Dielectric thin films for GaN-based high-electron-mobilitytransistors

Yan-Rong Li*, Xing-Zhao Liu, Jun Zhu,

Ji-Hua Zhang, Lin-Xuan Qian, Wan-Li Zhang

Received: 24 December 2014 / Revised: 13 January 2015 / Accepted: 13 January 2015

� The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2015

Abstract The effects of dielectric thin films on the per-

formance of GaN-based high-electron-mobility transistors

(HEMTs) were reviewed in this work. Firstly, the nonpolar

dielectric thin films which act as both the surface passiv-

ation layers and the gate insulators of the high-frequency

GaN-based high-electron-mobility transistors were pre-

sented. Furthermore, the influences of dielectric thin films

on the electrical properties of two-dimensional electron gas

(2DEG) in the AlGaN/GaN hetero-structures were ana-

lyzed. It was found that the additional in-plane biaxial

tensile stress was another important factor besides the

change in surface potential profile for the device perfor-

mance improvement of the AlGaN/GaN HEMTs with

dielectric thin films as both passivation layers and gate

dielectrics. Then, two kinds of polar gate dielectric thin

films, the ferroelectric LiNbO3 and the fluorinated Al2O3,

were compared for the enhancement-mode GaN-based

HEMTs, and an innovative process was proposed. At last,

high-permittivity dielectric thin films were adopted as

passivation layers to modulate the electric field and

accordingly increase the breakdown voltage of GaN-based

HEMTs. Moreover, the polyimide embedded with Cr par-

ticles effectively increased the breakdown voltage of GaN-

based HEMTs. Finally, the effects of high-permittivity

dielectric thin films on the potential distribution in the drift

region were simulated, which showed an expanded electric

field peak at the drain-side edge of gate electrode.

Keywords GaN-based HEMTs; Surface passivation;

Gate dielectrics; Enhancement-mode HEMTs;

High-permittivity field plate

1 Introduction

Nowadays, the mass production of silicon crystals with

high purity and excellent crystalline can be easily realized

owing to the development of processing technology.

Moreover, silicon dioxide, as the native oxide of silicon,

can function as a high-quality dielectric thin film for the

present silicon complementary metal–oxide–semiconduc-

tor (CMOS) field-effect transistors (FETs) and even the

future devices after the continued scaling in critical

dimension. Meanwhile, silicon-based devices dominate the

present semiconductor technology in various related

applications. However, silicon still has many drawbacks

due to its own physical properties. As a result, researchers

have been exploring the alternative semiconductor mate-

rials with supreme physical and electrical properties.

Within them, compound semiconductor GaAs, as one of

the second-generation semiconductors, is regarded as a

very promising candidate for photonics and high-frequency

electronic devices. Since early 1960s, intensive efforts

have been made to seek electrically and thermodynami-

cally stable insulators for GaAs metal–oxide–semicon-

ductor field-effect transistors (MOSFETs). For example,

the findings about the mixed oxide Ga2O3(Gd2O3) depos-

ited by molecular beam epitaxy (MBE) on GaAs surface

made a great progress in such a research field, which solves

the problems bothering researchers in the past 35 years [1–7].

Y.-R. Li*, X.-Z. Liu, J. Zhu, J.-H. Zhang, L.-X. Qian,

W.-L. Zhang

State Key Laboratory of Electronic Thin Films and Integrated

Devices, School of Microelectronics and Solid-State Electronics,

University of Electronic Science and Technology of China,

Chengdu 610054, China

e-mail: yrli@uestc.edu.cn

123

Rare Met. RARE METALSDOI 10.1007/s12598-015-0451-3 www.editorialmanager.com/rmet

Consequently, III–V MOSFETs explored a brand new field

for the integrated circuit (IC) industry, providing a timely

innovation to replace Si CMOS at 16 nm node.

In addition, GaN-based hetero-structure, such as AlGaN/

GaN, can realize an ultra-high-power density operation with

a low power loss due to its high carrier mobility in the two-

dimensional electron gas (2DEG) and large breakdown

voltage with a strong critical electric field. As a result, GaN-

based hetero-structure FETs, as represented by the GaN-

based high-electron-mobility transistors (HEMTs), were

considered as almost the most promising candidates for the

high-frequency and high-power microwave applications. In

the last decade, GaN-based HEMTs were highly developed

and successfully fabricated into some prototypes of mono-

lithic microwave IC. Moreover, various dielectric thin films

were investigated in GaN-based HEMTs, and it was proved

that dielectrics played a very important role in their device

performance [8–16]. In this work, the effects of dielectric

thin films on the electrical properties of AlGaN/GaN hetero-

structures were systematically analyzed in order to better

control the device performance of GaN-based HEMTs.

2 Nonpolar dielectric thin films for high-frequency

GaN-based HEMTs

Both gate insulation and surface passivation are widely

adopted in GaN-based HEMTs. Moreover, many kinds of

nonpolar dielectric thin films, such as SiO2 [8], Si3N4 [14],

AlN [12], Al2O3 [13], Sc2O3 [9] and HfO2 [11], were

investigated for the applications of surface passivation and/

or gate insulation. Recently, GaN-based metal–insulator–

semiconductor (MIS)-HEMTs have attracted considerable

attention since they can exhibit excellent direct-current

(DC) characteristics, especially for reducing the gate

leakage current which often appears in some conventional

metal–semiconductor (MES)-HEMTs [17–20]. In addition,

radio frequency (RF) characteristics must be examined as

well to demonstrate the feasibility of MIS-HEMTs, and

excellent RF performance and power stability were repor-

ted in GaN-based MIS-HEMTs [8, 21, 22]. Moreover, the

Al2O3/Si3N4 bilayer gate insulator was proposed to

simultaneously utilize (1) the high-quality interface

between Si3N4 and AlGaN, and (2) the high resistivity and

large dielectric constant of Al2O3 [23–27]. In these papers,

the simulation results showed that the electron tunneling

through the Schottky barrier, due to a high density of

donor-like defects on the surface of AlGaN barrier layers,

was the dominant mechanism of gate leakage current in the

conventional MES-HEMTs, and an Al2O3/Si3N4 bilayer

gate insulator can substantially reduce the donor-like sur-

face defect density and then suppress the gate leakage

current in GaN-based MIS-HEMTs.

Moreover, the potential application of GaN-based

HEMTs is also limited by the charge trapping effect [18,

20, 28–30], particularly in the access region between gate

and drain. The trapped surface charges degrade device

performance and reliability. For example, the drain current

under a large source–drain voltage decreases, which is

called current collapse effect and believed to be due to the

traps both on the exposed surface and in the underlying

GaN buffer. Therefore, in addition to gate insulator,

dielectric thin films are often used for surface passivation

to suppress the surface charge trapping effect, which is

specific to GaN-based HEMTs. By means of adopting an

additional dielectric thin film as the passivation layer on

the top surface of GaN-based HEMTs, the current collapse

effect can be effectively suppressed due to the reduction in

surface electron traps.

It is one of the most fundamental studies for GaN-based

HEMTs to investigate the effects of dielectric passivation

on the electrical properties of 2DEG in the AlGaN/GaN

hetero-structures. For instance, the influence of depositing

dielectric thin films on the electrical properties of 2DEG in

GaN-based hetero-structures was investigated for the Si-

and Al-based insulators (Si3N4, SiO2, AlxNy and Al2O3),

which are widely adopted for surface passivation as well as

gate insulation. A significant increase in 2DEG concen-

tration (Ns) was observed for all the insulators. Moreover,

Al2O3 exhibited a lower sheet resistance compared with

others. In addition, a band engineering was proposed to

study the physical mechanism of dielectric deposition

effects [31].

In our experiment, Al2O3 thin film was deposited on the

surface of AlGaN/GaN hetero-structure through MBE. The

Hall effect measurement was conducted at room tempera-

ture, and the result is shown in Table 1 [31]. Although the

sheet carrier concentration (Ns) and mobility (ls) of Al-

GaN/GaN hetero-structure vary from sample to sample,

both a Ns increase of about 30 % and a ls increase of about

20 % were observed in all the samples with the deposition

of Al2O3 thin film.

The AlGaN/GaN MIS-HEMTs with Al2O3 thin film for

both surface passivation and gate insulation are fabricated

Table 1 Effects of Al2O3 dielectric thin film on sheet carrier con-

centration and mobility of AlGaN/GaN hetero-structure

Parameters 400 �C 500 �C 600 �C

Sheet carrier concentration/1012cm-2

MES-HEMT 6.94 6.83 6.42

MIS-HEMT 8.73 8.78 8.32

Sheet carrier mobility/(cm2�V-1�s-1)

MES-HEMT 947 950 1,030

MIS-HEMT 1,150 1,090 1,260

Y.-R. Li et al.

123 Rare Met.

as shown in the inset in Fig. 1 [31]. In addition, the AlGaN/

GaN MES-HEMTs without dielectric thin film were fab-

ricated as well on the same wafer for the comparison

purpose. As shown in Fig. 1, the measured results of output

characteristics of MIS-HEMTs and MES-HEMTs exhibit

the maximum saturated drain currents (IDSSmax) of 790 and

590 mA�mm-1 at a gate bias of 2 V, and the saturated

drain currents (IDSS) of 700 and 360 mA�mm-1 at a gate

bias of 0 V, respectively [31]. Accordingly, MIS-HEMTs

yielded a 34 % increase in IDSSmax and a 94 % increase in

IDSS compared with MES-HEMTs. The increase in IDSSmax

revealed a larger output power density of MIS-HEMTs

than that of MES-HEMTs.

Besides, MIS-HEMTs possessed a peak extrinsic trans-

conductance of 170 mS�mm-1 at drain–source voltage

VDS = 10 V, higher than that of MES-HEMTs (120

mS�mm-1) as shown in Fig. 2 [31]. The extrinsic trans-

conductance is described by the equation: Gm;ext ¼Gm;int=ð1þ Gm;intRSÞ, where Rs is the series resistance, and

Gm,int is the intrinsic trans-conductance. Gm,int can be

described by Gm;int ¼ ldCGSðVGS � VthÞ=LGS, where CGS is

the gate capacities, LGS is the gate length, Vth is the

threshold voltage, ld is the carrier mobility in the channel

and VGS is the gate voltage. Because of a larger gate-

channel distance and a smaller gate capacitance, one would

expect a lower trans-conductance for MIS-HEMTs com-

pared with that for MES-HEMTs. So, the enhanced trans-

conductance of the MIS-HEMTs could be explained by the

increased sheet carrier mobility in the AlGaN/GaN hetero-

structure after the deposition of Al2O3 thin film as revealed

by the result of Hall effect measurement. Moreover, high

resolution X-ray diffraction (XRD) patterns were also uti-

lized to study the effects of depositing Al2O3 thin film on

the crystal structure of AlGaN/GaN hetero-structure. As

shown in Fig. 3, the diffraction peak from the (0001) plane

of the AlGaN barrier layer changed from 34.97� to 35.04�after the deposition of Al2O3 thin film [31]. Accordingly, a

smaller c-axis lattice constant of AlGaN barrier layers was

observed, which implies an expanded a-axis and/or b-axis

lattice constant. In other words, an additional in-plane

biaxial tensile stress was introduced into the AlGaN/GaN

barrier layers after the deposition of Al2O3 thin film.

The energy band gap, band offset, effective electron

mass and polarization were studied after taking into

account of the additional in-plane biaxial tensile stress.

Then, both concentration and mobility of sheet carrier were

calculated by a self-consistent method based on the charge

control model. As listed in Table 2 [32], the values of the

stress in AlGaN barrier layer before and after the deposi-

tion of Al2O3 thin film were 5.0 and 6.5 GPa, respectively

[31]. On the one hand, the additional in-plane biaxial ten-

sile stress enhanced the band offset and polarization.

Accordingly, the calculated sheet carrier concentration

Fig. 1 Transfer characteristics of AlGaN/GaN MES-HEMTs and

AlGaN/GaN MIS-HEMTs. Inset being cross-sectional illustration of

AlGaN/GaN MIS-HEMTs with Al2O3 thin films for both surface

passivation and gate dielectrics. S source, G gate, D drain

Fig. 2 Trans-conductance characteristics of AlGaN/GaN MES-

HEMTs and AlGaN/GaN MIS-HEMTs with Al2O3 thin films for

both surface passivation and gate dielectrics

Fig. 3 High resolution XRD patterns of AlGaN/GaN hetero-structure

with and without Al2O3 thin film coatings

GaN-based high-electron-mobility transistors

123Rare Met.

could be improved by about 50 %, which well explained its

experimental increase of 32 %. On the other hand, the

additional in-plane biaxial tensile stress deepened the

potential well at the AlGaN/GaN interface, resulting in a

decrease in the inter-sub-band scattering and thus an

improvement of electron mobility.

Furthermore, the influence of the interface between

dielectric thin film and the AlGaN barrier layer on the device

performance of AlGaN/GaN HEMTs was in-depth studied

[32–34]. It was reported that the amount of interface traps

was effectively reduced accompanied by the decrease in the

lattice mismatch between dielectric thin film and AlGaN,

revealing the enhanced passivation effect of dielectric thin

film on the interface traps. For example, an excellent surface

passivation on HEMTs was achieved after employing a

lattice-matched crystalline MgCaO thin film as the passiv-

ation layer [33]. Recently, the high-crystallinity AlN thin

film grown by atomic layer deposition has been demon-

strated to effectively passivate the surface of AlGaN/GaN

HEMTs [34]. It is believed that AlN thin film can provide

high dielectric constant, wide band gap, high thermal con-

ductivity and small lattice mismatch to GaN. In that work,

the GaN-based HEMTs adopted an ultrathin (4 nm) high-

crystallinity AlN thin film as the passivation layer,

improving the properties of 2DEG sheet carrier density, gate

leakage current, off-state drain leakage current, sub-

threshold slope and breakdown voltage, and also suppress-

ing the degradation of dynamic on resistance during the

pulsed off-state voltage switching stress.

3 Polar dielectric thin films for enhancement-mode

GaN-based HEMTs

Nowadays, semiconductors, including Si, GaAs and GaN,

dominate the electronic devices, such as transistor, diode

and other light emitters. However, the responsivity to

perturbation is quite poor if semiconductor is adopted in

sensor. Polar dielectrics, such as BaTiO3 (BTO), Pb(Zr,Ti)O3

(PZT) and LiNbO3 (LN), possess excellent piezoelectric,

pyroelectric, ferroelectric and electro-optic properties, which

make them favorable for the application in sensor. A small

perturbation, such as stress and temperature, in polar

dielectrics can induce the polarization change which can be

easily detected by some semiconductor-based devices.

So far, there are many publications in which polar

dielectrics are adopted in semiconductor device. Within the

related devices, ferroelectric random-access memory (Fe-

RAM) is the most important one [35, 36]. It was reported

that ferroelectric capacitors (FE) were integrated with sil-

icon devices by 1T-1C configuration, where the polariza-

tion state was measured through a transistor. The hetero-

structure which consists of polar dielectric and semicon-

ductor can possess the properties of both materials at the

same time, which provides some possible applications in

sensor. For instance, the device called ‘‘FET-sensor’’, in

which FET adopted polar dielectric as gate oxide, was

theoretically studied [37]. The calculation result shows that

such a functional device can have the properties of both

sensor and transistor. Accordingly, the ferroelectric polar-

ization can be directly coupled to the transistor channel

based on the polar dielectric/semiconductor hetero-struc-

ture. Moreover, FET-sensor itself is a power amplifier, and

thus it is easy to integrate it into some electronic circuits or

microprocessors. Until now, some significant progresses

have been made although the complicated interface state

between polar dielectric and semiconductor causes some

difficulties [38–41].

The effect of polar dielectrics on the characteristics of

2DEG in the AlGaN/GaN interface was theoretically

investigated based on the first-principle calculation and

charge control model [42]. The dependence of 2DEG

density on polarizability and the polar dielectric thickness

is shown in Fig. 4 [42]. It is clear that the sheet carrier

concentration in the AlGaN/GaN interface increases under

a positive polarization of polar dielectric. On the contrary,

a negative polarization of polar dielectric can result in the

decrease in sheet carrier concentration in the AlGaN/GaN

interface, which might be beneficial to realize the

enhancement-mode AlGaN/GaN HEMTs. However, the

result exceeds our expectation as exhibited by the limited

modulation of sheet carrier concentration, which could be

due to the large dielectric constant of the polar dielectric

adopted in this study. Accordingly, polar dielectric LN was

selected to experimentally modulate the 2DEG character-

istics in the AlGaN/GaN interface due to its low dielectric

constant and large polarizability.

c-axis epitaxial LN thin film with c? ferroelectric

spontaneous polarization (pointing from substrate to the

surface of LN thin film) was deposited on AlGaN/GaN by

Table 2 Calculated lattice parameters, band gap (Eg), conduction

band offset (DEc), polarization (P) and optical phonon energy (�hxpop)

under different tensile stresses

Parameters GaN Al0.25Ga0.75N

Relaxed 5.0 GPa 6.5 GPa

a/nm 0.3189 0.3168 0.3180 0.3219

c/nm 0.5185 0.5154 0.5127 0.5117

Eg/eV 3.420 3.930 4.048 4.096

DEc/eV – 0.35 0.44 0.47

mn (m0) 0.230 0.256 0.279 0.282

�hxpop/meV 92.5 94.5 96.3 96.7

P/(lC�cm-2) 2.90 4.40 5.17 6.34

N2DEG/1013 cm-2 – 0.7025 1.2730 1.9070

Y.-R. Li et al.

123 Rare Met.

pulsed laser deposition, which causes a negative polari-

zation [40, 41]. Based on the calculation result as shown

in Fig. 4, the sheet carrier concentration can be depleted

under a negative polarization. Therefore, it is expected

that the enhancement-mode AlGaN/GaN HEMTs can be

realized. As exhibited in the inset in Fig. 5, the AlGaN/

GaN MIS-HEMTs with the ferroelectric LN as gate

dielectrics were fabricated [40, 41]. As mentioned above,

the interface characteristics between dielectrics and the

AlGaN barrier layer is one of the most key factors

affecting the device performance of AlGaN/GaN HEMTs.

Therefore, a 5-nm-ZnO-buffer layer was introduced to

improve the epitaxial quality of LN gate dielectric [41]. In

addition, the AlGaN/GaN MES-HEMTs without LN gate

dielectric were also fabricated for the comparison purpose

[41]. The transfer characteristics of the AlGaN/GaN

MES-HEMTs with and without ZnO buffer layer are

shown in Fig. 5a [41], revealing the threshold voltage

(Vth) values of both samples are almost at the same level

(-2.2 V). Their negative Vth values prove that the Al-

GaN/GaN MES-HEMTs operate in the depletion mode. In

Fig. 5b, the transfer characteristics of the AlGaN/GaN

MIS-HEMTs with LN gate dielectrics are exhibited [41].

On the one hand, the electrical properties of the AlGaN/

GaN MIS-HEMTs are improved by adding a ZnO buffer

layer. For instance, the maximum trans-conductance (Gm)

increases from 27 to 46 mS�mm-1, and the IDSSmax is

improved from 97 to 204 mA�mm-1. The improved

electrical properties are possibly attributed to the ame-

liorated interface properties by inserting ZnO buffer layer.

On the other hand, the device operation mode is changed

into the enhancement mode due to the adoption of LN

gate dielectric, and the Vth values of such AlGaN/GaN

MIS-HEMTs with and without ZnO buffer layer are ?0.4

and ?0.3 V, respectively. In general, the enhancement

mode is more desirable for the real application of elec-

tronic devices, simplifying the design of the GaN-based

IC and guaranteeing the fail-safe operation of GaN-based

power devices, for example, power switch. In addition,

the normally-off state implies the depletion of 2DEG due

to the negative polarization of LN thin film.

Besides the polarization in ferroelectrics which origi-

nates from the dipole formed by negative and positive ions,

there is another kind of polarization, the space-charge

polarization, which is due to the inhomogeneity of

dielectric charges. As shown in the inset in Fig. 6, the

AlGaN/GaN MIS-HEMTs with selectively fluorinated

Al2O3 gate dielectric were fabricated [38, 39]. During the

fluorination treatment by the aid of a fluorine-based plasma

immersion, negative charges are incorporated in the near-

surface layer of Al2O3 gate dielectric. Moreover, both

higher plasma activation power and longer immersion time

can incorporate more negative charges in gate dielectric.

As shown in Fig. 6, Vth positively shifts due to the incor-

porated negative charges in gate dielectric [38]. With a

proper dose of the incorporated negative charges, Vth

changes from a negative value to a positive one, which

means that the device operation transfers from the deple-

tion mode to the enhancement one.

Fig. 4 Calculated modulation effect of polar dielectrics on sheet

carrier density in AlGaN/GaN interface

Fig. 5 Transfer characteristics of AlGaN/GaN MES-HEMTs a with-

out LN as gate dielectrics and b with LN as gate dielectrics. Inset

being cross-sectional illustration of AlGaN/GaN MIS-HEMTs with

ZnO buffered LN ferroelectric thin films as gate dielectrics

GaN-based high-electron-mobility transistors

123Rare Met.

As mentioned above, it is an interesting research topic

to adopt the AlGaN/GaN HEMTs in high-power switch.

In this application, a completely turn-off state at a zero

bias is required to guarantee the operation safety. Thus,

the AlGaN/GaN HEMTs must possess a sufficiently high

Vth. There are two methods conducted in order to further

increase the Vth of AlGaN/GaN MIS-HEMTs. The first

one is to incorporate more negative charges in the near-

surface layer of Al2O3 gate dielectric by increasing the

plasma power during the fluorination treatment. The

second one is to successively deposit another Al2O3 layer,

which acts as a blocking oxide layer, on the fluorinated

Al2O3 thin film as shown in the inset in Fig. 7 [43]. The

enhancement-mode MIS-HEMTs with and without a

blocking oxide layer were fabricated by the first and

second method mentioned above, respectively, and their

transfer characteristics are exhibited in Fig. 7. For the

convenience of comparison, the Vth values of both sam-

ples should be almost the same. It shows that the device

performance is degraded due to more severe damage

induced by the high-energy ions in plasma although Vth is

successfully further increased by the first method. On the

contrary, the blocking oxide layer formed by the second

method yields an excellent device performance, including

high Vth, large IDSS and moderate Gm.

Furthermore, both surface potential and barrier height

of the fluorinated-Al2O3/AlGaN/GaN structure were

studied by X-ray photoelectron spectroscopy (XPS) [39].

It shows that the reduction in the surface Fermi level (EF)

relative to valance-band maximum (VBM), VBM - EF is

about 0.2 eV after the fluorine-based plasma treatment,

which is consistent with the modification value of AlGaN

barrier height. Hence, the main reason causing the Vth to

increase after the fluorination treatment on Al2O3 gate

dielectric is not the surface potential modification but the

negative charges incorporation due to the partial substi-

tution of oxygen atoms by fluorine ones. In addition, the

effects of negative charges in gate dielectric on energy

band and carrier concentration were calculated by a self-

consistent method in the framework of charge-control

mode. The negative-charge concentration was estimated

through the fluorine ion distribution profile measured by

XPS, which was expressed as a Gaussian distribution:

ND = N0exp(-x/k), where ND is the fluorine-ion con-

centration, N0 is the maximum value of fluorine-ion

concentration on Al2O3 surface, x is the depth from the

surface of the Al2O3 thin films and k is the diffusion depth

which is estimated to be about 1 nm according to the

measured fluorine ion distribution profile. The calculated

result shows that the negative charges in Al2O3 thin film

increase the conduction band. Moreover, when the nega-

tive-charge concentration increases to a critical value, for

example, N0 = 191021 cm-3, the 2DEG in the AlGaN/

GaN interface is completely depleted as shown in Fig. 8,

achieving the enhancement-mode AlGaN/GaN MIS-

HEMTs [39].

Fig. 6 Dependence of Vth on process parameters of fluorine-based

plasma immersion. Inset being cross-sectional illustration of AlGaN/

GaN MIS-HEMTs with fluorinated Al2O3 thin films as gate dielectrics

Fig. 7 Transfer characteristics of enhancement-mode MIS-HEMTs

with and without blocking oxide layer using fluorinated Al2O3 thin

films as gate dielectrics

Fig. 8 Conduction band and carrier concentration of 2DEG in

AlGaN/GaN hetero-structure covered by fluorinated-Al2O3 thin films

Y.-R. Li et al.

123 Rare Met.

4 High-permittivity thin films for high-voltage

GaN-based HEMTs

Besides the normally-off state, an enough high reverse

breakdown voltage is also required when the AlGaN/GaN

HEMTs is adopted in power electronic devices, as

explained by the following equation: Pout ¼ 18

IDSS max

ðVbr � VkneeÞ, where Pout is the maximum output power of

HEMTs, Vknee is the knee voltage and Vbr is the reverse

breakdown voltage. Although the AlGaN/GaN hetero-

structure possesses a large breakdown electric field in

theory, the electric fields on the surface and in the channel

of HEMTs are inhomogeneous. In other words, there is an

electric field peak located at the drain-side edge of gate

electrode, which can lead to a premature breakdown of

HEMTs. Thus, it is necessary to optimize the potential

distribution in the drift region and weaken the peak electric

field in the AlGaN/GaN HEMTs so as to enhance the

reverse breakdown voltage. The metal field plate was

reported to reduce the electric field peak in AlGaN/GaN

HEMTs by offering additional edges for the termination of

electric field lines [44]. Hence, one strong electric field

peak can be split into several weak electric field peaks by

multiple metal field plates. Although the AlGaN/GaN

HEMTs with high breakdown voltage is achieved by this

method, the process is too complex.

Chen [45] reported that the optimization of the potential

distribution in the drift region can be achieved by covering

a high-permittivity (HK) dielectric on the drain-gate region

of AlGaN/GaN HEMTs as shown in the inset in Fig. 9. The

electric field distribution in the AlGaN/GaN HEMTs with

and without a HK thin film covering the drain-gate region

was simulated by integrated system engineering technol-

ogy computer Aided design (ISE TCAD) as shown in

Fig. 9 [46]. The permittivity of HK thin film in this sim-

ulation is about 100. Because of the large difference in

permittivity between HK thin film and AlGaN barrier layer,

HK thin film can transmit an electric flux into or extract an

electric flux from AlGaN surface. As a result, the electric

field peak at the drain-side edge of gate electrode is wid-

ened by expanding to the entire gate-drain region, and thus

the breakdown voltage increases. In this method, HK thin

film acts as a field plate.

Furthermore, the effects of dielectric permittivity on the

peak electric field are simulated as shown in Fig. 10 [46,

47]. It shows that the peak electric field gradually decreases

with the increase in dielectric permittivity. However, it

should be noticed that an obvious electric field modulation

can be achieved only when the dielectric permittivity is

high enough. Taking the peak electric field without HK

dielectrics as 1.0, the relative peak electric field of 0.5 is

achieved when adopting the HK dielectrics with a per-

mittivity of 70. However, the relative peak electric field

reaches as high as 0.9 when the permittivity of HK thin

film is 20. In this case, the electric field modulation is

negligible if adopting the most commonly used HK

dielectrics, such as Al2O3 and HfO2.

It was reported that the breakdown voltage of Si LDMOS

was increased from 95 to 360 V by covering PZT HK thin

film in the drift region [48]. In theory, the permittivity of

inorganic multi-component dielectric, for example, PZT, is

very high ([100). However, an annealing treatment in the

oxygen ambient is required during a real fabrication to

guarantee the high permittivity of such multi-component

dielectric. Besides, the difficulties in both the etching process

and the stoichiometry control during thin-film deposition

need to be overcome as well. Hence, it is hard to incorporate

the inorganic multi-component dielectrics into HEMTs.

In addition, polyimide (PI) is widely used in electronic

devices because of its excellent thermal stability and

chemical resistance. It was reported that a high permittivity

was achieved by embedding the Cr particles into the PI

matrix. Moreover, the PI/Cr composite thin film with

8 vol% Cr particles acting as a passivation layer is dem-

onstrated in AlGaN/GaN HEMTs as shown in the inset of

Fig. 11 [46, 47]. The permittivity of the PI/Cr composite

thin film is estimated to be about 70. For the comparison

Fig. 9 Simulated electric field distribution in AlGaN/GaN HEMTs

with and without HK thin films covering drain-gate region Fig. 10 Simulated effect of permittivity on relative peak electric field

GaN-based high-electron-mobility transistors

123Rare Met.

purpose, both the AlGaN/GaN HEMTs only with PI pas-

sivation and without any passivation were investigated as

well. The transfer characteristics and trans-conductances of

AlGaN/GaN HEMTs are shown in Fig. 11 [46, 47]. It

shows that the passivation can benefit the DC performance,

and that the difference between PI and PI/Cr on the pas-

sivation effect is negligible. Furthermore, the measured

result related to the breakdown properties of AlGaN/GaN

HEMTs is shown in Fig. 12 [46, 47]. The breakdown

voltage is defined as the off-state source–drain voltage with

a source–drain current of 1 mA�mm-1. The breakdown

voltages of the AlGaN/GaN HEMTs without any, with PI

and with PI/Cr passivation are about 122, 156 and 248 V,

respectively.

In order to clarify the difference between PI and PI/Cr

passivations, the dependence of breakdown voltage on the

thickness of dielectric thin film was studied. As for the PI-

passivated AlGaN/GaN HEMTs, a weak relationship

between the breakdown voltage and the thickness of

passivation layer is observed as shown in Fig. 13 [46].

However, the breakdown voltage almost linearly increa-

ses with adding the passivation layer thickness in the PI/

Cr-passivated AlGaN/GaN HEMTs. Therefore, the

improvement in the breakdown voltage properties can be

attributed to the surface effect for the PI-passivated Al-

GaN/GaN HEMTs, while that for the PI/Cr-passivated

AlGaN/GaN HEMTs is ascribed not only to the surface

effect but also to the HK field plate, which is a bulk effect

and thus contributes the most to the breakdown voltage

enhancement.

5 Summaries

To sum up, several experimental innovations in device

process were achieved for GaN-based power HEMTs in

our past works. Firstly, the nonpolar dielectric thin films,

for example, Al2O3, were adopted as surface passivation

layer as well as gate insulator to improve the device per-

formance of AlGaN/GaN MISHFETs. Then, the enhance-

ment-mode GaN HEMTs were successfully fabricated

using polar gate dielectric thin films, such as the ferro-

electric LiNbO3 and the fluorinated Al2O3. In addition, the

polyimide embedded with nano-sized Cr particles as a HK

dielectric thin film with a passivation function was utilized

in GaN-based HEMTs so as to increase the breakdown

voltage.

Acknowledgments The work was financially supported by the

National Nature Science Foundation of China (No. 50932002) and the

Research Foundation for the Doctoral Program of Higher Education

of China (No. 2012018530003).Fig. 11 Transfer a and trans-conductance b characteristics of

AlGaN/GaN HEMTs (VDS = 10 V)

Fig. 12 Off-state current–voltage characteristics of AlGaN/GaN

HEMTs

Fig. 13 Effect of passivation layer thickness on breakdown voltage

of AlGaN/GaN HEMTs

Y.-R. Li et al.

123 Rare Met.

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Professor Yan-Rong Li acade-

mician of the Chinese Academy

of Engineering, president of

University of Electronic Science

and Technology of China, was

born in 1962 in Sichuan Prov-

ince. In 1992, he got the Ph.D.

degree from Changchun Insti-

tute of Applied Chemistry,

Chinese Academy of Sciences.

He is currently the director of

the State Key Laboratory of

Electronic Thin Films and Inte-

grated Devices. He is engaged

in electronic thin films and

devices. He invented a novel bi-axial rotation method to fabricate

large-area double-sided YBCO superconducting thin films by inverted

cylindrical sputtering. Based on the study of the initial growth

mechanism, self-buffered architecture was proposed to improve the

reliability of pyroelectric thin film sensors for nondispersion infrared

detection. Now he is working at dielectric thin films for GaN HEMTs

and MMICs, thin film sensors for aircraft engine, and HTSC coated

conductors. He twice won the prize of National Technology Invention

Award. He has got more than 20 issued Chinese patents, published

more than 200 journal articles and three books.

Y.-R. Li et al.

123 Rare Met.