IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007
23
A Low Phase-Noise X -Band MMIC VCO Using
High-Linearity and Low-Noise Composite-Channel
Al0:3Ga0:7N/Al0:05Ga0:95N/GaN HEMTs
Zhiqun Q. Cheng, Yong Cai, Jie Liu, Yugang Zhou, Kei May Lau, Fellow, IEEE, and
Kevin J. Chen, Senior Member, IEEE
Abstract—A low phase-noise
-band monolithic-microwave
integrated-circuit voltage-controlled oscillator (VCO) based
on a novel high-linearity and low-noise composite-channel
Al0 3 Ga0 7 N/Al0 05 Ga0 95 N/GaN high electron mobility transistor (HEMT) is presented. The HEMT has a 1 m 100 m gate.
A planar inter-digitated metal–semiconductor–metal varactor is
used to tune the VCO’s frequency. The polyimide dielectric layer
is inserted between a metal and GaN buffer to improve the
factor of spiral inductors. The VCO exhibits a frequency tuning
range from 9.11 to 9.55 GHz with the varactor’s voltage from 4 to
6 V, an average output power of 3.3 dBm, and an average efficiency
of 7% at a gate bias of 3 V and a drain bias of 5 V. The measured
phase noise is 82 dBc/Hz and 110 dBc/Hz at offsets of 100 kHz
and 1 MHz at a varactor’s voltage ( tune ) = 5 V. The phase
noise is the lowest reported thus far in VCOs made of GaN-based
HEMTs. In addition, the VCO also exhibits the minimum second
harmonic suppression of 47 dBc. The chip size is 1.2 1.05 mm2 .
Index Terms—Al0 3 Ga0 7 N/Al0 05 Ga0 95 N/GaN high electron
mobility transistor (HEMT), monolithic, phase noise, voltage-controlled oscillator (VCO).
I. INTRODUCTION
HE AlGaN/GaN high electron mobility transistors
(HEMTs), with their high breakdown voltage and
high-frequency operation, are promising candidates for power
amplifiers to be used in next-generation wireless base stations
and military applications [1]–[5]. The AlGaN/GaN HEMTs
are also showing good linearity and excellent microwave
T
Manuscript received May 11, 2006; revised August 18, 2006. This work was
supported in part by the Hong Kong Research Grant Council and National Science Foundation of China under Grant N_HKUST616/04 and by the National
Science Foundation of China under Grant 60476035.
Z. Q. Cheng is with the Department of Electronic and Computer Engineering,
Hong Kong University of Science and Technology, Hong Kong, and also with
the Department of Information Engineering, Hangzhou Dianzi University,
Hangzhou, 310018, China.
Y. Cai and J. Liu were with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong. They
are now with the Material and Packing Technologies Group, Hong Kong Applied Science and Technology Research Institute Company Ltd., Hong Kong.
Y. Zhou was with the Department of Electronic and Computer Engineering,
Hong Kong University of Science and Technology, Hong Kong. He is now with
Advanced Packaging Technology Ltd., Hong Kong.
K. M. Lau and K. J. Chen are with the Department of Electronic and Computer
Engineering, Hong Kong University of Science and Technology, Hong Kong
(e-mail: eekjchen@ust.hk).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMTT.2006.888942
noise performance [2], [6]–[9], which are attractive not only
for low-noise amplifier (LNA) development, but also for low
phase-noise voltage-controlled oscillators (VCOs). The VCOs,
if monolithically integrated with the power amplifiers, can
provide flexible high-power signal sources for a broader range
of applications [10]–[12]. The VCOs are also indispensable in
fully integrated AlGaN/GaN HEMT transceivers. For improved
device performances, advanced device structures are also being
investigated. For example, a composite channel high electron
mobility transistor (CC-HEMT), which features Al Ga N
as the main channel and GaN as the minor channel, was recently demonstrated. CC-HEMTs exhibit enhanced linearity
and low noise [13], [14], both of which are favorable for low
phase-noise VCOs. In particular, the enhanced linearity enables
noise up-conversion factor, which is favorable for
a lower
phase-noise reduction. In this paper, we demonstrate a fully
integrated -band VCO using the CC-HEMT technology that
features improved noise characteristics. On-chip interdigitated
metal–semiconductor–metal (MSM) varactors and planar
spiral inductors form the tunable LC tank. The VCO with a
100- m-wide single HEMT exhibits a tuning range from 9.106
to 9.55 GHz with a maximum output power of 4.2 dBm at a
of 5.0 V. Low phase noise of 82 and
supply voltage
110 dBc/Hz were obtained at offsets of 100 kHz and 1 MHz,
respectively. In addition, the VCO shows excellent second
harmonic suppression of 47 dBc. To our best knowledge, this
is the highest harmonic suppression reported on AlGaN/GaN
HEMT VCOs.
II. MONOLITHIC INTEGRATION TECHNOLOGY
As shown in Fig. 1, the VCO was designed and fabricated in
a fully integrated MMIC process that includes an active HEMT,
air-bridge interconnects, spiral planar inductors, metal–insulator–metal (MIM) capacitors (with SiN as the dielectric), and
the interdigitated MSM varactors [15].
A. Composite-Channel HEMT
The epitaxial structure of the composite-channel HEMT was
grown by metal-organic chemical vapor deposition (MOCVD)
on a
sapphire substrate. The epitaxial layer structure
contains a 2.5- m undoped GaN buffer layer, a 6-nm undoped
Al Ga N layer, 3-nm undoped spacer, a 21-nm doped
2 10 cm
carrier supplier layer, and a 2-nm undoped
cap layer. Different from the conventional AlGaN/GaN HEMT,
0018-9480/$25.00 © 2006 IEEE
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007
Fig. 1. Cross section of the GaN-based HEMT MMIC process.
a thin layer (6 nm) of AlGaN with 5%Al composition is incorporated between the Al Ga N barrier and the GaN buffer,
aimed at reducing the alloy scattering at the barrier interface.
All the HEMTs have a gate length of 1 m and the fabrication
details can be found in [13]. The reduced scattering leads to
noise reduction, with a detailed explanation given in [13] and
[14]. The HEMT devices exhibit a maximum drain current
density of 910 mA/mm and dc extrinsic transconductance
of 175 mS/mm. A maximum oscillation frequency
of 35 GHz was obtained. An output third-order intermodulation
intercept point (OIP3) of 33.2 dBm was obtained at 2 GHz. The
minimum noise figure was measured to be less than 3.5 dB up
to 10 GHz [14].
Fig. 2. Photograph and high-frequency characteristics of the on-chip spiral inductor.
B. On-Chip Passive Components
Since the phase noise of the VCO is predominantly determined by the intrinsic noise of HEMT and the
factor of
the resonant tank, low-noise HEMT and high- inductors
factor
and varactors are essential. In order to improve the
of inductors, it is essential to reduce the coupling to the GaN
buffer layer, which still exhibit nonperfect insulating behavior
10
cm after many
with a resistivity in the range of 10
years’ optimization. A low- polyimide dielectric layer (5- m
thick) was inserted between the major metal traces (transmission lines and inductor metal) and the GaN buffer to reduce
buffer coupling. On-wafer -parameters were measured using
Agilent Technologies’ 8722ES network analyzer and Cascade
microwave probes. One-port inductors [as shown in Fig. 2(a)]
ranging from 1 to 4.5 nH were designed and measured. The
pad-only characteristics were measured on the “open” pad
pattern [as shown in Fig. 2(b)] to extract the pad’s parasitics.
The pad’s parasitics were then deembedded from the overall
inductor characteristics by subtracting the -parameters of the
“open” pad from the -parameter of the overall inductor. A
one-port equivalent-circuit model, as shown in Fig. 2(c), was
used to extract the inductance value and factor. Inductance is
, and the factor is calculated
calculated using
using
. As shown in Fig. 2(d), a
factor of 11 was achieved at 10 GHz in a 2.5-nH inductor.
factor of 14 was achieved at a frequency of
A maximum
6.5 GHz. The self-resonance frequency of the inductor is over
20 GHz. For a 2.5-nH inductor with metal traces directly on top
of the GaN buffer, the factor is approximately 5 at 10 GHz.
Fig. 3. (a) Microphotograph of a fabricated MSM varactor with a close-up view
factor as a function of the
of the interdigital fingers. (b) Capacitance and
varactor’s bias, measured at 10 GHz.
Q
The MSM planar inter-digitated varactors [15], which
factor compared to conventional
showed an improved
metal–insulator–semiconductor varactors, were also fabricated
on the HEMT structure. The varactors were characterized by
on-wafer -parameter measurement with a similar deembedding procedure as that used for inductors. In Fig. 3, an MSM
CHENG et al.: LOW PHASE-NOISE
-BAND MMIC VCO
25
Fig. 5. Circuit schematics of the X -band VCO.
Fig. 4. Photograph and high-frequency characteristics of the on-chip spiral inductor.
planar varactor’s microphotograph together with its characteristics at 10 GHz were presented. The dimensions of the
of 2 m, finger width
varactor include finger length
of 20 m, finger-to-finger spacing
of 1.5 m, and number
of fingers of 20. The varactor’s capacitance varies from 2.0 to
0.5 pF when its bias voltage changes from 4 to 5 V.
MIM capacitors are made of bottom metal (0.3 m), upper
metal (3 m) and SiN dielectric layer (0.2 m). The capacitor
[as shown in Fig. 4(a)] and “open” pad], as shown in Fig. 4(b)]
for deembedding are designed and measured such as that used
for inductors. The capacitance and factor are extracted using
Fig. 4(c) shows the measured result for a 2-pF capacitor according to the above equations.
It should be noted that the factor of both the varactor and
MIM capacitor are relatively low, between 8–10 GHz, seemingly creating obstacles in achieving low phase noise. However,
in our VCO circuit, both of them are in series with the input
of the transistor, which presents a small capacitance value and
small resistance. As the overall capacitance of series-connected
capacitors is dominated by the smaller capacitor, the overall capacitance in the resonant tank is reduced. The overall effective
capacitor together with an inductor form an LC tank that resonates in -band. The reduced effective capacitance also results in a higher factor in the effective capacitor according to
, even though the resistive loss ( ) remains the
same. More discussions are provided in Section III when the design of the VCO is discussed.
III. DESIGN OF VCOS
The VCO circuit was designed using Agilent Technologies’
Advanced Design System (ADS). The schematic circuit of the
Fig. 6. Stability factor K as a function of the length of
the shunt capacitor C .
T
with and without
-band VCO is shown in Fig. 5. In the design of VCO, the measured small-signal -parameters of the CC-HEMT was used. A
stub
was connected between the CC-HEMT’s source and
the ground, providing a positive feedback to make the HEMT
pf
more unstable. A small MIM shunt capacitor
was in parallel with the stub
in order to shorten the stub’s
length and reduce the size of the VCO chip. Fig. 6 shows the
simulated stability factor’s ( ’s) variation with the length of
stub with or without the shunt capacitor. Without the cathe
, the
factor is large (not favorable for oscillator
pacitor
circuits) and the minimum value is larger than 0.2 at a length
, the
factor is reduced and
of 5 mm. With the capacitor
length of 1 mm. As a result, the addition
less than 0.2 at a
not only makes the oscillation more
of the shunt capacitor
likely, but also helps shorten the length of the feedback stub.
The resonant tank was designed and connected to the gate of the
CC-HEMT. The resonator circuit includes the spiral inductor
, the MIM capacitor
, transmission line ( , ), and
an interdigitated MSM varactor
. The MIM capacitor
pF is used to decouple the negative gate bias of the
,
, and
HEMT and the varactor’s control voltage.
are applied through off-chip inductors of 10 nH.
In the design of the VCO, in order to reduce the phase noise of
(tuning frequency divided by
the VCO, the tuning gain
tuning voltage) of the VCO must be minimized [16]. However,
minimizing tuning gain usually results in reduced frequency
tuning range. Therefore, the tuning gain was capped limited to
299.5 MHz/V (from small-signal simulation) when the tuning
range was specified to be approximately 500 MHz in our design. After tuning the elements’ values (it is mainly inductance)
26
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007
Fig. 8. Measured output spectrum at V
Fig. 7. Microphotograph of the X -band VCO chip. Its size is 1.2
2 1.05 mm .
5 V.
= 5 V, V
= 03 V, and V
=
of the resonant tank in simulation, a large negative resistance
is presented at the drain output. An
pF and ) connecting the
output matching network (
drain port of the CC-HEMT and a 50- load was then designed
according to the oscillation conditions as follows:
(1)
Real
(2)
is the real part of the negative resistance
where Real
at the drain output of the HEMT and
is the
and
are the real and imaginary
imaginary part.
parts of the impedance looking into the load network with the
50- load included. is the resonant frequency.
As mentioned in Section III, the impedance
looking
at 9.4 GHz,
into the gate of the HEMT is
. When
yielding a small effective capacitance of 0.108 pF
is combined with
and
in series, the overall capacitance
of the resonant loop at the transistor’s input
, which indicates that the tuning range of
is dominated by
will be much less than that of
. However, the reduced capacitance improves the factor of the resonant circuit
significantly, leading to low phase noise in the -band VCO,
as shown in Section IV. The microphotograph of the fabricated
-band VCO is shown in Fig. 7. The feedback stub is designed
with the meander layout to minimize the chip size.
IV. MEASUREMENT RESULTS
The VCO was characterized with Agilent Technologies’
E4440A PSA spectrum analyzer. The HEMT’s gate and drain
bias were set to 3 and 5 V, respectively, which was experimentally found to be optimal for both phase noise and the
tuning frequency range. A typical output power spectrum is
of 5 V. The output
shown in Fig. 8 at a tuning voltage
power at the resonance is 3.3 dBm.
In general, the VCO’s phase noise is dominated by two
major factors: the thermal noise of the passive components
noise of the active
(e.g., inductors and capacitors) and the
transistor. The thermal noise contribution from the passive
elements is minimized by reducing the loss of the passive elements through implementation of thick metal and the insertion
of a low- (polyimide) dielectric between the metal traces and
Fig. 9. Oscillation frequency and output power versus the varactor control
voltage.
substrate. As for the
noise of the AlGaN/GaN HEMTs,
it can affect the phase noise of RF/microwave VCOs after
frequency up-conversion. As a result, high-linearity HEMTs
are preferred for VCO application because they provide a lower
noise up-conversion factor. It has also been reported that the
AlGaN/GaN HEMTs have a corner frequency in the range
of 70–100 kHz [17]. The Hooge parameter of AlGaN/GaN
HEMTs have been reported to be in the range of 10 [18],
comparable to or slightly lower than that in conventional AlGaAs/GaAs HEMTs. The phase noise realized in this study
is approximately 10 dBc/Hz higher than that achieved in an
-band VCO using a 0.6- m GaAs MESFET [19]. It can be
expected that further improvement in the material quality for
suppressing the surface states can lead to competitive noise
performance in AlGaN/GaN HEMT VCOs.
The output power at the fundamental resonance and the
oscillation frequency at a different tuning voltage are plotted in
was measured to be 222 MHz/V.
Fig. 9. The tuning gain
Compared to the previously reported AlGaN/GaN HEMT
VCOs [11], the VCO reported here features a smaller active
HEMT, a lower supply voltage of 5 V, and eventually lower
output power. It should be pointed out that while high-power
AlGaN/GaN HEMT VCOs takes advantage of the power-handling capability of AlGaN/GaN HEMTs, low-power VCOs
also have their own suitable applications. For example, for a
single-chip AlGaN/GaN HEMT transceiver, the low-power
VCOs can be used in the receiver front-end. In addition, compared to high-power VCOs, low-power VCOs integrated with
amplifier buffers can provide lower frequency pulling.
CHENG et al.: LOW PHASE-NOISE
-BAND MMIC VCO
27
The sensitivity of the oscillation frequency upon the load
impedance variation is characterized by a pulling figure measurement. The change of oscillation frequency is measured as a
function of the phase of the load reflection coefficient for a constant return loss of 12 dB, corresponding to a constant magnitude of 0.25 for the load reflection coefficient. The measurement
was carried out using Maury Microwave’s 982B01 load–pull
system, and the results are plotted in Fig. 12. The maximum
frequency shift from the frequency in the “matched load” situation is 5 MHz.
Fig. 10. Oscillation frequency and the phase noise (at 100-kHz and 1-MHz
offset, respectively) versus varactor control voltage.
V. CONCLUSION
A MMIC -band VCO using Al Ga N Al Ga
N/GaN composite-channel HEMTs was designed, fabricated,
and characterized. The MMIC features monolithically integrated passive components including planar inductors, MSM
varactors, and MIM capacitors. The VCO exhibits phase noise
of 82- and 110-dBc/Hz at offsets of 100 kHz and 1 MHz. In
addition, the VCO also exhibits a maximum second harmonic
suppression of 47 dBc. The low phase noise and high harmonic
suppression of the CC-HEMT-based VCOs are attributed to the
inherent high linearity and low noise of the CC-HEMT.
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Fig. 11. Measured output spectrum of fundament and second harmonic at
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0
Fig. 12. Pulling-figure measurement of the AlGaN/GaN CC-HEMT VCO.
V = 5 V, V = 3 V, V
= 5 V. The return loss of the load is set
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0
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The phase noise at 100-kHz and 1-MHz offset is characterized within the entire tuning range, as shown in Fig. 10. It is
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second harmonic suppression reported in VCOs based on GaN
HEMTs. The high harmonic suppression and the low phase
noise of the VCOs reported here can be attributed to the high
linearity and low noise performances of the CC-HEMTs.
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Q
X
Zhiqun Q. Cheng received the B.S. and M.S. degrees in microelectronics from Hefei University of
Technology, Hefei, China, in 1986 and 1995, respectively, and the Ph.D. degree in microelectronics and
solid-state electronics from the Shanghai Institute of
Microsystem and Information Technology, Chinese
Academy of Sciences, Shanghai, China, in 2000.
He is currently a Professor with the Department
of Information Engineering, Hangzhou Dianzi
University, Hangzhou, China, and a Senior Visiting
Scholar with the Department of Electronic and
Computer Engineering, Hong Kong University of Science and Technology
(HKUST), Hong Kong. Prior to joining HKUST, he was a Lecturer with the
Hefei University of Technology, Hefei, China (1986–1997) and an Associate
Professor with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China (2000–2005). He has
carried out research on GaAs HBTs and GaN HEMTs and has been involved
with the design of compound semiconductor digital integrated circuits and RF
transceivers. He has authored or coauthored 50 technical papers in journals and
conference proceedings. His current interests focus on III–V high-power and
low-noise devices and circuits for microwave and millimeter applications and
high-speed Si and SiGe devices and T/R systems for wireless communications.
Yong Cai was born in Nanjing, Jiangsu Province,
China, in 1971. He received the B.S. degree in
electronics engineering from Southeast University,
Nanjing, China, in 1993, and the Ph.D. degree from
the Institute of Microelectronics, Peking University,
Beijing, China, in 2003.
From 2003 and 2006, he was a Post-Doctoral Research Associate with the Department of Electronic
and Computer Engineering, Hong Kong University
of Science and Technology, where he was involved
with wide-bandgap GaN-based devices and circuits.
In August 2006, he joined the Material and Packing Technologies Group, Hong
Kong Applied Science and Technology Research Institute (ASTRI) Company
Ltd., Hong Kong, where he is a Senior Engineer.
Jie Liu received the B.S. and M.S. degrees in
physics from Nanjing University, Nanjing, China, in
2000 and 2003, respectively, and the Ph.D. degree
in electrical and computer engineering from the
Hong Kong University of Science and Technology
(HKUST), Hong Kong, in 2006.. His master’s thesis
concerns the Schottky contacts of III-nitride devices.
In August 2003, he joined the Department of
Electronic and Computer Engineering, HKUST,
where he is involved with device technologies
and high-frequency characterization techniques of
III-nitride HEMTs. In particular, he has focused on the channel engineering of
III-nitride HEMTs and developed highly linear composite-channel HEMT and
low-leakage current AlGaN/GaN/InGaN/GaN double-heterojunction HEMT.
In October 2006, he joined the Hong Kong Applied Science and Technology
Research Institute (ASTRI) Company Ltd., Hong Kong, where he is involved
with advanced wireless packaging technologies.
Yugang Zhou was born in Hubei Province, China,
in 1975. He received the B.S. and Ph.D. degrees in
physics from Nanjing University, Nanjing, China, in
1996 and 2001, respectively.
From September 2001 to September 2004, he was
a Post-Doctoral Research Associate with the Department of Electronic and Computer Engineering,
Hong Kong University of Science and Technology.
In September 2004, he joined Advanced Packaging
Technology Ltd., Hong Kong. He was mainly
involved with MOCVD growth, device fabrication,
and device physics of GaN-based heterostructure field-effect transistors
(HFETs) prior to September 2004. Since that time, he has been focused on the
fabrication of GaN-based high-power LEDs.
Kei May Lau (S’78–M’80–SM’92–F’01) received
the B.S. and M.S. degrees in physics from the
University of Minnesota at Minneapolis–St. Paul, in
1976 and 1977, respectively, and the Ph.D. degree
in electrical engineering from Rice University,
Houston, TX, in 1981.
From 1980 to 1982, she was a Senior Engineer
with M/A-COM Gallium Arsenide Products Inc.,
where she was involved with epitaxial growth
of GaAs for microwave devices, development
of high-efficiency and millimeter-wave IMPATT
diodes, and multiwafer epitaxy by the chloride transport process. In Fall 1982,
she joined the faculty of the Electrical and Computer Engineering Department,
University of Massachusetts at Amherst, where, in 1993, she became a Full
Professor. She initiated metalorganic chemical vapor deposition (MOCVD)
and compound semiconductor materials and devices programs at the University
of Massachusetts at Amherst. Her research group has performed studies on
heterostructures, quantum wells, strained-layers, and III–V selective epitaxy,
as well as high-frequency and photonic devices. In 1989, she spent her first
sabbatical leave with the Massachusetts Institute of Technology (MIT) Lincoln
Laboratory. From 1995 to 1006, she developed acoustic sensors with the
DuPont Central Research and Development Laboratory, Wilmington, DE,
during her second sabbatical leave. In Fall 1998, she was a Visiting Professor
with the Hong Kong University of Science and Technology (HKUST), Hong
Kong, where, in Summer 2000, she joined the regular faculty. She established
the Photonics Technology Center for research and development efforts in
wide-gap semiconductor materials and devices. In July 2005, she became a
Chair Professor of electronic and computer engineering with HKUST.
Prof. Lau served on the IEEE Electron Devices Society Administrative Committee and was an editor for the IEEE TRANSACTIONS ON ELECTRON DEVICES
(1996–2002). She also served on the Electronic Materials Committee of the
Minerals, Metals and Materials Society (TMS) of AIME (American Institute
of Materials Engineers). She was a recipient of the National Science Foundation (NSF) Faculty Award for Women (FAW) Scientists and Engineers in the
U.S.
CHENG et al.: LOW PHASE-NOISE
-BAND MMIC VCO
Kevin J. Chen (M’96–SM’06) received the B.S. degree in electronics from Peking University, Beijing,
China, in 1988, and the Ph.D. degree from the University of Maryland at College Park, in 1993.
From January 1994 to December 1995, he was a
Research Fellow with NTT LSI Laboratories, Atsugi,
Japan, where he was engaged in the research and
development of functional quantum effect devices
and heterojunction field-effect transistors (HFETs).
In particular, he developed device technologies for
monolithic integration of resonant tunneling diodes
and HFETs on both GaAs and InP substrates for applications in ultrahigh-speed
signal processing and communication systems. He also developed the Pt-based
buried gate technology that is widely used in enhancement-mode HEMT and
pseudomorphic high electron-mobility transistor (pHEMT) devices. From
1996 to 1998, he was an Assistant Professor with the Department of Electronic
Engineering, City University of Hong Kong, where he carried out research
29
on high-speed device and circuit simulations. In 1999, he joined the Wireless
Semiconductor Division, Agilent Technologies, Santa Clara, CA, where he
was involved with enhancement-mode pHEMT RF power amplifiers used in
dual-band global system for mobile communication (GSM)/digital communication system (DCS) wireless handsets. His work with Agilent Technologies
has covered RF characterization and modeling of microwave transistors, RF
integrated circuits (ICs), and package design. In November 2000, he joined the
Department of Electronic and Computer Engineering, Hong Kong University
of Science and Technology (HKUST), Hong Kong, as an Assistant Professor
and, in 2006, became an Associate Professor. He has authored or coauthored
over 140 publications in international journals and conference proceedings.
With HKUST, his group has carried out research on novel III-nitride device
technologies, III-nitride and silicon-based microelectromechanical systems
(MEMS), silicon-based RF/microwave passive components, RF packing
technology, and microwave filter design.