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INV ITEDP A P E R
GaN-Based RF Power Devicesand AmplifiersGallium nitride power transistors can operate at millimeter wave and beyond
to meet future needs of cell phones, satellites, and TV broadcasting.
By Umesh K. Mishra, Fellow IEEE, Likun Shen, Thomas E. Kazior, and Yi-Feng Wu
ABSTRACT | The rapid development of the RF power
electronics requires the introduction of wide bandgap mate-
rial due to its potential in high output power density, high
operation voltage and high input impedance. GaN-based RF
power devices have made substantial progresses in the last
decade. This paper attempts to review the latest develop-
ments of the GaN HEMT technologies, including material
growth, processing technologies, device epitaxial structures
and MMIC designs, to achieve the state-of-the-art microwave
and millimeter-wave performance. The reliability and manu-
facturing challenges are also discussed.
KEYWORDS | Gallium nitride; High Electron Mobility Transistors
(HEMTs); microwave transistors; millimeter wave transistors;
MMICs; reliability
I . INTRODUCTION
With the recent upsurge of the wireless communication
market, as well as the steady but continuous progress of
traditional military applications, microwave transistors are
playing critical roles in many aspects of human activities.
The requirements for the performance of microwavetransistors are becoming more and more demanding. In
the personal mobile communication applications, next
generation cell phones require wider bandwidth and
improved efficiency. The development of satellite com-
munications and TV broadcasting requires amplifiers
operating at higher frequencies (from C band to Ku
band, further to Ka band) and higher power to reduce the
antenna size of terminal users. The same requirement
holds for broadband wireless internet connections as well
because of the ever increasing speed or data transmission
rate. Because of these needs, there has been significant
investment in the development of high performance
microwave transistors and amplifiers based on Si/SiGe,GaAs, SiC and GaN. Table 1 lists the major parameters of
these materials and the Johnson’s figure of merit (JM)
calculated to compare the power-frequency limits of
different materials [1]. The JM gives the power-frequency
limit based solely on material properties and can be used to
compare different materials for high frequency and high
power applications.
The requirement for high power and high frequencyrequires transistors based on semiconductor materials with
both large breakdown voltage and high electron velocity.
From this point of view, wide bandgap materials, like GaN
and SiC, with higher JM are preferable. The wide bandgap
results in higher breakdown voltages because the ultimate
breakdown field is the field required for band-to-band
impact ionization. Moreover, both have high electron
saturation velocities, which allow high frequency opera-tion. The ability of GaN to form heterojunctions makes it
superior compared to SiC, in spite of having similar
breakdown fields and saturation electron velocities. GaN
can be used to fabricate high electron mobility transistors
(HEMTs) whereas SiC can only be used to fabricate metal
semiconductor field effect transistors (MESFETs). The
advantages of the HEMT include its high carrier concen-
tration and its higher electron mobility due to reducedionized impurity scattering. The combination of high
carrier concentration and high electron mobility results in
a high current density and a low channel resistance, which
are especially important for high frequency operation and
power switching applications.
From the amplifier point of view, GaN-based
HEMTs have many advantages over existing production
Manuscript received February 5, 2007; revised August 22, 2007.
U. K. Mishra and L. Shen are with the Department of Electrical and Computer
Engineering, University of California, Santa Barbara, CA 93106 USA
(e-mail: mishra@ece.ucsb.edu; lkshen@ece.ucsb.edu).
T. E. Kazior is with the Raytheon RF Components, Andover, MA 01810 USA
(e-mail: Thomas_E_Kazior@raytheon.com).
Y.-F. Wu is with the Santa Barbara Technology Center, CREE Inc., Goleta,
CA 93117 USA (e-mail: yifeng_wu@cree.com).
Digital Object Identifier: 10.1109/JPROC.2007.911060
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 2870018-9219/$25.00 �2007 IEEE
technologies (e.g. GaAs) [2]. The high output power
density allows the fabrication of much smaller size devices
with the same output power. Higher impedance due to the
smaller size allows for easier and lower loss matching inamplifiers. The operation at high voltage due to its high
breakdown electric field not only reduces the need for
voltage conversion, but also provides the potential to obtain
high efficiency, which is a critical parameter for amplifiers.
The wide bandgap also enables it to operate at high
temperatures. At the same time, the HEMT offers better
noise performance than that of MESFET’s.
These attractive features in amplifier applicationsenabled by the superior semiconductor properties make
the GaN-based HEMT a very promising candidate for
microwave power applications.
In this article we discuss the key components of GaN
HEMT technology. In Section II we review growth of high
purity device layers by metal organic chemical vapor
deposition (MOCVD) and molecular beam epitaxy (MBE).
In Section III we present device engineering andprocessing technologies that are being developed to realize
state-of-the-art GaN HEMT performance. The reliability
and manufacturing challenges are also discussed. In
Section IV, we highlight some of the GaN HEMT hybrid
amplifiers and monolithic microwave integrated circuit
(MMIC) that have recently been achieved.
II . GaN EPITAXIAL LAYER GROWTH
Numerous teams have been developing the MOCVD and
MBE techniques for growth of group-III nitride materials
such as GaN, AlN, AlGaN, and InGaN [3]–[8]. In the
MOCVD process, Ga, Al, and In are supplied using
corresponding metal organic compounds, usually tri-
methylgallium, trimethylaluminum and timethylindium.
The metal-organic compounds are then transported by acarrier gas, most often hydrogen. Thereby the concentra-
tion of the compound in the carrier gas is determined by its
vapor pressure. The most commonly used nitrogen source
is ammonia. In the RF-MBE technique reactive nitrogen
atoms and molecules are produced by passing a nitrogen
flow (N2 gas) through a plasma discharge. A variant of this
process uses ammonia ðNH3Þ as the nitrogen source gas
[8]. The column III growth fluxes are provided by
evaporation of high purity elemental sources. The growth
efforts of both techniques have been focused on developing
high power microwave and millimeter-wave AlGaN/GaNHEMT structures. SiC has been extensively employed as
substrates due to its excellent thermal conductivity [9],
while sapphire and Si are also used because of the low cost
[10], [11]. Device isolation from the SiC and Si substrate is
provided by a resistive AlN nucleation layer, in which the
growth conditions are adjusted to prevent silicon out
diffusion [12].
Excellent material quality has been achieved for GaNHEMT films. The impurity concentrations in semi-
insulating GaN films are below the detection limit when
characterized by SIMS. AlGaN/GaN, AlN/GaN [13], GaN/
AlN/GaN [14] and AlGaN/AlN/GaN [15] heterostructures
with smooth and abrupt interfaces have been demonstrat-
ed, leading to the formation of 2DEGs with electron
mobilities as high as 2000 cm2=Vs at room temperature
[16]. Non-uniformites of G 2% on 4-inch diameter SiCsubstrates are routinely achieved (for example, see
Fig. 1(a)Va sheet resistivity map of a GaN DHFET
(Double Heterostructure Field-Effect Transistors) [17].
Mercury probe capacitance-voltage (C-V) measure-
ments of AlGaN/ GaN HEMT structures grown on semi-
insulating SiC substrates reveal high quality material. The
C-V profile exhibits a sharp pinch-off and extremely low,
flat capacitance at high reverse bias (equal to thecapacitance of the SiC substrate) indicative of negligible
GaN buffer and epi/SiC interface charge/doping [as shown
in Fig. 1(b)] [18].
Both MOCVD and MBE techniques are capable of
growing thin layers. The use of a thin, �10A, AlN
interlayer between the AlGaN barrier and GaN channel
has been demonstrated to reduce sheet resistance by
increasing the mobility and sheet density of the HEMTstructure [15]. The increase in mobility is attributed to the
reduction in alloy scattering and the increase in sheet
charge due to the larger conduction band discontinuity at
the AlGaN/GaN interface. Fig. 2 is an x-ray spectrum of a
250 A Al0:26Ga0:74N=10 A AlN/GaN HEMT grown on a
SiC substrate. The presence of the thin, AlN layer
enhances the strength of the Pendellosung oscillations.
Table 1 Material Properties Related to the Power Performance at High Frequencies for Various Materials
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
288 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
(The Pendellosung oscillations are a measure of thequality (flatness and abruptness) of the hetero-interface.)
The AlN interlayer lowered the sheet resistance from 400
to 285 ohm/sq. and the mobility was increased to greater
than 2000 cm2=Vs.
Using the MOCVD and MBE techniques, growers have
demonstrated more complex device structures similar to
GaAs pHEMTs, such as quantum well or double hetero-
junction (DH) FETs. Some of these devices operate up toW-band frequencies. The quantum well or DH structures
provide improved electron confinement to mitigate short
channel effects associated with smaller gate lengths as well
as better substrate isolation resulting in higher gaindevices and improved device efficiency. AlGaN buffer
layers [19] and InGaN backside barrier layers [20]–[22]
have been used to create conduction band discontinuities
(double quantum wells similar to GaAs pHEMTs and InP
HEMTs) that inhibit the injection of electrons into the
buffer layer. Improved channel confinement/buffer iso-
lation and reduced buffer leakage current by Fe, Be, or C
doping of the GaN buffer layer (similar to fully depletedburied p-layers commonly used in GaAs MESFETs and Si
nMOS devices) has also been demonstrated [23]–[26].
Finally, highly doped cap layers are being added to the
epi structure to reduce device access (source) resistance,
which results in increased device gain and efficiency
[19], [27].
III . ADVANCED DEVICE DESIGNS ANDPROCESSING TECHNOLOGIES
While several electronic devices have been investigated
(for example, HBTs [28], MESFETs [29], MISFETs [30],
HEMTs [31]), most of the research work has been focused
on HEMTs [including MOSHEMT [32] (Metal-oxide-
semiconductor HEMT)], because HEMTs have better
carrier transport properties than MESFETs and thedifficulty of p-doping in GaN impedes the development
of bipolar transistors. A typical AlGaN/GaN HEMT is
shown in Fig. 3.
The polarization doping effect in GaN HEMTs was
predicted by Bykhovski et al. [33]. The first observation of
a Two-Dimensional Electron Gas (2DEG) with a carrier
concentration of the order of 1011 cm�2 and a room
temperature mobility of 400–800 cm2=Vs in an AlGaN/GaN heterojunction was reported in 1992 [31]. The first
Fig. 1. (a) Sheet resistance map and (b) capacitance-voltage plot for GaN HEMT grown on a 4-inch SiC substrate.
Fig. 2. (0002) x-ray spectrum of a AlGaN=10 A AlN/GaN HEMT
on a SiC substrate.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 289
DC performance of AlGaN/GaN HEMT was shown in 1993
with the saturation drain current of 40 mA/mm [34]. First
RF power data of 1.1 W/mm at 2 GHz for an AlGaN/GaN
HEMT was demonstrated in 1996 [35]. In the early stage of
the development of the GaN devices, many AlGaN/GaN
HEMTs suffered a discrepancy between the predicted
output power from static I-V curves and load pull
measurements of output power, referred to as BDC-to-RFdispersion.[ As seen in Fig. 4, current collapse occurs in
the pulsed I-V measurement. It is believed to be a trap-
related phenomenon where both surface and bulk traps
contribute [36], [37]. The existence of the dispersion has
severely limited the microwave output power of GaN
HEMTs, until two innovations were proposed to overcome
this problem. One was the introduction of the SixN
passivation in 2000 [38], [39], which effectively reducedDC-to-RF dispersion caused by surface trap states, thereby
resulting in a significant increase in output power to 9 and
11 W/mm [40], [41]. Another was the adoption of the field
plate in 2003 [10], [42]. In addition to the traditional
function of the field plate to increase the breakdown
voltage, it also reduced the dispersion beyond what SixNpassivation offered. Since then, the output power density
has further increased with the help of steadily improved
growth techniques, material qualities, enhanced proces-
sing technologies and more optimum device designs.
The latest record for power density is over 40 W/mm at
4 GHz [43].
The trend of the GaN-based device is towards higher
output power density, higher Power-Added-Efficiency(PAE), higher operation frequencies and improved reli-
ability. In order to achieve these requirements, novel
device designs and processing technologies are being
developed. Recently, much progress has been made and
will be discussed below. The first subsections focus on
improvements to the performance of microwave transis-
tors. The last subsection addresses the unique challenges
of optimizing the device for millimeter wave applications.
A. Field-Plated GaN HEMTsImplementing a field plate on a dielectric layer at the
drain side of the GaN HEMTs has resulted in some of the
most significant and exciting improvements [10], [42],
[43]. The performance and tradeoffs of the field plate (FP)
configurations have been investigated in an attempt to
extract the best gain and power characteristics.
Gate Connected FP (GC-FP): Fig. 5(a) shows the cross
section of a gate-connected field-plated GaN HEMT. The
function of a FP is to modify the electric field profile and to
decrease its peak value, hence reducing trapping effect and
increasing breakdown voltages. Initial FPs were either
constructed as part of the gate or tied to the gate
externally. This has been effective in improving largesignal (or power) performance and enabling high voltage
operation as seen in Fig. 6(a) and (b) [44]. Up to a certain
value, the longer the FP, the more output power was
achieved.
However, in this configuration the capacitance be-
tween the FP and drain becomes gate-to-drain capacitance
ðCgdÞ, resulting in negative Miller feedback. This causes
reduction in current-gain and power-gain cutoff frequen-cies ðft=fmaxÞ as seen in Fig. 7.
Source-Connected FP (SC-FP): A close look into the
device operation reveals that, since the voltage swing
across the gate and source is only 4–8 V for a typical GaN
HEMT, much less than the dynamic output swing up to
230 V, terminating the FP to the source [shown in
Fig. 5(b)] also satisfies the electrostatics for it to befunctional. In this configuration, the FP-to-channel
capacitance becomes the drain-source capacitance, which
could be absorbed in the output-tuning network. The
drawback of additional Cgd by the FP is hence is
eliminated. Depending on the implementation, the
source-connected field plate can add parasitic capacitance
to the device input. However, this can also be absorbed
Fig. 3. A schematic of a typical AlGaN/GaN HEMT.
Fig. 4.DCandpulsed I-VcharacteristicsofanunpassivatedAlGaN/GaN
HEMT on SiC substrate. Obvious current collapse (dispersion) could be
observed in the pulsed mode.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
290 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
into the input tuning circuit, at least for narrow band
applications.
SC-FP, GC-FP and non-FP Devices were fabricated on
the same wafer for a direct evaluation. Compared to the
non-FP device, the reveres power transfer ðS12Þ of the
device with GC-FP increased by 71% at 4 GHz, while thatof the device with SC-FP actually reduced by 28%. The
reduction in S12 for the latter is attributed to the Faraday
shielding effect by the grounded field plate. As a result,
at 10 V drain bias and 4 GHz the SC-FP device exhibited
a maximum-stable-gain (MSG) 1.3-dB higher than the
non-FP device and 5.2 dB higher than the GC-FP device.
As a result, the SC-FP devices shows a significant (9 5 dB
at 4 GHz) improvement in maximum stable gain, This
advantage for SC-FP devices was maintained for biases
from 10 though 60 V as seen in Fig. 8(a). Fig. 8(b) lists
the change of the capacitance components in GC- and
SC-FP devices, respectively.
Large-signal performance was characterized by load-
pull power measurement at 4 GHz. Both the GC-FP andthe SC-PF devices outperformed the non-FP devices in
both output power and PAE at 48 V and above, while
the SC-FP device consistently delivered large-signal gains
5–7 dB higher than that of the GC-FP device.
As successful high-voltage designs, both FP devices
were able to operate at 118 V dc bias as shown in Fig. 9,
where tuning was optimized for the best combination of
gain, power-added-efficiency (PAE) and output power at
Fig. 5. Cross section of a GaN HEMT with (a) gate-connected field plate; (b) source-connected field plate.
Fig. 6. (a) Power density vs. drain voltage for various FP lengths. Device dimension: 0:5� 246 �m2. (b) Power performance of a
GaN HEMTs with gate-connected field plates, showing 32.2 W/mm output power at 120 V drain bias.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 291
3-dB compression ðP3 dBÞ. While both devices generate
power densities around 20 W/mm, the SC-FP device
distinguishes itself by 7-dB higher associated gain. With
the achieved large-signal gain of 21 dB at 4 GHz and theestimated voltage swing of 224 V, the voltage-frequency-
gain product (Johnson’s voltage-frequency figure of merit
[1]) for the SC-FP is approaching 10 kV-GHz, the highest
ever shown for any semiconductor device.
The above studies were for operation at C-band and
below. For applications at X-band and above, dimensions
for the field plates need to be reduced accordingly to
manage the parasitic capacitances.
B. Deep-Recessed GaN HEMTsSiNx passivation has been used to reduce the disper-
sion, but reproducibility of breakdown voltage, gate
leakage, and effectiveness of dispersion elimination is
strongly process related. Recently, solutions to thedispersion problem had been addressed at the epitaxial
level [45], [46]. One of these approaches, which has made
substantial progress, is the deep-recessed GaN HEMT
using a thick cap layer to eliminate dispersion [47], as
shown in Fig. 10.
The effect of the surface to the channel is inversely
proportional to the distance between surface and channel.
The thick AlGaN or GaN cap layers in the deep-recessedHEMTs increase the surface-to-channel distance, the
dispersion caused by surface traps is therefore reduced
or eliminated without surface passivation because now
only a smaller portion of the channel charge is affected
compared to the conventional AlGaN/GaN HEMTs. The
graded AlGaN layer is Si-doped to compensate the negative
polarization charge and prevent hole accumulation.
The processing flow was similar to that of the standardHEMT except for the deep ohmic and gate recess. A
fluorine plasma treatment of the recessed surface before
gate metallization was found to be very effective to reduce
the gate leakage (up to two orders of magnitude) and
increase breakdown voltage (9 200 V) [48]. A record
output power density Pout of more than 17 W/mm with an
associated power added efficiency (PAE) of 50% was
measured at VDS ¼ 80 V at 4 GHz (without SiNx
passivation as shown in Fig. 11). This is believed to be
the highest power generated from a GaN transistor with-
out surface passivation to date. At lower bias of 30 V,
an excellent PAE of 74% with output power density of
5.5 W/mm was achieved.
In order to control the recess depth accurately and
improve the manufacturability, a selective dry etch tech-
nology of GaN over AlGaN using BCl3=SF6 has beendeveloped [49]. The presence of fluorine decreases the
etch rate of AlGaN due to the formation of a non-volatile
Fig. 7. ft=fmax as functions of FP length Lf .
Fig. 8. (a) MSG as a function of drain voltage; (b) change of the capacitance components in GC- and SC-FP devices.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
292 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
AlF3 residue on the AlGaN surface. The compatible deep-
recessed structure has a GaN cap (9 200 nm) and an
abrupt GaN/AlGaN interface to clearly define the etch-
stop position, seen in Fig. 12. Selectivity of around 25 ofGaN over Al0:22Ga0:78N was achieved. The selectivity
increased with Al composition in AlGaN, up to about 50–
100 between GaN and AlN. The devices processed with
selective etch technology demonstrated significantly
reduced processing variations as well as excellent
microwave power performance. At 10 GHz, a high PAE
of 63% with an output power density of 5 W/mm was
achieved at VD ¼ 28 V, while 10.5 W/mm with 53% PAEwas achieved at VD ¼ 48 V, shown in Fig. 13. The power
performance of these devices with gate length of 0.6 �m
is comparable to state-of-the-art conventional SiNx-
passivated AlGaN/GaN HEMTs at 10 GHz.
C. Metal-Oxide-Semiconductor HEMT (MOSHEMT)The MOSHEMT design combines the advantages of the
MOS structure, which suppresses the gate-leakage current,
and an AlGaN/GaN heterointerface that provides high-density high-mobility 2DEG channel [50]. The
MOSHEMT approach also allows for application of high
positive gate voltages to further increase the sheet electron
density in the 2-D channel and, therefore, the peak device
current. The MOSHEMT built-in channel is formed by the
high-density 2DEG at the AlGaN/GaN interface as in
regular AlGaN/GaN HEMTs. However, in contrast to a
regular HEMT, the gate metal is isolated from the AlGaNbarrier layer by a thin dielectric film such as SiO2, AlO,
ZrO, NbO, AlN, HfO2 and so on, as seen in Fig. 14. Thus,
the MOSHEMT gate behaves more like a MOS gate
structure rather than a Schottky barrier gate used in
regular HEMTs. Since the properly designed AlGaN
barrier layer is fully depleted by electron transfer to the
adjacent GaN layer, the gate insulator in the MOSHEMT
consists of two sequential layers: the SiO2 film and AlGaNepilayer. This double layer ensures an extremely low gate-
leakage current and allows for a large negative to positive
gate voltage swing.
The suppression of the gate-leakage current is one of
the most important features of the MOSHEMT. In
Fig. 15, the gate-leakage currents for the 1:5 �m �200 �m gate MOSHEMT at different temperatures is
shown. The data shows that the MOSHEMT leakagecurrent is as low as 1 nA/mm at 20-V gate bias at room
temperature and is approximately six orders of magnitude
smaller than for the regular HEMT with similar gate
Fig. 9. Power sweeps with a SC-FP device and a GC-FP device at 118 V drain bias and 4 GHz. Device dimension: 0:5� 500 �m2.
Fig. 10.Devicestructureofadeep-recessedGaNHEMTwithAlGaNcap.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 293
dimensions. Even at 300 �C, the gate-leakage current for
MOSHFET remains 3–4 orders of magnitude lower than
for regular HEMTs.The maximum DC saturation drain current at positive
gate voltages is a key parameter controlling maximum
output RF power. For conventional AlGaN/GaN HEMTs,
gate voltages in excess of 1.2 V result in excessive forward
current. In a MOSHEMT, the gate voltages as high as 10 V
could be applied. This results in significant increase in
maximum channel current. The gate leakage, however,
remains well below 1 nA/mm. Fig. 16 shows the transfercharacteristics for the 1.5 �m-gate-length MOSHEMT and
HEMT measured at the drain voltage sufficient to shift the
operating point into the saturation regime.
With the SixN surface passivation and field plate, the
MOSHEMT demonstrated an output power density of
18.6 W/mm with a PAE of 49.5% at drain bias of 55 V at
2 GHz, seen in Fig. 17. Moreover, there was no degra-
dation after the RF-stress at such a high output powerdensity for 100 hours [51]. The application of the
MOSHEMT to higher frequencies (e.g. 26 GHz) has also
been demonstrated [52]. The gate leakage was much
lower and the maximum output power was 3 dB higher
than a HEMT fabricated by the same group. A more
careful scaling of the gate length and gate oxide
thickness, or adoption of high-K dielectrics, could extend
the MOSHEMT into the millimeter-wave frequencies.
D. Process and Device Technology for GaN HEMTsfor mm-Wave Applications
New applications are demanding high output power and
efficiency at higher frequencies, especially Ka-band (26–
40 GHz) and beyond, with the aim to replace or complement
traveling wave tube amplifiers. Satellite and broad-band
wireless communications as well as advanced radars are only
a few of the many applications that would greatly benefitfrom the increased reliability, reduced size and noise of these
solid-state based amplifiers. In order to achieve the goal of
working at mm-Wave frequencies and beyond, new process
technologies and device structures have to be utilized.
The gate-to-source spacing of mm-Wave HEMT must
be minimized, to keep the source access resistance low.
Fig. 11. Power performance at 4 GHz without SiNx passivation.
Fig. 12. Device structure of a deep-recessed GaN HEMT with GaN cap,
which is compatible with selective etch technology.
Fig. 13. Power performance at 10 GHz without SixN passivation.
Fig. 14. Device structure of an AlGaN/GaN MOS-HEMT.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
294 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
However, the conventional alloyed ohmic contacts have
rough morphology and edges, which limits the reduction of
the gate-to-source spacing. Therefore, a non-alloyed ohmic
contact is preferred for the high frequency devices. Ion
implantation has been used in the GaN device fabricationto form non-alloyed ohmic contacts [53], [54]. In the past,
a high temperature ð1200 � 1500 �CÞ annealing process
was employed using protective surface layers during the
implant activation annealing including SiO2 [55], Si3N4
[56] and AlN [54], as well as high pressure (�100 bar N2).
However, the use of a high temperature, high pressure and
capped annealing processes limits the manufacturability of
this process for AlGaN/GaN HEMTs. Recently investiga-tors began applying this technique to selectively Si dope
the source and drain contact region of the GaN HEMT in
order to reduce the contact resistance and enable the
creation of non-alloyed ohmic contacts (see Fig. 18) [57].
The non-alloyed ohmic contacts formed on the implanted
region have much smoother surfaces than alloyed contacts,as shown in Fig. 19. The smooth edges of the ohmic
contacts allow the reduction of the gate-drain spacing, thus
further lowering the access resistance, which is important
to high frequency devices. The same investigators have also
demonstrated a capless implant activation anneal with a
reduced thermal budget and improved the manufactur-
ability [57].
Devices fabricated with the non-alloyed ohmic con-tacts exhibit performance comparable to control devices,
indicating that the implant and capless anneal process
do not degrade the HEMT material characteristics. Re-
cently, a non-alloyed ohmic contact resistance lower
than 0.3 �–mm was achieved with the optimization of
the ion implantation process including reduction of the
spacing between the implant and ohmic edge. The HEMT
showed an excellent PAE of 60% with an output powerdensity of 7.3 W/mm at 10 GHz when VD ¼ 35 V [58].
In the past few years, the power performance at Ka-
band has made steady progress. For instance, an output
power density of 2.8 W/mm was reported at 40 GHz in
2003 [59] and 5.7 W/mm at 30 GHz in 2004 [60].
Fig. 15. Gate-leakage currents for the MOSHEMT 1:5 �m� 200 �m
gate at different temperatures and the baseline HEMT at room
temperature measured in diode mode (drain disconnected).
Fig. 16.Maximumsaturation and gate-leakage currents in 1.5-�mgate
MOSHEMT and HEMT devices.
Fig. 17. Power sweep at 2 GHz for a 200-�m-wide device. Device
dimensions are Lsd ¼ 6 �m, Lg ¼ 1:1 �m, LFP ¼ 2:1 �m with a 1.1-�m
overlap with the gate.
Fig. 18. Schematicepitaxial structureof the implanted-S/DAlGaN/GaN
HEMT [57].
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 295
Recently, an output power of 10.5 W/mm with a PAE of
34% was demonstrated at 40 GHz at drain bias of 30 V, asshown in Fig. 20 [61]. The device had a gate length of
160 nm and showed a current gain cut-off frequency ðfTÞof 70 GHz and a maximum power gain cut-off frequency
ðfmaxÞ of 100 GHz. The very high output power is the
result of the combination of both very high current
densities (�1.4 A/mm at VGS ¼ þ2 V) and breakdown
voltages (9 80 V) with negligible knee walk-out and
current collapse.Higher fT and fmax are required for operation beyond
Ka-band and are attracting much research efforts [62]. The
traditional methods, for instance, shorter gate length,
multiple fingers to reduce gate resistance and �-shaped
gate to decrease gate-to-drain capacitance, are still
effective to further boost the device performance. A fT of
180 GHz has been achieved with 30-nm-gate, thin AlGaN
barrier layer and CAT-CVD-deposited SiN thin layer [63].In order to improve the confinement of the electrons to
reduce the output conductance and improve fmax, the
concept of the back-barrier has attracted some research
recently. The DHFET utilizing a low Al content
Al0:04Ga0:96N buffer achieved three orders of magnitude
lower sub-threshold drain leakage and demonstrated 30%
improvement in output density and 10% improvement in
PAE [64]. Another InGaN back-barrier design used the
unique strong polarization property of GaN to improve thechannel charge confinement [65]. The sample structure was
shown in Fig. 21. The ultra-thin InGaN layer is 1 nm thick
and has an In composition of 10%. As shown in Fig. 22(b),
the pinch-off characteristics of the sample with an InGaN
back-barrier are excellent for drain voltages as high as 50 V,
much better than the control sample without InGaN back-
barrier [Fig. 22(a)]. This led to an improvement of output
Fig. 19. (a) Rough surface morphology of the alloyed ohmic contact
of GaN HEMTs (b) smooth surface morphology of the nonalloyed
ohmic contact.
Fig. 20. Power sweep of a mm-wave MOCVD AlGaN/GaN HEMT
showing a maximum power of 10.5 W/mm and PAE of 33% at 40 GHz.
Thedrainvoltagewas30Vand thedrainbiascurrentwas500mA/mm.
Fig. 21. Schematic and band diagram of the InGaN back-barrier
sample.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
296 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
resistance. Standard HEMTs with a gate length of 200 nm
have an output resistance of 20 � 5 � � mm, while in the
sample with an InGaN back-barrier the output resistance is
35 � 5 � � mm. An average 18% increase in fmax wasmeasured as a result of the improved confinement. A record
fmax of 230 GHz and fT of 150 GHz were achieved from
unpassivated devices, shown in Fig. 23. More work is needed
to confirm the benefit of the InGaN back barrier devices
under large-signal operation.
E. Linearity of GaN HEMTsFor all high-data-rate communication applications,
device linearity is a key performance specification. Due
to their large operation space in the I-V plane as compared
to the lower band-gap semiconductors, GaN-based HEMTshave the potential to offer high linearity for the stringent
requirements. The linearity criteria for high power
transmitters are usually expressed in terms of associated
output power and efficiency at a certain distortion level.
Although there exist many modulation schemes, the basic
evaluation of a device technology can always be done with
a 2-tone inter-modulation measurement. Up to now the
best-reported 2-tone linearity-efficiency combination wasachieved with a field plate GaN HEMT [66].
These devices were similar to those with gate-
connected field plates in Section III-A. The gate dimen-
sions were 0:5 � 246 �m2. The lengths of the FP ðLFÞ,defined as the extension of the FP over the gate edge on the
drain side, were set at 0 (i.e. non-FP), 0.7 and 1.1 �m.
All devices with and without FP had similar dc
characteristics including 9 1 A/mm open-channel currentand a �4 V gate pinch-off voltage. Yet the FP devices
showed higher breakdown voltages of 9 140 V compared
to �100 V for the non-FP devices. When cutoff fre-
quencies were investigated against bias current, all devices
showed extremely sharp turn-on in power-gain cutoff
frequencies ðfmaxÞ as the device channel opened up as seen
in Fig. 24. Such gain characteristics are well suited for
class-B or deep class-AB operation for high efficiency.When biased at 48 V with a small quiescent current
of 20 mA/mm and driven by a 2-tone signal with 100 kHz
spacing at 4 GHz, a non-FP device generated 3.4 W/mm
with 56% PAE and 15.8 dB gain at IM3 of �30 dBc. The
FP devices exhibited improved linear power due to the
benefit of field shaping. However, as LF was increased
the gain was reduced, which negatively affected PAE. As a
compromise, LF ¼ 0:7 �m was found to be optimum at
Fig. 22. Change in gm and pinch off with VDS in a standard AlGaN/GaN
HEMT (a) and a HEMT with InGaN back-barrier (b).
Fig. 23. Small signal performance of an AlGaN/GaN HEMT with InGaN back-barrier with bias voltages optimized for maximum fTand maximum fmax.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 297
this bias voltage, achieving a linear power density of
3.7 W/mm with 57% PAE and 13.7 dB gain at IM3 of
�30 dBc as shown in Fig. 25.A more significant advantage of the FP devices is their
capability for higher voltage operation. At 78 V, 7 W/mm
linear power was obtained with 50% PAE and 15.2 dB gain
from a device with LF ¼ 0:7 �m. As shown in Fig. 26,
with further increased bias voltages, a longer LF was
needed. At 108 V, a device with LF ¼ 1:1 �m produced
10 W/mm linear power with 41% PAE and 14.3 dB gain at
IM3 of �30 dBc. The combination of high linear powerdensity and PAE is a dramatic improvement over pre-
viously reported performance, very promising for future
communication applications.As an example of commercial applications, CREE Inc. has
started offering GaN HEMT products for stringent WiMax
transmitter amplifiers. Instead of using three amplifiers to
manage a bandwidth of 3.3–3.9 GHz in Si technologies, a
single GaN HEMT amplifier not only covers the whole band
but does so with much higher drain efficiencies of 23–28% at
a specified distortion level of 2.5 EVM, as compared to
around 18% for Si devices (see Fig. 27).Another important advantage is the reduced memory
effect found with the GaN HEMTs, which is beneficial to
the modern digital pre-distortion transmitters [67].
Recently, using all advantageous features of the GaN
HEMTs, a remarkable average wall-plug efficiency of 50%
was achieved in a WCDMA base-station amplifier with an
average output power of 37.2 W and with a normalized
power RMS error of 0.7% and ACLR of �52 dBc at anoffset frequency of 5 MHz [68], which was at least twice
improvement from conventional technologies.
F. Reliability and Manufacturing ChallengesOne of the last remaining hurdles to the commercial-
ization of GaN technology has been the demonstration of
reliability consistent with system requirements. To address
reliability, research has focused on reducing or eliminatingdevice drift and leakage currents under the high field and
high power conditions associated with device operation.
These include:
1) improving material quality (reducing defect den-
sity) of both substrates and epitaxially grown
device layers
Fig. 24. Ft and Fmax vs. current for a GaN HEMT showing extremely
sharp turn-on, very suitable for class B or deep class AB operations.
Fig. 25. A device with LF of 0.7 mm achieves 57% PAE at�30 dBc IM3
with associated output power of 3.7 W/mm. Device dimensions:
0:5� 246 mm2. Vds ¼ 48 V, deep class-AB biaswith IQ ¼ 20 mA=mm.
Single-tone power at 3 dB compression was: P3 dB ¼ 8:8 W=mm
PAE ¼ 71%.
Fig. 26. A device with LF of 1.1 mmproduces 10W/mmat�30 dBc IM3
with associated PAE of 41%. Device dimensions: 0:5� 246 mm2.
Vds ¼ 108 V, deep class-AB bias with IQ ¼ 20 mA=mm. Single-tone
power at 3 dB compression was: P3 dB ¼ 24 W=mm PAE ¼ 48%.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
298 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
2) epi-engineering such as backside barrier layers to
reduce subthreshold leakage currents [64]
3) surface stabilization/passivation to eliminate de-vice drift due to changes in surface charge and
gate leakage currents [69], [70]
4) device/process engineering, such as gate recess
and field plate technology, to reduce peak electric
fields in the channel;
5) development of robust ohmic and gate contacts
Recent progress in the reliability of wide bandgap
devices have been presented at different workshops andconferences. (For example, see the proceeding of the 2005
and 2006 ROCS [Reliability of Compound Semiconductors]
Workshops.) Numerous laboratories [70]–[75] have re-
ported reliable device operation under industry standard
reliability testing. Three temperature DC Arrhenius accel-
erated life testing predicts lifetimes in excess of a million
hours at standard operating (channel) temperatures for GaN
HEMTs operating at 28 V (Fig. 28). Similar results have
been achieved for RF accelerated life testing and threetemperature RF Arrhenius accelerated life testing. Recent
RF operational life testing of X-band GaN MMICs (single
stage MMIC amplifier biased at 28 V and driven 3 dB into
compression) show stable operation (no change in output
power) in excess of 10 000 hours (Fig. 28). Extrapolation of
these results predict stable operation in excess of one million
hours under realistic operating conditions Similar results
have also been demonstrated for large periphery GaNdiscrete transistors at S-band (for base stations) [75]. These
reliability results have been obtained for devices fabricated
on epitaxial material grown by both MOCVD and MBE and
for SiC and alternate substrates such as silicon, highlighting
the dramatic improvement in material quality over the past
few years.
Fig. 27. Drain efficiency of a WiMax amplifier using a CGH35015S-DS GaN HEMT by CREE Inc.
Fig. 28. DC 3 temperature Arrhenius accelerated life test (left) and RF operational life test (right) show that GaN MMIC are stable under
realistic operating conditions.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 299
For use in MMICs, achievement of transistor reliabilityis necessary but not sufficient. Passive components must
also be reliable under high voltage and power conditions.
To this end, reliable high voltage, high power on chip MIM
capacitors have also been demonstrated, which have
MTTF larger than 108 hours and � of 4.0 MV/cm.
Thus wide bandgap semiconductors have matured to the
point where they can be seriously considered for insertion
into systems.Once performance and reliability have been demon-
strated, the last hurdle is producing wide bandgap
semiconductors RF devices and circuits in a manufacturing
environment at a cost that is affordable for the various
system insertion opportunities (i.e., if a technology is
10 times better than the incumbent technology but is
10 times more expensive, will anyone buy it?). Since
most of the processing steps for the manufacture of widebandgap semiconductors are similar to or compatible with
other compound semiconductors, most development has
focused on scaling the technology to take advantage of
existing semiconductor manufacturing infrastructure. The
most pressing need is for cost effective 100 mm or larger
diameter substrates. This need has driven the scaling of SiC
substrates (the most commonly used, although expensive,
substrate due to its excellent thermal properties) as well asthe use of alternate lower cost substrates, such as GaN on
Si. Epitaxial growth requires dedicated reactorsVmulti-
wafer 100 mm (or larger) GaN reactors, for example, are
becoming readily available. With the availability of 100 mm
diameter substrates and epitaxial growth, wideband gap
semiconductors can either be produced in existing
compound semiconductor wafer fabs (and thus take
advantage of compound semiconductor wafer loadings todrive down cost) or on dedicated lines using readily
available fabrication equipment. As a result many wide
bandgap pilot and production lines exist today (e.g, Cree,
Eudyna, Nitronix, RFMD, Raytheon, TriQuint, NGST) and
thus an infrastructure for the low cost manufacture of widebandgap devices and circuits is emerging.
IV. APPLICATIONS
As GaN technology is maturing and migrating from
university and industry research labs into foundries,
wide bandgap semiconductors are attracting interest in a
wide range of applications ranging from cell phone andwireless infrastructure (base stations) to high performance
military electronics. The wide bandgap semiconductors are
being used as discrete devices in hybrid assemblies and in
MMICs. While the majority of the applications are for
power amplification, the wide bandgap semiconductors,
particularly GaN, also provide significant advantages for
robust low noise receivers and switching power supplies.
For base station applications a number of manufac-turers have reported reliable, high power large periphery
discrete transistors [75]–[82]. An example (see Fig. 29) is
the Eudyna GaN hybrid power amplifier capable of
efficiently delivering 9 200 W of power at 2.1 GHz for
W-CDMA applications [78]. To provide margin for reliable
operation, these hybrid amplifiers are designed/optimized
for relatively lower power densities (3–4 W/mm) (i.e.,
backed off from the peak power densities).CREE Inc. has also demonstrated compact, high-power
microwave amplifiers taking advantage of the high-voltage
and high power density of GaN HEMTs [82]. The devices
used had 28.8-mm periphery with through via holes
employed under the source ohmic contacts for minimum
grounding inductance and elimination of air bridges. A
peak power of 550 W (57.40 dBm) is achieved at 3.45 GHz
with 66% DE and 12.5 dB associated gain. An outstandingpower-efficiency combination of 521 W and 72.4% is
obtained at 3.55 GHz. Such power levels, accompanied
by the high efficiencies, are believed to be the highest at
around 3.5 GHz for a fully-matched, single-package
Fig. 29. Picture and output power of a large periphery 2.1 GHz GaN HEMT hybrid assembly for W-CDMA base station applications (Eudyna).
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
300 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
solid-state power amplifier, attesting the great potentialof the GaN HEMT technology.
Numerous companies are developing GaN MMICs for
applications ranging from L- through W-band. The high
power density of GaN has several advantages in MMIC
design: 1) The higher power density results in lower
parasitic capacitance per watt of output power; and 2) the
higher operating voltage results in higher output imped-
ance. These two factors enable the design of simpler, lowerloss, and wider bandwidth matching networks enabling
higher power, higher efficiency, and wider bandwidth
amplifiers than possible with GaAs pHEMTs.
Two different MMIC topologies are being pursuedVmicrostrip and co-planar waveguide (CPW). Each approach
has its inherent advantages and both are capable of yielding
high performance MMIC HPAs. CPW based MMICs avoid
the additional fabrication steps associated with backsideprocessing (wafer thinning and via hole etching) and take
full advantage of thermal spreading in the high thermal
conductivity SiC substrates to maintain low device channel
temperatures and reliable operation. Examples of CPW GaN
MMIC HPAs are shown in Fig. 30. High power, high
efficiency, high gain, multistage CPW GaN MMICs have
been demonstrated from L thru Ka band in a fraction of the
footprint of GaAs pHEMT MMICs of comparable outputpower. CPW devices may also be the preferred approach for
the heterogeneous integration of GaN and Silicon transistors
since silicon technology typically relies on topside metalli-
zation schemes for interconnects.
With the demonstration of a production worthy SiC
through wafer via hole technology, microstrip GaN
MMICs are also being designed and fabricated. The GaN
microstrip MMIC design approach leverages the experi-ence and infrastructure (e.g., design and modeling
methodologies) of GaAs microstrip MMICs. The via
technology, in general, and individual source finger via
holes in particular, provides an added degree of freedom in
device and component grounding (as opposed to highdensity of air bridge grounding straps used in CPW
designs). However, these advantages come at the expense
of reduced thermal spreading in the thinner (50–100 �m)
SiC substrate and added constraints in device thermal
management (packaging) to maintain reliable operation.
Nevertheless, GaN MMICs with similar levels of perfor-
mance have been achieved with each circuit topology, and
it is up to the MMIC designer to determine whichapproach provides the best solution for a given application.
As discussed earlier, GaN HEMTs have also proven to be
very attractive and viable as a power source for millimeter
wave applications [84]–[88]. Similar to microwave frequen-
cies, microstrip and CPW MMICs have been demonstrated.
Fig. 31 shows the performance of a microstrip Ka band GaN
MMIC power amplifier capable of delivering 11 W of
output power [84]. Wu et al. announced an amplifier withan 1.5-mm-wide device produced 8.05 W output power at
30 GHz with 31% PAE and 4.1 dB associated gain. This is
believed to be the highest power generated from a GaN
transistor at millimeter wave frequencies to date. The
output power matches that of a GaAs-based MMIC with a
14.7-mm-wide output device but with a 10 times smaller
size. Recently, GaN MMIC performance has been
demonstrated at W-band as well (Fig. 32) [88]. The W-band MMIC is based on an MBE grown device structure
and relies on individual source via holes, similar to GaAs
pHEMTs, to achieve 2 W/mm at 80 GHz.
While there is a long history of microstrip and CPW
GaAs pHEMT, MHEMT and InP HEMT devices and
circuits operating at millimeter wave frequencies, these
devices can not support the power, linearity and efficiency
requirements of next generation systems such as radar,satellite communications and active self protect systems.
The demonstration of GaN devices and MMICs with high
power densities and usable gain will enable the proliferation
of solid state solutions at millimeter wave frequencies.
Fig. 30. Examples of 2 stage CPW GaN MMIC Pas [83]). Design of the MMIC on the right funded under the DARPA WBGS Phase 2 Program.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 301
V. CONCLUSION
The rapid development of the RF power electronics
requires the introduction of wide bandgap material (for
instance, GaN and SiC) due to its potential in high output
power density, high operation voltage and high input
impedance. GaN-based RF power devices have made
substantial progresses in many aspects, from material
growth, processing technology, device structure, to MMIC
design in the last decade. Output power density has
reached 30–40 W/mm, more than one order of magnitudehigher than GaAs. The extremely high power density does
demand stringent thermal management but could be
Fig. 32. Picture, and RF performance of a 3 stage W-band GaN MMIC (epi layers grown by MBE). The MMIC exhibited over 2 W/mm of
output power at 80 GHz [88].
Fig. 31. Picture and performance of a 31–36 GHz balanced GaN MMIC Power Amplifier. 11 W of output power at 34 GHz were achieved [84].
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
302 Proceedings of the IEEE | Vol. 96, No. 2, February 2008
mitigated by achieving a higher efficiency. At the sametime, fT and fmax of about 200 GHz have been achieved,
extending the application of GaN devices to millimeter
wave and beyond. With the reliability issues being sortedout, GaN-based devices will soon offer new solutions to
future electronic applications. h
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ABOUT THE AUTHORS
Umesh K. Mishra (Fellow, IEEE) received the
B.Tech. from the Indian Institute of Technology
(IIT) Kanpur, India, in 1979, the M.S. degree from
Lehigh University, Bethlehem, PA, in 1980, and
the Ph.D. degree from Cornell University, Ithaca,
NY, in 1984, all in electrical engineering. He has
been with various laboratory and academic
institutions, including Hughes Research Labora-
tories, Malibu, CA, The University of Michigan at
Ann Arbor, and General Electric, Syracuse, NY,
where he has made major contributions to the development of
AlInAsGaInAs HEMTs and HBTs. He is currently a Professor in the
Department of Electrical and Computer Engineering and the Associate
Dean of the School of Engineering, University of California at Santa
Barbara (UCSB). He has authored or coauthored over 450 papers in
technical journals and conferences. He holds nine patents. His current
research interests are in oxide-based IIIV electronics and IIIV nitride
electronics and opto-electronics. Dr. Mishra was a recipient of the
Presidential Young Investigator Award from the National Science
Foundation, the Hyland Patent Award presented by Hughes Aircraft,
the Young Scientist Award presented at the International Symposium
on GaAs and Related Compounds and the David Sarnoff Award from
the IEEE.
Likun Shen received the B.S. and M.S. degrees in
physics and electrical engineering from Fudan
University, Shanghai, China, in 1995 and 1998,
respectively, and the Ph.D. degree in electrical
engineering from the University of California,
Santa Barbara, U.S.A., in 2004. He is currently
an Assistant Project Scientist in the Department
of Electrical Engineering in the University of
California, Santa Barbara, U.S.A.
His research interests focus on design, fabri-
cation, characterization, and development of compound semiconductor
devices, especially GaN-based high frequency high power electronic
devices and optical devices. He is also interested in semiconductor device
physics, modeling and simulation. Dr. Shens contributions to the
development of GaN HEMTs included the novel passivation method,
novel epitaxial structure and processing technologies. Dr. Shen has
authored and coauthored more than 40 papers in technical journals and
international conferences as well as several patents in pending.
Thomas E. Kazior received a BSEE from Tufts
University and the Ph.D. degree from the Depart-
ment of Material Science and Engineering at the
Massachusetts Institute of Technology specializ-
ing in electronic materials. Upon completing his
studies at MIT, Dr. Kazior joined Raytheon and is
presently a Principal Engineering Fellow and
Technical Director of Advanced MMIC Technology
at Raytheon RF Components, Raytheons MMIC
foundry. His research focuses on the development
of next generation material, process, device, component and circuit
technologies for microwave and millimeter wave applications, including
GaAs based HEMTs, metamorphic device technology, GaN HEMTs,
and heterogeneous integration of compound semiconductors and
silicon. Dr. Kazior has authored or coauthored over 70 papers,
conference presentations, invited talks, and lectures on process and
device technology and holds numerous patents on process technology
innovations. Dr. Kazior is a 2001 recipient of Raytheons Excellence in
Technology Award for his leadership role in the development of
advanced compound semiconductor device technology as well as a
participant in the National Academy of Engineerings 2002 Frontiers in
Engineering Symposium.
Yi-Feng Wu obtained his B.E. degree in Engineer-
ing Thermal Physics in 1985 from Tsinghua
University, Beijing, China. He received his M.S.
degree in Mechanical Engineering and Ph.D.
degree in Electrical Engineering from University
of California at Santa Barbara in 1995 and 1997,
respectively. He joined Witech LLC in 1997, a
successful start-ups in Nitride Semiconductors
which was merged with CREE Inc. in year 2000.
Dr. Wu has been a forefront researcher in Wide-
gap Microwave Electronics since 1996. His achievements include the first
demonstration of microwave power capability from an AlGaN/GaN
transistor and maintaining the records of the highest power densities
for solid-state FETs. His recent work has been focused on improving
device reliability as well as applying the GaN devices to the millimeter-
wave regime.
For the last 11 years, Dr. Wu has continuous presentations in either
IEEE DRC (Device Research Conference) or IEDM (International Electron
Device Meeting). He has authored many high-impact papers resulting in
greater than 1000 citations. He served in the technical committee of
the IEEE DRC from 2004 to 2006. He is a technical committee member
of ICNS-7 (International Conference on Nitride Semiconductors, 2007).
Dr. Wu holds 6 U.S. patents in GaN related devices.
Mishra et al. : GaN-Based RF Power Devices and Amplifiers
Vol. 96, No. 2, February 2008 | Proceedings of the IEEE 305
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