Journal of ELECTRONIC MATERIALS, Vol. 45, No.

6, 2016
DOI: 10.1007/s11664-016-4435-3
 2016 The Minerals, Metals & Materials Society

GaN Technology for Power Electronic Applications: A Review

TYLER J. FLACK,1,2 BEJOY N. PUSHPAKARAN,1

and STEPHEN B. BAYNE1

1.—Department of Electrical and Computer Engineering, Texas Tech University, 1012 Boston
Avenue, Lubbock, TX 79409, USA. 2.—e-mail: tyler.flack@ttu.edu

Power semiconductor devices based on silicon (Si) are quickly approaching

their limits, set by fundamental material properties. In order to address these
limitations, new materials for use in devices must be investigated. Wide
bandgap materials, such as silicon carbide (SiC) and gallium nitride (GaN)
have suitable properties for power electronic applications; however, fabrica-
tion of practical devices from these materials may be challenging. SiC tech-
nology has matured to point of commercialized devices, whereas GaN requires
further research to realize full material potential. This review covers funda-
mental material properties of GaN as they relate to Si and SiC. This is fol-
lowed by a discussion of the contemporary issues involved with bulk GaN
substrates and their fabrication and a brief overview of how devices are fab-
ricated, both on native GaN substrate material and non-native substrate
material. An overview of current device structures, which are being analyzed
for use in power switching applications, is then provided; both vertical and
lateral device structures are considered. Finally, a brief discussion of proto-
types currently employing GaN devices is given.

Key words: Gallium nitride (GaN), power electronics, power semiconductors

INTRODUCTION switching applications. Wide bandgap materials,

such as silicon carbide (SiC) and gallium nitride
In general, power electronic systems are used to
(GaN) have suitable properties for power electronic
process and control the flow of electric power. The
applications; however, fabrication of practical
primary goal of a power electronic system is to
devices from these materials is challenging.
provide power of optimum form for a user load.1 A
SiC has, since its discovery, matured significantly
key element of any power electronic system is the
as a semiconductor material and has subsequently
power switching device, the characteristics of which
made a lot of progress in the area of high power
will largely govern what frequencies and power
devices for power electronics and applications. GaN,
levels at which a power electronic system may
on the other hand, has encountered significant
operate. Silicon (Si) has been the material of choice
technical drawbacks in the realization of practical
for power devices for quite some time, due to ease of
device structures which have characteristics close to
processing, availability and the wealth of informa-
material limits.6 In this review, a summary of GaN
tion available about its material properties. Si
material properties is provided as well as contrasted
devices are, however, reaching their operational
against those of Si and SiC. This is followed by a
limits, set by intrinsic material properties. In light
discussion of the contemporary issues involved with
of this, new materials for the fabrication of power
bulk GaN substrates and their fabrication and a
semiconductors must be evaluated; specific atten-
brief overview of how devices are fabricated, both on
tion should be paid to material properties which
native GaN substrate material and non-native
directly impact the devices performance in power
substrate material. An overview of current device
structures, which are being analyzed for use in
power switching applications is then provided; both
(Received November 11, 2015; accepted February 25, 2016; vertical and lateral device structures are
published online March 10, 2016)

2673
2674 Flack, Pushpakaran, and Bayne

considered. Finally, a brief discussion of prototypes 4H-SiC. In addition, GaN exhibits critical electric
currently employing GaN devices is given. field strength of about 11 times greater than Si and
only slightly smaller than 4H-SiC. GaN is not,
MATERIAL PROPERTIES COMPARISON: however, without disadvantages as a material for
Si, SiC AND GaN use in power devices, specifically thermal conduc-
tivity. As illustrated in Table I, GaN’s thermal
Properties of semiconductor materials relevant to
conductivity is less than both 4H-SiC (roughly four
power devices include bandgap (Eg), critical field
times lower) and Si.
strength (Ec), carrier mobility (l), and thermal
conductivity.2 The bandgap of a material determi-
nes the carriers generated in the depletion region of BULK GaN FABRICATION
a semiconductor device; a larger bandgap correlates
to lower intrinsic carrier concentration, thus, lower Due to inherently beneficial material properties,
levels of leakage current during blocking condi- as detailed in the previous section, it is reasonable
tions.3 Critical field strength directly impacts the to see why there is so much focus on GaN as a
on-state resistance of the drift region in a vertical prospect for the manufacture of wide bandgap
power device: the larger the critical field strength, power switching devices. There is, however, a
the smaller the drift region thickness, and hence, disparity between theoretical material properties
the lower the on-state resistance; leading to a and realizable device properties for GaN. Specifi-
reduction in conduction losses.4 The relationships cally for the fabrication of GaN power devices, the
of critical field strength of a material to the on-state lack of bulk-quality GaN for native device growth is
resistance of a vertical unipolar power device and a a big issue. Additionally, GaN fabrication suffers
lateral high electron mobility transistor (HEMT) typical fabrication issues (i.e. economics of available
device are shown in Eqs. 1 and 2, respectively.5,6 processes, packaging concerns, etc.) In order to
address these concerns, research is continuously
4  BV ongoing to produce affordable, high-quality bulk
Ron ðverticalÞ ¼ ð1Þ GaN for fabrication of devices9,10 as well as the
es  l  E3c
resulting vertical device profiles.11,12 Additionally,
research into GaN devices fabricated on non-native
BV2
Ron ðHEMTÞ ¼ ð2Þ substrate materials (i.e. Si and SiC) is being con-
q  l  Qs  E2c ducted.13–17 Research conducted on high-quality
bulk GaN is necessary for the realization of vertical
A standard method of summarizing the efficacy of power structures (i.e. power PIN diodes, vertical
a material as a power switching device is to use MOSFETs, etc.) Vertical unipolar power device
baliga’s figure-of-merit (BFOM) given by Eqs. 3 and structures (e.g. power MOSFETs) are desirable for
4 for vertical devices and lateral high-electron their ease of control and high voltage capabilities
mobility transistors, respectively.6 These equations (without sacrificing current capabilities)18 Vertical
indicate how fundamental material properties influ- GaN p-n diodes were tested for avalanche capabil-
ence the resistance of a power switching device.5 ities in Ref. 12, and a general overview of research
into GaN p-n devices based on extremely low defect
BFOM ðverticalÞ ¼ es  l  E3c ð3Þ (104–106 cm2) GaN native substrates has been
discussed in Ref. 11.
BFOMðHEMTÞ ¼ q  l  Qs  E2c ð4Þ For the fabrication of high-quality GaN to be used
as substrate material epitaxial growth processes are
where es is the dielectric constant of the semicon- used. This is due to traditional methods of crystal
ductor material, l is the mobility of free carriers growth (e.g. Czochralski growth) are not viable for
(electrons in the case of a unipolar vertical device), nitride materials.14 Common examples of these
Qs is the sheet carrier density (in HEMT structures) processes are ammonothermal growth, liquid phase
and Ec is the critical electric field of a given epitaxy (LPE) (or solution growth) and hydride
semiconductor material. Thermal conductivity is vapor phase epitaxy (HVPE).9,19–23 Each of these
paramount when designing power devices, as it is processes has inherent advantages and disadvan-
the ability of a material to dissipate generated heat; tages associated with growth rates, dislocation
materials with high thermal conductivity require densities and repeatability.
minimal external heat sinking, thus lowering sys- HVPE is an epitaxial growth process which
tem cost. Material properties of Si, 4H-SiC and GaN utilizes a halide vapor and a hydride as group III
are provided in Table I.7,8 and group V precursors, respectively.24 Halides
Table I illustrates GaN’s inherent benefits when exhibit large surface migration and good thermal
compared with Si (traditional power devices) and stability which, paired with pure seed materials
4H-SiC (currently the wide bandgap material of allow the HVPE process to produce high growth
choice for power devices). GaN exhibits a bandgap rates relative to LPE and ammonothermal growth
roughly three times that of Si and 1.1 times that of techniques. HVPE is currently a dominant force in
GaN Technology for Power Electronic Applications: A Review 2675

Table I. Material properties of Si, SiC and GaN7,8

Material property Si 4H-SiC GaN

Bandgap (eV) 1.12 3.26 3.4

Critical field (106 V/cm) 0.3 3.5 3.3
Carrier mobility (cm2/V 9 sec) 1500 650 990, 2000a
Electron saturation velocity(106 cm/sec) 10 20 25
Thermal conductivity (W/cm2 9 K) 1.5 5 1.3
a
In bulk GaN/2D electron gas region of GaN/AlGaN HEMT, respectively.

commercialized GaN bulk materials. Boasting high (10 lm/h). It is imperative that the inherent
growth rates (£300 lm/h), control of impurities scalability of this process be exploited to combat
(doping), high volume output, relatively large wafer these very low growth rates.
size and high purity of resulting substrates are the
primary advantages of this process.23 GaN sub- DEVICE FABRICATION
strates produced using HVPE are not without their
Several thin film growth techniques are currently
limitations, specifically material uniformity is not
used for the heteroepitaxial growth of GaN on non-
quite up to par with some of the competing growth
native substrates; namely HVPE, molecular beam
mechanisms. Additionally, HVPE GaN substrates
epitaxy (MBE) and metalorganic chemical vapor
suffer from high strain resulting in bowing and
deposition (MOCVD); MOCVD is currently the most
cracking at the interface of GaN and the non-native
widely implemented method.27 These techniques
substrate material used for growth due to mis-
were used, at first, to grow thin-films of GaN
matches in their thermal expansion coefficients.
directly onto non-native substrate materials (e.g.
Recent research advances in GaN-on-sapphire and
sapphire or SiC), typically resulting in large
GaN-on-Si processing to reduce cracking have been
amounts of defects extending through the thin-film
able to reduce some of the limitations due to bowing
material and rough surfaces. However, circa 1986,
and cracking. Results of this experimentation on
Amono et al. published results of a great technical
thick GaN films to be used as freestanding substrate
innovation in GaN device fabrication using the
material using the HVPE process, presented in,19
MOCVD technique to grow GaN epitaxially on a
yield a 600-lm-thick (crack-free) GaN film with
sapphire substrate. Through the use of an alu-
dislocation densities lower than 1 9 106 cm2. In
minum nitride (AlN) buffer layer, the GaN surface
this research, two commercial 2-in. (c.5-cm) sap-
manifested in a much smoother form, substantially
phire substrate were processed simultaneously. The
increasing both the optical and electrical properties
first sapphire substrate was processed using a laser-
of thin-film GaN layers grown.28 Important to note
induced stress process, detailed in Refs. 25 and 26,
is the required high source-drain activation tem-
while the second substrate was processed without
perature of 1100–1200C,29 which requires special
using the laser process.
attention to desired gate-oxide materials for use in
The ammonothermal growth process is quite
MOSFET structures.
similar to other processes used to produce other
materials, such as quartz, at very high volumes
LATERAL DEVICE STRUCTURES
with great quality.23 The process uses a solvent
comprised of supercritical ammonia with added GaN-based power semiconductors are largely
mineralizers to grow GaN via recrystallization at lateral structures, such as HEMTs shown in
seed crystals.23 Ammonothermal growth has the Fig. 1. This structure is a heterostructure which
benefit of being a true-bulk process, meaning the utilizes the inherently polarized nature of GaN to
process does not require a non-native starting develop a 2-degree electron gas layer to achieve high
substrate.22 Other benefits of the ammonothermal electron mobility.30,31
process include the potentially huge scalability and GaN HEMTs have numerous operational benefits,
reduction of manufacturing cost.23 Detriments to including high-frequency operation in power appli-
the ammonothermal process include very slow cations, large breakdown fields and good transport
growth rates (72 times slower than HVPE), small properties.32 GaN HEMT structures are not without
resulting wafer size and greater complexity of deficiencies, one of which is that, in order for this
growth chemistry. Research in the production of structure to obtain suitably high breakdown volt-
large-size substrate quality GaN wafers, via the ages, a large amount of space must be left between
ammonothermal process, have yielded 1-in. (c.2.5- the gate and drain of the device. This causes the
cm) GaN wafers with defect densities of 5 9 103 current density ratings of HEMT devices to be
cm2.22 It is important to note even in these inherently low, thereby increasing the cost to
promising results, growth rates are still very low current density ratio.30 Additionally, HEMT
2676 Flack, Pushpakaran, and Bayne

Fig. 3. Schematic view of a cascode packaged HEMT device34 

2015 IEEE. Reprinted, with permission, from Ref. 34.

Fig. 1. Generic AlGaN/GaN HEMT structure30  2015 IEEE. Rep- off operation. Important to note is that this device
rinted, with permission, from Ref. 30. structure is not without setbacks, namely high-
temperature operation is expected to be severely
limited due to the inclusion of a Si device.

VERTICAL DEVICE STRUCTURES

For power switching applications, lateral device
structures are less desirable than vertical struc-
tures. This is due in part to vertical structures
exhibiting superior on-state resistance (for a given
breakdown voltage).36 Until recently, vertical
devices based on GaN technology have not been
practical, due to lack of bulk GaN substrates with
acceptable defect densities (<106 cm3).37 Signifi-
cant research and development efforts have taken
place recently on vertical GaN device structures;
Fig. 2. Cross-section of an enhancement mode p-GaN cap HEMT these structures include a vertical p–n diode, HFET
device35  2014 IEEE. Reprinted, with permission, from Ref. 35. with p-GaN islands and a HFET with p-GaN buried
buffer layer.36–39
The first of these structures being investigated is
structures are typically depletion mode field-effect the vertical p–n diode,37 which is the most basic of
transistors (FETs), which means that HEMT switching devices. This diode design operates in the
devices exhibit normally-on operation and must be same regime as any power diode (i.e. low resistance,
supplied a negative gate-bias in order to enter the high breakdown, etc.), with the exception of the use
blocking condition. Recently, there have been of an edge-termination region; this region is used to
numerous efforts to alter the structure of a typical spread the electric field experienced at the edges of
HEMT to produce enhancement mode FET devices the device. This field is spread across an area which
(i.e. normally-off operation).33–35 One such modifi- is greater than the thickness of the lightly doped
cation is the p-type GaN cap layer device,35 illus- drift region. Important to note is the necessity of a
trated in Fig. 2. This structure facilitates normally- specific design of edge termination per desired
off operation via trapping negative charges at the p- blocking voltage. The devices presented in37 exhi-
type cap layer’s interface with the i-AlGaN, this bits a specific on-state resistance of 2 mXÆcm2 for
causes a discontinuity in the 2-D electron gas layer. 2.6-kV breakdown capability and a specific on-state
Another device which modifies typical HEMT resistance of 2.95 mXÆcm2 for 3.7-kV breakdown
device operation is a cascode packaged HEMT capability; the structure presented in37 is illus-
device, manufactured by Transphorm. It is impor- trated in Fig. 4.
tant to note this device is not a structural modifi- The second device structure, presented in Ref. 36,
cation to the basic GaN HEMT, like the p-cap GaN is a heterostructure field effect transistor (HFET)
HEMT, but rather a pairing of a typical GaN HEMT structure utilizing p-type GaN islands, illustrated
with a low voltage Si FET device. This device is in Fig. 5. To facilitate conduction, this device struc-
manufactured by Transphorm; a schematic view of ture transports electrons from the source of the
the cascode packaged HEMT device is illustrated in device, through the current aperture, to the n-type
Fig. 3. buffer region. During the off-state (i.e. voltage is
This device is a combination of a 600-V GaN applied from drain to source) the p-type GaN
HEMT device which exhibits normally-on operation regions block applied voltages. Under high applied
and a Si MOSFET.34 The benefit of this configura- drain voltages, the n-type buffer region is allowed to
tion is two-fold: gate control of the device is identical fully deplete, due to the p-type island, thus becom-
to the gate control of a low-voltage Si MOSFET, and ing similar to an intrinsic region. The high voltage
the device, in this configuration, exhibits normally- is then supported across the junction of the p-type
GaN Technology for Power Electronic Applications: A Review 2677

Fig. 4. Vertical p–n diode structure37  2015 IEEE. Reprinted, with

permission, from Ref. 37.

Fig. 6. HFET with buried p-type GaN buffer layers38  2015 Else-
vier. Reprinted, with permission, from Ref. 38.

structure which is a HFET structure employing p-

type buffer layers is presented in Ref. 38. This
structure is referred to by the authors as the PBL-
HFET device. This device is illustrated in Fig. 6.
This device’s on-state operation is identical to that
discussed for the HFET device utilizing p-type GaN
islands, discussed in the previous section. During
low voltage off-state conditions, voltage is blocked
by the p-type CBL layers. Under applied high drain
voltage blocking conditions, the buried p-type buffer
layers essentially create three punch-through
regions which allow for a more uniform breakdown
electric field across the depletion region. The PBL-
HFET device exhibits a specific on-state resistance
of 3.13 mXÆcm2 for a breakdown voltage of 3-kV.38
The takeaway being the significant increase in
Fig. 5. GaN HFET with ap-type GaN island36  2015 Elsevier. breakdown voltage capability (50% increase),
Reprinted, with permission, from Ref. 36.
while keeping on-state resistance low.36
The final vertical device structures based on GaN
discussed in this review are the 1.2-kV vertical
current blocking layers (CBL) layers, buffer layer junction field effect transistors (JFETs) presented in
and substrate (i.e. p–i–n-type junction). Important Ref. 39. There are two vertical JFET structures
to note is this device has not yet been experimen- proposed the first has a vertical channel, illustrated
tally evaluated, as of,36 all results disseminated in Fig. 7, while the second has a lateral channel,
here are simulation results. The HFET device with shown in Fig. 8. Each of these structures is simu-
p-type islands exhibits a specific on-state resistance lated using Silvaco’s ATLAS software to investigate
of 2.79 mXÆcm2 for a breakdown voltage of 3.2-kV. their performance. Parameters used for the simula-
The takeaway being the significant increase in tions are from experimental values obtained in
breakdown voltage capability (50% increase), previous research.40,41
while keeping on-state resistance low.36 In both of these JFET structures, the n-type GaN
Following up on the addition of a p-type island drift region supports the majority of the reverse
layer included in the HFET device, a device voltage during the off-state. The difference in
2678 Flack, Pushpakaran, and Bayne

operation between the two JFET device structures the resistance of the device during the on-state. As
has to do with the channel structures. In the for the lateral channel structure, the amount the
vertical channel structure, the width and thickness gate contact overlaps the p-type layer controls the
of the current aperture are used to control the amount of modulation of electrons transported
amount of leakage current in the off-state as well as through the channel. Thus, this metric is used to
control the amount of leakage current in the off-
state of the device.
The performance of devices discussed in this
review are summarized in Table II, additionally
this table provides a comparison of these two
devices with available power devices (both GaN
and SiC).39,42 In addition this research used these
two structures to investigate the impact of mobility
on breakdown voltage and RON, it is shown that as
mobility goes down breakdown voltage increases;
the same is true for RON, as mobility decreased RON
increased.

POWER ELECTRONICS USING GAN

The inherent superiority of GaN over other
semiconductor material in the area of high fre-
quency power electronics has accelerated the
Fig. 7. JFET with vertical channel39  2015 IEEE. Reprinted, with demand for device research and prototype develop-
permission, from Ref. 39. ment. In order to assess the viability of GaN power
semiconductor devices, several universities and
selected enterprises are developing system proto-
types and comparing its performance to silicon and
SiC counterparts. This section will discuss major
GaN-based power electronic system prototypes.

5 kW High Efficiency DC–DC Boost Converter

A high-frequency high-efficiency DC–DC Boost
converter prototype rated for 2–5 kW has been
developed by Arkansas Power Electronics Inc.
(APEI) and GaN Systems using GaN HEMT switch.
The power converter was designed for a switching
frequency of 1 MHz and the switch turn ON and
turn OFF times were found to be 8.25 ns and
3.72 ns, respectively. The ultra-high-speed switch-
ing frequency of GaN device resulted in reducing
the size of the energy storage components thereby
aiding in the miniaturization of the complete sys-
Fig. 8. JFET with lateral channel39  2015 IEEE. Reprinted, with tem. The completed system prototype is shown in
permission, from Ref. 39. Fig. 9.43

Table II. JFET comparison with other power switching devices39,42

Device Manufacturer Breakdown (V) RON (mX cm2) Threshold (V)

SiC JFET SemiSouth 1900 2.8 14.1

SiC MOSFET Mitsubishi 1200 5 0
SiC DMOSFET CREE 10000 111 3.5
CAVET Avogy 1500 2.2 0.5
Vertical PN diode 37 2600 2 N/A
Vertical PN diode 37 3700 2.95 N/A
HFET p-type island 36 3200 2.79 0.95
PBL-HFET 38 3000 3.13 0.95
Vertical channel JFET 39 1260 5.2 0.8
Lateral channel JFET 39 1310 1.7 1
GaN Technology for Power Electronic Applications: A Review 2679

Fig. 11. PWM power waveform from a state-of-the-art silicon IGBT-

based inverter motor drive operating at 15-kHz switching frequency47
 2013 Transphorm. Reprinted, with permission.

Fig. 9. High efficiency DC–DC boost converter prototype based on

GaN transistor technology developed by APEI.43  2013 Wolfspeed.
Reprinted, with permission.

Fig. 12. Sine wave power output from a GaN-based inverter motor
drive operating at 100-kHz switching frequency47  2013
Transphorm. Reprinted, with permission.
Fig. 10. 4-kW three-phase inverter evaluation kit developed46 
2013 Transphorm. Reprinted, with permission.
half-bridges. The inverter operates at a switching
frequency of 100 kHz which is beneficial in reducing
The converter was designed for an output bus harmonics and increasing overall operational effi-
voltage of 400 V. The converter was able to main- ciency. Figure 10 shows the three-phase motor drive
tain an efficiency of over 98% in the entire range of evaluation unit.46
output load varying from 700 W to 2 kW. An The high frequency operation helps in reducing
efficiency of over 99% was obtained for power output the size of energy storage elements thereby result-
in the range 2–4 kW. At an output power of 5 kW, ing in a compact filter design. The GaN inverter
the converter was able to achieve an efficiency of motor drive was compared to an equivalent system
98.5%. The complete hardware unit had a mass of designed using state-of-the-art silicon IGBT. The
approximately 487 g which corresponds to a gravi- sinusoidal output waveforms obtained from both the
metric power density of approximately 10 kW/kg. systems have been compared in Figs. 11 and 12.47
The high system efficiency translates to lower losses The spike-free pure sine wave output from the
thereby requiring only passive air cooling.43–45 GaN-based inverter significantly improved the
motor efficiency. The efficiency of the GaN-based
4-kW Three-Phase Inverter Evaluation Kit inverter including filer loss was higher than the
The 4-kW three-phase inverter was developed by silicon IGBT-based system (w/o filter loss) as shown
Transphorm to evaluate the performance advan- in Fig. 13. An overall increase in the system effi-
tages of GaN semiconductor in motor drive applica- ciency (including motor efficiency) was observed in
tions. The three-phase inverter is formed by six the GaN-based inverter drive as shown in
GaN HEMTs in a module configured as three 600-V Fig. 14.47,48
2680 Flack, Pushpakaran, and Bayne

Fig. 13. Comparison of efficiency and power loss with respect to the
power output of a GaN-based inverter and a Si IGBT-based in-
verter48  2014 Transphorm. Reprinted, with permission. Fig. 16. Comparison of system efficiency with respect to the power
output of GaN-based PFC circuit to silicon power MOSFET-based
version 50  2014 Transphorm. Reprinted, with permission.

Fig. 14. Comparison of the overall system efficiency with respect to

the power output of a GaN-based inverter to a Si IGBT-based in-
verter48  2014 Transphorm. Reprinted, with permission. Fig. 17. LLC DC–DC converter prototype rated for 1 kW using a
GaN power transistor  2012 Fraunhofer ISE. Reprinted, with per-
mission.

High Switching Frequency 320 W Power Fac-

tor Correction (PFC) Evaluation Board
In order to evaluate the high frequency perfor-
mance of 600 V/29 mX GaN HEMT (TPH3002PS), a
320-W PFC evaluation board was developed by
Transphorm. The PFC circuit was designed for a
switching frequency of 750 kHz. The evaluation
board can be used with a 115 V AC or 230 V AC
supply. The PFC board is shown in Fig. 15.49,50
The performance of the GaN-based system was
compared with a similar system implemented using
Infineon silicon CoolMOS 650-V/38.5-mX power
MOSFET (IPP60R385CP). The comparison of sys-
tem efficiency as a function of power output (shown
in Fig. 16) for an input voltage of 115 VAC clearly
Fig. 15. GaN-based 300-W PFC evaluation board operating at 750- shows a significant increase in efficiency for the
kHz switching frequency49  2014 Transphorm. Reprinted, with GaN-based system as compared to its silicon
permission. counterpart.49,50
GaN Technology for Power Electronic Applications: A Review 2681

a viable option to replace Si devices and provide

greater system performance.
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