in dB, the gain is found to decrease at a rate of 20 dB/decade and becomes

unity at the transition frequency fT:

𝑓𝑇 =𝑔𝑚

2𝜋𝐶𝑔𝑠 (2.8)

Therefore, for 𝑓>𝑓𝑇 the transistor no more amplifies the current. In the ideal case, all the

gate-source capacitance is used to modulate the 2DEG electrons. If<𝑣𝑒> is the mean

electron velocity under the gate electrode, we have 𝑔𝑚 = 𝐶𝑔𝑠 < 𝑣𝑒 > 𝐿𝑔 .

Writing now < 𝑣𝑒 >= 𝐿𝑔/ 𝜏 , where 𝜏 is the transit time for electrons under the gate, we

obtain:

𝑓𝑇 =1

2𝜋𝜏 (2.9)

Therefore, the faster the electrons transit under the gate, the higher 𝑓𝑇 will be. High 𝑓𝑇 can

be thus obtained by reducing 𝐿𝑔 and by increasing the electric field under the gate so that

electrons reach their saturation velocity. The highest possible value for 𝑓𝑇 is therefore

𝑓𝑇,𝑚𝑎𝑥 =𝑣𝑠𝑎𝑡

2𝜋𝐿𝑔 (2.10)

Considering that for GaN 𝑣𝑠𝑎𝑡 = 2.5𝑋107 cm/s, and taking 10 nm as the lower bound for

Lg, transition frequencies in excess of 1 THz can be theoretically reached. In real devices,

however, 𝐶𝑔𝑠 contains also fringing parasitic components that lower the 𝑔𝑚/ 𝐶𝑔𝑠 ratio . A

second point is that the electron velocity cannot be 𝑣𝑠𝑎𝑡 under the whole gate. Electrons

enter the gate at relatively low velocities and are accelerated to 𝑣𝑠𝑎𝑡 only at the right side

36

of the gate. This further lowers the maximum value of 𝑓𝑇. Current gain vs frequency

characteristics is simulated for the device to determine the cutoff frequency as shown in

figure 2.11.Furthermore, the effect of the other parasitic elements is not completely

negligible. An expression for 𝑓𝑇 taking into account the parasitic components is the

following one:

𝑓𝑇 = 𝑔𝑚2𝜋

1

(𝐶𝑔𝑠+𝐶𝑔𝑑)+(𝑅𝑠+𝑅𝑑

𝑅𝑑𝑠)(𝐶𝑔𝑠+𝐶𝑔𝑑)+𝑔𝑚𝐶𝑔𝑑(𝑅𝑠+𝑅𝑑)

(2.11)

Figure 2.11:Current gain vs cutoff frequency

The cutoff frequency of this device is determined as 23GHz.

2.3.8 Maximum Oscillation Frequency(fmax)

Parasitic components can have a significant impact on 𝑓𝑇 , especially in deeply scaled

transistors where the low 𝐿𝑔 makes 𝐶𝑔𝑠 small, enhancing the effect of the parasitic source

and drain resistances. Regarding the power gain, the situation is more complicated as many

definitions have been given for this parameter. The Mason‟s unilateral power gain U is the

most commonly used and is defined as the power gain when the drain to gate feedback is

suppressed. Its frequency behavior is not different from the current gain. It also decreases

37

with frequency at a rate of 20 dB/decade and becomes equal to one at a frequency indicated

as 𝑓𝑚𝑎𝑥 . Therefore, as for the current gain, the higher 𝑓𝑚𝑎𝑥 is, the higher the gain at a given

frequency will be. However, 𝑓𝑚𝑎𝑥 can be quite different from 𝑓𝑇 (Schwierz et al. 2002):

𝑓𝑚𝑎𝑥 =𝑓𝑇

2√𝑅𝑠+𝑅𝑔+𝑅𝑔𝑠

𝑅𝑑𝑠+2𝜋𝑓𝑇𝐶𝑔𝑑𝑅𝑔

(2.12)

For power amplifier applications, 𝑓𝑚𝑎𝑥 is more critical than 𝑓𝑇 as amplifiers have to amplify

power rather than current. In order to obtain a high 𝑓𝑚𝑎𝑥 , it is required to have a high 𝑓𝑇

and low parasitic resistances.The power gain vs frequency of this device is simulated and

shown in figure2.12.

The frequency of this device is improved by:

Si3N4 passivation,which decrease the effect of parasitic capacitances

GaN doping,which reduces contact resistances

Figure 2.12:Power gain vs frequency characteristics

The maximum oscillation frequency of this device is extracted as 100GHz.

38

CHAPTER 3

InAlN/GaN HIGH ELECTRON MOBILITY

TRANSISTORS(HEMTs)

3.1 Basic Structure

The structure is similar to the structure mentioned in previous chapter(AlGaN/GaN) except

the barrier layer is replaced with In0.17Al0.83 N as shown in figure 3.1 and the device

characteristics are simulated.The simulated structure and band diagram are simulated and

shown in figure 3.2 and 3.3 respectively.

Kuzmik proposed an alternative approach of replacing AlGaN by InAlN as the barrier layer

material in the GaN high-electron-mobility transistor (HEMT) structure to improve the

device performance. The improvement is mainly due to the superior material properties of

In0.18Al0.82N, which is lattice-matched to GaN and has a stronger polarization effect.

Without lattice strain, In0.18Al0.82N grown on GaN has a lower possibility of defect

formation. Additionally, the larger spontaneous polarization field in the InAlN barrier layer

also induces higher sheet carrier density at the InAlN/GaN interface than that in the

AlGaN/GaN structure. Nevertheless, for nitride-based HEMTs, the issue of high leakage

current resulting from the low-resistivity GaN layer has restricted the device performance.

In particular, GaN grown by metal organic vapor deposition (MOCVD) exhibits n-type

conductivity owing to the incorporation of oxygen (O), silicon (Si), and other impurities.

On the other hand, high-resistivity GaN is required to ensure complete channel pinch-off,

high-voltage operation, and proper drain–source current saturation on a GaN HEMT

device.

InAlN/GaN-based high-electron mobility transistors (HEMTs) are excellent candidates for

high power and frequency applications, primarily because of the very high polarization-

induced two-dimensional gas (2DEG) in the channel (n2DEG). On the other hand

AlGaN/GaN-based HEMTs are known to suffer from the trapping effects related to the

surface states.It was suggested that the existence of surface states is coupled with

polarization effects because the surface states provide the charge neutrality on the device

39

surface and eliminate strong electric field in AlGaN. Filling of surface states in the gate–

drain area by electrons emitted from the reverse-biased gate may deplete n2DEG in the

AlGaN/GaN quantum well (QW) channel. Subsequently a gate-lag (GL) i.e. collapse in

drain current is observed if positive pulse on the gate is applied. Alternatively it was

suggested that interface states behave as hole traps during the GL transients but that

happens also in the source-gate area. Strong modulation of the AlGaN surface potential

due electrons emitted from the ohmic contacts was reported elsewhere and this resembles

situation during the pulse applied on the drain called a drain lag (DL) effect.

Figure3.1:InAlN/GaN HEMT structure with field plated

40

Figure3.2:Simulated result of InAlN/GaN HEMT with field plate length 2.25μm

Figure 3.3:Band diagram of InAlN/GaN HEMT

41

3.2 Results and Discussions

3.2.1 Drain Current Characterisctics

When a dc voltage applied to the drain electrode with the source electrode grounded, a

current Id, whose magnitude depends on the total resistivity of the 2DEG channel, flows

between two electrodes. The gate electrode, by acting on the 2DEG density below it,

modulates the channel resistivity and thus Ids. The V-I characteristics, also known as the

output characteristics, can be obtained by imposing the constant current condition along

the channel at a given gate voltage Vgs.

The Id vs Vds characteristics of this device is simulated and shown in figure 3.4.At constant

Vgs ,Vds is swept from 0V to 20V.The device remained in off-state(i.e Ids=0 A)

when Vgs=-7V.The drain current when Vgs=0V is 3.8A/mm.

Figure 3.4:Drain current charcteristics

Vgs=-7V

Vgs=2V

42

3.2.2 Transfer characteristics

The Ids vs Vgs characteristics of this device is simulated and shown in figure 3.5 which

describes the Ids-Vgs dependence at a fixed 𝑉𝑑𝑠, usually in saturation regime. At constant

Vds(the value where current is saturated),Vgs is swept from -10V to 4V.The minimum gate-

to-source voltage VGS (th) that is needed to create a conducting path between the source and

drain terminals is called threshold voltage(Vth).

The Vth for this device is determined as -8.5V.

Figure 3.5:Transfer characteristics

Vds=15V

Vds=17V

43

3.2.3 Log Id vs Vg Characteristics

The log Id vs Vgs characteististics of this device is simulated and shown in figure 3.6.

The slope of this curve is called subthreshold slopeand detrmined as 13.91mV/dec.

Short channel effects can be also important causes of non-ideal behaviour. If the gate

does not have a very strong electrostatic control over the channel, the drain voltage can

still strongly influence the drain-source current even at the saturation regime. This

happens particularly when the gate to channel distance,i.e. the barrier thickness, is not

negligible with respect to Lg. Short channel effects have a strong effect in the pinch-off

region. At high Vds, the gate can no longer block the current flow and a non negligible

Ids can appear. Short channel effects can be therefore assimilated to a Vds dependent Voff

and are measured through the drain induced barrier lowering (DIBL) parameters.

𝐷𝐼𝐵𝐿 =𝜕𝑉𝑜𝑓𝑓

𝜕𝑉𝑑𝑠 (3.1)

DIBL of this device is determined as 0.116297

Ion/Ioff of theis device is 1014

Figure 3.6:Log Id vs Vg characteristics

44

3.2.4 Gate lekage current Characteristics

The gate lekage current charcteristics of this device is simulated and shown in figure 3.7.

The gate leakage current surges as a consequence of surface processing and passivation

issues.

Figure:3.7 Log Ig vs Vg Characteristics

We found a strong correlation between the gate leakage current and the transistor

subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and

the steeper the subthreshold slope. In order to reduce off-state power consumption, it is

necessary to reduce leakage current.

The gate leakage current of this device is determined as 10-17 A/mm.

45

3.2.5 Breakdown Characteristics

The breakdown voltage is the drain-source voltage, which causes the transistor to enter the

breakdown region.It is the region where the device receives too much voltage across its

drain-source terminal, which causes the drain-source terminal to break down, which makes

the grain current drastically increase.The breakdown characteristics of this device is

simulated and shown in figure3.8.

The breakdown down charcteristics is also simulated for 400o k and 5000k for 1.75 micro

meter fieldplte length and the breakdown volatage values are determined as:

Vbr=85.8771V (300ok)

Vbr=89.4763V (400ok)

Vbr=95.428V(500ok)

Figure 3.8:Breakdown characteristics of this device for different field plated lengths

46

3.2.6 Transconductance(gm)

Transconductance is the ratio of the change in drain current to the change in gate voltage

over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.

𝑔𝑚 =𝜕𝐼𝑑𝑠

𝜕𝑉𝑔𝑠 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉𝑑𝑠 (3.2)

As for the output characteristics, what is usually represented is the Vgs dependence of gm

in the saturation regime. The transconductunce vs Vgs characteeristics for this device is

simulated and shown in figure 3.9.

In an ideal HEMT, both Ids and gm increase indefinitely with Vgs. In real devices,

however, several non linearities contribute to modify the output and transfer

characteristics.

Figure 3.9:gm vs Vgs characteristics

The gm is maximum when Vgs=-2V with a value of 0.0625S/mm.

47

3.2.7 Cutoff frequency(fT)

The fT is based on the short-circuit current gain (h21) which is the ratio of the small-signal

output current (drain current) to input current (gate current) with the output short-circuited.

Generally, the magnitude of the current gain rolls off with a slope of -10 dB/decade at high

frequency range.The current gain vs frequency characteristics of this device is simulated

and shown in figure 3.10.

The fT is the frequency that the magnitude of the current gain becomes unity.

𝑓𝑇 = 𝑔𝑚2𝜋

1

(𝐶𝑔𝑠+𝐶𝑔𝑑)+(𝑅𝑠+𝑅𝑑

𝑅𝑑𝑠)(𝐶𝑔𝑠+𝐶𝑔𝑑)+𝑔𝑚𝐶𝑔𝑑(𝑅𝑠+𝑅𝑑)

(3.3)

Neglcting the parasitic capacitances and resistances by GaN doping and Si3N4 passivation

the cutoff frequency is improved.

𝑓𝑇 =𝑔𝑚

2𝜋𝐶𝑔𝑠 (3.4)

Figure 3.10:Current gain vs Cutoff frequency

The cutoff frequency of this device is 90GHz.

48

3.2.8 Maximum Oscillation Frequency(fmax)

Fmax is defined as the frequency that the unilateral power gain (U) equals unity.The power

gain vs frequency characteristic of this device is simulated and shown in figure 3.11.The

unilateral power gain (U) is the power gain under the assumption of no feedback from the

output to the input and it rolls off with the -20 dB/decade slope at high frequency range.

𝑓𝑚𝑎𝑥 =𝑓𝑇

2√𝑅𝑠+𝑅𝑔+𝑅𝑔𝑠

𝑅𝑑𝑠+2𝜋𝑓𝑇𝐶𝑔𝑑𝑅𝑔

(3.5)

Figure 3.11:Power gain vs frequency characteristics

The maximum oscillation frequency(fmax) is determined as 160GHz.

49

Table 3.1:DC and microwave performance of GaN based HEMTs

The comparison with various experimental results of GaN-HEMTs along with

our work are presented in table 3.1.The high Vbr along with high fmax make the

asymmetric recess technology particulary well-suited for the design of high-

speed and efficient power amplifiers.This result exhibits that these devices are

excellent candidates for ultra-high-frequency power applications.

Parameters AlGaN/

GaN

[this

work]

AlGaN/

GaN

[15]

AlGaN/

GaN

[13]

AlGaN/

GaN

[14]

AlGaN/

GaN

[13]

InAlN/

GaN

[this

work]

Gatelength

(Lg)(nm)

400 600 250 170 150 400

Drain current

(Ids) (A/mm)

1.8 1.59 1.40 - 2.7 3.8

Threshold

volage(Vth)(V)

-2.989 - -5.6 -2.6 -6.0 -8.5

Cutoff

frequency(fT)(GHz)

60 19 42 46 50 70

Maximum Oscillation

frequency(fmax)(GHz)

180 50 100 150 121 210

Transconductance(gm)

(mS/mm)

525 374 255 245 280 625

Breakdown voltage

(V)

819.443 - - - - 85.8771

50

CONCLUSION

In our project, the DC and microwave characteristics of GaN-based high

electron mobility transistors are analysed by using Silvaco TCAD tool. The primary

focus of our project is to enhance the drain and microwave characteristics of GaN-

based HEMTs with high breakdown voltage for future high power electronic

applications.

The essential key parameters for high power microwave applications is

maximum frequency oscillation (fmax). In this project, several optimization

techniques are adopted for improving the maximum frequency oscillation, such as

heavily doped n++ GaN source/drain region for drastic reduction in the source/drain

contact resistances,addition of fieldplate of length 2.25μm to the structure for

improving the breakdown voltage,Si3N4 passivation techniques for reducing the

parasitic capacitances for enhancing the high frequency performance of the HEMTs

for microwave electronics. The InAlN/GaN is of particular interest of this project

work due to their superior performance in terms of high frequency, high power. The

summary of this work is presented in Table 3.1.

The proposed novel GaN-based high electron mobility transistors shows

preeminent drain current and microwave characteristics and the objectives of this

project work is attained. These devices are expected to be the most optimistic future

forerunners of millimeter wave electronics such as, satellite telecommunications,

high power amplifiers for radar, microwave image sensing, military electronic

systems and low noise wide bandwidth amplifiers design for future high power

microwave communications.

51

FUTURE SCOPE

The rapid development of high power millimeter wave electronics such as next

generation cellular communication, satellite communication, high speed data

transmission and radar applications requires both high breakdown voltage and high

electron velocity transistors. GaN-based high electron mobility transistors offering

high electron mobility and high carrier concentration results in high current density

and high frequency operation.

Although the microwave performance of the GaN-based HEMTs is pushed towards

the sub-millimetre wave region, a novel device structures such as metal oxide

semiconductor high electron mobility transistors (MOSHEMT) are needed for

future ultra-high power, high frequency applications.

52

REFERENCES

[1] Bo Song, Mingda Zhu, Zongyang Hu, Meng Qi, Kazuki Nomoto, Xiaodong Yan,

Yu Cao, Debdeep Jena, and Huili Grace Xing,“Ultralow-Leakage AlGaN/GaN High

Electron Mobility Transistors on Si with Non-alloyed Regrown Ohmic Contacts”, IEEE

electron Device letters, 2015. DOI 10.1109/LED.2015.2497252,

[2] Y. Tang, K. Shinohara, D. Regan, A. Corrion, D. Brown, J. Wong, A. Schmitz, H.

Fung, S. Kim and M. Micovic, “Ultra-High-Speed GaN High-Electron-Mobility

Transistors with fT/fmax of 454/444 GHz”, 10.1109/LED.2015.2421311, IEEE Electron

Device Letters

[3] Annealing Mingchen Hou, Gang Xie, and Kuang Sheng “Improved Device

Performance in AlGaN/GaN HEMT by Forming Ohmic Contact with Laser”, DOI

10.1109/LED.2018.2844951, IEEE Electron Device Letters

[4] Gourab Dutta, Nandita DasGupta, and Amitava DasGupta, “Gate Leakage

Mechanisms in AlInN/GaN and AlGaN/GaN MIS-HEMTs and Its Modeling”, IEEE

TRANSACTIONS ON ELECTRON DEVICES, Digital Object Identifier

10.1109/TED.2017.2723932.

[5] Ronghui Hao , Weiyi Li, Kai Fu, Guohao Yu, Liang Song, Jie Yuan, Junshuai Li,

Xuguang Deng, Xiaodong Zhang, Qi Zhou, Yaming Fan, Wenhua Shi, Yong Cai, Xinping

Zhang, and Baoshun Zhang , “Breakdown Enhancement and Current Collapse Suppression

by High-Resistivity GaN Cap Layer in Normally-Off AlGaN/GaN HEMTs” IEEE

ELECTRON DEVICE LETTERS, VOL. 38, NO. 11, NOVEMBER 2017

[6] William M. Waller, Michael J. Uren, , Kean Boon Lee, Peter A. Houston, David J.

Wallis, Ivor Guiney, Colin J. Humphreys, Saurabh Pandey, Jan Sonsky, and Martin Kuball,

“Subthreshold Mobility in AlGaN/GaN HEMTs”, Digital Object Identifier

10.1109/TED.2016.2542588, IEEE TRANSACTIONS ON ELECTRON DEVICES.

[7] Yu-Shyan Lin , and Chi-Che Lu, “Improved AlGaN/GaN Metal–Oxide–

Semiconductor High-Electron Mobility Transistors With TiO2 Gate Dielectric Annealed

53

in Nitrogen”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 65, NO. 2,

FEBRUARY 2018, Digital Object Identifier 10.1109/TED.2017.2781141.

[8] F. Benkhelifa, S. Müller, V. M. Polyakov, and O. Ambacher, “Normally-off

AlGaN/GaN/AlGaN Double Heterostructure FETs with a Thick Undoped GaN Gate

Layer”, DOI 10.1109/LED.2015.2459597, IEEE Electron Device Letters.

[9] Sen Huang , Xinyu Liu, Xinhua Wang, Xuanwu Kang, Jinhan Zhang , Jie Fan,

Jingyuan Shi, Ke Wei, Yingkui Zheng, Hongwei Gao, Qian Sun, Maojun Wang, , Bo Shen,

and Kevin J. Chen, “Ultrathin-Barrier AlGaN/GaN Heterostructure: A Recess-Free

Technology for Manufacturing High-Performance GaN-on-Si Power Devices”IEEE

TRANSACTIONS ON ELECTRON DEVICES, VOL. 65, NO. 1, JANUARY 2018

[10] Li-Jiang Zhang, Jiang-Hui Mo, Yu-Xing Cui, Xing-Chang Fu, Guang Qian , Xian-

Jie Li, and Tong Zhang, “A 750-W AlGaN/GaN HEMT Operating at 80 V for L-Band

Applications”, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS,

Digital Object Identifier 10.1109/LMWC.2018.2813878.

[11] Ling Yang , Member, Meng Zhang , Bin Hou , Minhan Mi , Mei Wu, Qing Zhu,

Jiejie Zhu , Yang Lu , Lixiang Chen , Xiaowei Zhou, Ling Lv , Xiaohua Ma , and Yue Hao,

“High Channel Conductivity, Breakdown Field Strength, and Low Current Collapse in

AlGaN/GaN/Si _-Doped AlGaN/GaN:C HEMTs”, IEEE TRANSACTIONS ON

ELECTRON DEVICES, DOI:10.1109/TED.2018.2889786.

[12] P. Murugapandiyan, S. Ravimaran, J. William,” DC and Microwave Characteristics

of Lg 50nm T-Gate InAlN/AlN/GaN HEMT for Future High Power RF Applications”,

International Journal of Electronics and Communications, DOI:

http://dx.doi.org/10.1016/j.aeue.2017.05.004

[13] Mohammad A Alim,Ali A Rezazadeh and Christophe Gaquiere,”Thermal

Characterization of DC and small signal parameters of 150nm and 250nm gate-length

AlGaN/GaN HEMTs grown on a SiC substrate”,Semiconductor Science and

Technology,doi:10.1088/0268-1242/30/12/125005

[14] Hyung Sup Yoon,Byoung Gue Min,Jong Min Lee,Dong Min Kang,Ho Kyun

Ahn,Sung II Kim,Chul Won Ju,Hae Cheon Kim and Jong Won Lim,”DC and RF

54

Charecteristics of 100-nm mHEMT Devices Fabricated with a Two-Step Gate Recess”,The

Journal of Korean Institute of Electromagnetic Engineering and

science,doi:10.5515/kjkiees.2019.30.4.282

[15] Vassil Palankovski,Stanislav Vitanov,and Rudiger Quay,”Field-Plate Optimization

of AlGaN/GaN HEMTs”,Advanced materials and Device analysis Group, inst. For

Microelectronics, TU Wien, Gußhausstr. 27–29, 1040 Wien, Austria

[16] Amano, H, Sawaki,N, Akasaki, I & Toyoda, Y 1986 "Metalorganic vapor phase

epitaxial growth of a high quality GaN film using an AlN buffer layer," Appl. Phys. Lett.,

vol. 48, no. 5, pp. 353.

[17] Amano, H, Kito, M, Hiramatsu, K & Akasaki, I 1989 "P-Type Conduction in Mg-

Doped GaN Treated with Low-Energy Electron Beam Irradiation (LEEBI)”, Jpn. J. Appl.

Phys., vol. 28, no. Part 2, No. 12, pp. L2112-L2114.

[18] Pankove, J, I, Miller, E,A, & Berkeyheiser, J, E 1971 "GaN electroluminescent

diodes," Electron Devices Meeting, vol. 17, pp. 78-78.

[19] Nakamura, S, Mukai, T, & Senoh, M 1991 "High-Power GaN P-N Junction Blue-

Light-Emitting Diodes," Jpn. J. Appl. Phys., vol. 30, no. Part 2, No. 12A, pp. L1998-L2001.

[20] Monroy, E, Calle, F, Angulo, C, Vila, P, Sanz, A, Garrido, J,A, Calleja,E, Munoz, E,

Haffouz, S, Beaumont, B, Omnes, F, & Gibart, P 1998 "GaN-Based Solar-Ultraviolet

Detection Instrument," Appl.Opt., vol. 37, no. 22, pp. 5058-5062.

[21] Nakamura, S, Senoh, M, Nagahama, S, Iwasa, N, Yamada,T, Matsushita, T, Kiyoku,

H, & Sugimoto, Y 1996 "InGaN-Based Multi-Quantum-Well-Structure Laser Diodes,"

Jpn. J. Appl. Phys., vol. 35, no. Part 2, No. 1B, pp. L74-L76.

[22] Asif Khan, M, Bhattarai, A, Kuznia, J, N & Olson, D, T 1993 "High electron mobility

transistor based on a GaN-AlIGai-,N heterojunction," Appl. Phys. Lett., vol. 63, no. 9, pp.

1214.

[23] Asif Khan, M, Kuznia, J, N, Bhattarai, A, R & Olson, D, T 1993 "Metal semiconductor

field effect transistor based on single crystal GaN," Appl. Phys. Lett., vol. 62, no. 15, p.

1786.

55

[24] 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, & J, Hilsenbeck 1999

"Two dimensional electron gases induced by spontaneous and piezoelectric polarization

charges in Nand Ga-face AlGaN/GaN heterostructures," J. Appl. Phys., vol. 85, no. 6, pp.

3222-3233.

[25] Khan, M, A, Chen, Q, Sun, C, J, Shur, M, & Gelmont, B 1995 "Two-dimensional

electron gas in GaN-AlGaN heterostructures deposited using trimethylamine-alane as the

aluminum source in low pressure metalorganic chemical vapor deposition," Appl. Phys.

Lett., vol. 67, no. 10, pp. 1429-1431.

[26] Khan, M, A, Kuznia, J, N, Van Hove, J, M, Pan, N & Carter, J 1992 "Observation of

a twodimensional electron gas in low pressure metalorganic

[27] Sakalauskas, E, Behmenburg, H, Hums, C, Schley, P, Rossbach, G, Giesen, C,

Heuken, M, Kalisch, H, Jansen, R, H, Bläsing, J, Dadgar, J, Krost, A & Goldhahn, R 2010

“Dielectric function and optical properties of Al-rich AlInN alloys pseudomorphically

grown on GaN”, J. Phys. D: Appl. Phys., vol.43, pp.365102.

[28] Vurgaftman, I & Meyer, J, R 2003 “Band parameters for nitrogen-containing

semiconductors”, J. Appl. Phys., vol. 94. pp.3675.

[29] Ambacher, O, Majewski, J, Miskys, C, Link, A, Hermann, M, Eickhoff, M,

Stutzmann, M, Bernardini, F, Fiorentini, V, Tilak, V, Schaff, B, & Eastman, L, F 2002

“Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures”, J. Phys.:

Condens. Matter, vol. 14, pp.3399.

[30] Baliga, B, J 1989 “Power semiconductor device figure of merit for high-frequency

applications”, IEEE Electron Device Lett., vol. 10, pp.455.

[31] Kizilyalli, I, C, Edwards, A, P, Nie, H, Bour, D, Prunty, T & Disney, D 2014 “ 3.7

kV vertical GaN PN diodes”, IEEE Electron Device Lett., vol.35, pp.247.

[32] Das, J, Everts, J, Van den Keybus, J, Van Hove, M, Visalli, D, Srivastava, P,

Marcon, D, Kai Cheng, Leys, M, Decoutere, S, Driesen, J & Borghs, G 2011 “ A 96%

efficient high-frequency DC-DC converter using E-mode GaN DHFETs on Si. IEEE

Electron Device Lett., vol.32, pp.1370.

56

[33] Kizilyalli, I, C, Edwards, A, P, Nie, H, Disney, D, & Bour, D 2013 “High voltage

vertical GaN p-n diodes with avalanche capability” IEEE Trans. Electron Devices, vol.60,

pp. 3067.

[34] Morita, T, Tamura, S, Anda, Y, Ishida, M, Uemoto, Y, Ueda, T, Tanaka, T & Ueda,

D 2011 “99.3% efficiency of three-phase inverter for motor drive using GaN-based gate

injection transistors” Proceeding of the IEEE Appl. Power Electron. Conf. Expo., pp. 481.

[35] Sheridan, D, Ritenour, A, Bondarenko, V, Burks, P & Casady, J 2009 “Record 2.8

m-cm2 1.9 kV enhancement-mode SiC VJFETs”, Proc. of Intl. Power Semiconductors

(ISPSD), pp.335.

[36] Balachandran, S, Li, C, Losee, P, A, Bhat, I, B & Chow, T, P 2007 “6 kV 4H -SiC

BJTs with specific on-resistance below the unipolar limit using a selectively grown base

contact process”, Proc. of Intl. Power Semiconductors (ISPSD), pp.293.

[37] Furukawa, A, Kinouchi, S, Nakatake, H, Ebiike, Y, Kagawa, Y, Miura, N, Nakao, Y,

Imaizumi, M, Sumitani, H & Oomori, T, 2011 “Low on-resistance 1.2 kV 4H-SiC

MOSFETs integrated with current sensor”, Proc. of Intl. Power Semiconductors (ISPSD),

pp.288.

[38] Nakamura, T, Nakano, Y, Aketa, M, Nakamura, R, Mitani, S, Sakairi, H, &

Yokotsuji, Y 2011 “High performance SiC trench devices with oultra low RON” Proc. of

Intl. Power Semiconductors (ISPSD), pp. 26.5.1.

[39] Nie, H, Diduck, Q, Alvarez, B, Edwards, A, P, Kayes, B ,M, Zhang, M, Ye, G,

Prunty, T, Bour, D, & Kizilyalli, I, C 2014 “1.5-kV and 2.2-m-cm 2 vertical GaN

transistors on bulk-GaN substrates”, IEEE Electron Device Lett., vol.35, pp.939.

[40] Wu, Y, -F, Moore, M, Saxler, A, Wisleder, T, & Parikh, P 2006 “40-W/mmdouble

field-plated GaN HEMTs” , Proc. IEEE Device Res. Conf., pp.152.

[41] Wu, Y, F, Moore, M, Abrahamsen, A, Jacob-Mitos, M, Parikh, P, Heikman,S, &

Burk, A, 2007 “High-voltage millimeter-wave GaN HEMTs with 13.7W/mmpower

density” IEDM Tech. Dig., pp.405.

57

[42] Wu,Y, -F, Saxler, A, Moore, M, Smith, R, P, Sheppard, S.T, Chavarkar, P,M,

Wisleder, T, Mishra,U, K & Parikh, P 2004 “30-W/mmGaN HEMTs by field plate

optimization”, IEEE Electron Device Lett., vol.25, pp.117.

[43] Palacios, T, Chakraborty, A, Heikman, S, Keller, S, DenBaars, S, P & Mishra, U,

K 2006 "AlGaN/GaN high electron mobility transistors with InGaN back-barriers," IEEE

Electron Device Lett., vol. 27, no. 1, pp. 13 – 15.

[44] Shinohara, K, Regan, D, Milosavljevic, I, Corrion, AL, Brown, DF, Willadsen, PJ,

Butler, C, Schmitz, A, Kim, S, Lee, V, Ohoka, A, Asbeck, PM & Micovic, M 2011

“Electron velocity enhancement in laterally-scaled GaN DH-HEMTs with fT of 260 GHz,”

IEEE Electron Device Lett., vol. 32, no. 8, pp. 1074–1077.

[45] Johnson, E, O 1965 “Physical limitations on frequency and power parameters of

transistors”, RCA Review, vol.26, pp.163.

[46] Mishra, U, K, Shen, L, Kazior, T, E & Wu, Y, F 2007 “GaN-based RF power

devices and amplifiers” Proceeding of the IEEE, pp.96, vol.287.

[47] Hanson, C, M, Chu, P, Wieder, H & Clawson, A, R 1987 “InxAl1-xAs/InP

heterojunction insulated gate field effect transistors (HIGFET's)”, IEEE Electron Device

Lett., vol. 8, no. 2, pp. 53-54.

[48] Jae-Seung, L, Jong-Wook, K, Jae-Hak, L, Jin-Ho, S, Sung-Ho, H, Yong-Hyun, L,

Jessen, G, H, Fitch, R, C. Gillespie, J, K, Via, G, Crespo, A, Langley, D, J, Denninghoff,

D, Trejo, M, & Heller, E, R 2007 “Short-channel effect limitations on high-frequency

operation of AlGaN/GaN HEMTs for T-gate devices”, IEEE Trans. Electron Devices,

vol.54, pp. 2589.

[49] Palacios, T, Chakraborty, A, Rajan, S, Poblenz, C, Keller, S, DenBaars, S, P, Speck,

J, S & Mishra, U,K 2005 “High-power AlGaN/GaN HEMTs for Ka-band applications”

IEEE Electron Device Lett., vol.26, pp.781.

[50] DiSanto, D, W & Bolognesi, C, R 2006 “At-bias extraction of access parasitic

resistances in AlGaN/GaN HEMTs: Impact on device linearity and channel electron

velocity” IEEE Trans. Electron Devices, vol.53, pp.2914.