High breakdown voltage Schottky rectifier fabricated
on bulk n-GaN substrate
Yi Zhoua, Dake Wanga, Claude Ahyia, Chin-Che Tina, John Williamsa, Minseo Parka,*,
N. Mark Williamsb, Andrew Hanserb
aDepartment of Physics, Auburn University, Auburn, AL 36849, USA
bKyma Technologies, Inc., 8829, Midway West Road, Raleigh, NC 27617, USA
Received 14 April 2006; received in revised form 1 August 2006; accepted 11 September 2006
Available online 7 November 2006
The review of this paper was arranged by Prof. E. Calleja
Vertical Schottky rectifiers have been fabricated on a free-standing n-GaN substrate. Circular Pt Schottky contacts with different
diameters (50 lm, 150 lm and 300 lm) were prepared on the Ga-face and full backside ohmic contact was prepared on the N-face
by using Ti/Al. The electron concentration of the substrate was as low as ?7 · 1015cm?3. Without epitaxial layer and edge termination
scheme, the reverse breakdown voltages (VB) as high as 630 V and 600 V were achieved for 50 lm and 150 lm diameter rectifiers, respec-
tively. For larger diameter (300 lm) rectifiers, VBdropped to 260 V. The forward turn-on voltage (VF) for the 50 lm diameter rectifiers
was 1.2 V at the current density of 100 A/cm2, and the on-state resistance (Ron) was 2.2 mX cm2, producing a figure-of-merit (VB)2/Ronof
180 MW cm?2. At 10 V bias, forward currents of 0.5 A and 0.8 A were obtained for 150 lm and 300 lm diameter rectifiers, respectively.
The devices exhibited an ultrafast reverse recovery characteristics, with the reverse recovery time shorter than 20 ns.
? 2006 Elsevier Ltd. All rights reserved.
Keywords: GaN; Free-standing substrate; Schottky rectifier; Reverse breakdown voltage; Reverse recovery
Recent developments have allowed Si semiconductor
technology to approach the theoretical limits of this mate-
rial. For high power, high temperature and high frequency
applications, wide bandgap semiconductors, such as GaN
and SiC, are the most attractive alternatives over Si
because of inherent material advantages like larger energy
bandgap, higher critical electric field for breakdown and
higher saturation electron drift velocity . Some key mate-
rial properties and figure-of-merits (FOM) of several semi-
conductors are summarized in Table 1. Many of those
values are referred to reference [2,3]. The Johnson’s figure
of merit (JFOM) takes into account of the critical electric
field and saturation electron drift velocity in defining a
measure of the high frequency capability of the material
. For GaN, the JFOM is more than 1000 times that of
Si and 4 times that of SiC. The Baliga’s figure of merit
(BFOM) is based on dielectric constant, carrier mobility
and critical electric field to define a measure of minimizing
conduction losses in power field effect transistors (FETs)
. For GaN, the BFOM is more than 1000 times that of
Si and almost 3 times that of SiC. The combined figure
of merit (CFOM) is an ultimate measure of high power,
high temperatureand high
the material. Both GaN and SiC have much higher
CFOM than Si or GaAs. It is very obvious that for high
power, high frequency applications, GaN offers far better
0038-1101/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved.
*Corresponding author. Address: 303 Allison Lab, Auburn University,
Auburn AL 36849 USA. Tel.: +1 334 844 4270; fax: +1 334 844 4613.
E-mail address: email@example.com (M. Park).
Solid-State Electronics 50 (2006) 1744–1747
performance than conventional semiconductors like Si and
GaAs. Therefore, it is expected that GaN-based power
device will eventually outperform the SiC counterpart .
There have been tremendous interests in developing
GaN-based electronics for high power, high frequency
applications [6–10]. Due to its unipolar nature, a Schottky
diode does not exhibit the minority carrier storage effect,
thus faster switching speed and less reverse recovery cur-
rent can be achieved with Schottky diodes compared to
p–n junction diodes. Due to the poor availability of bulk
GaN substrates, GaN-based Schottky rectifiers have been
fabricated using GaN films epitaxially grown on foreign
substrates such as sapphire [11–17]. Zhang et al. has
reported a record VBvalue of 6350 V for Schottky rectifiers
fabricated laterally on GaN epitaxial layer grown on sap-
phire . The figure-of-merit (VB)2/Ronof these devices
are as high as 268 MW cm?2, which is also a record for
GaN Schottky rectifiers. Although excellent reverse break-
down voltages have been achieved, their application is lim-
ited due to some inevitable drawbacks such as limited
current density and poor thermal conductivity of the sap-
phire substrate. These drawbacks are expected to be over-
come by using a free-standing GaN substrate. Few initial
reports of GaN Schottky diodes on free-standing sub-
strates have been published recently [18–23]. Thanks to
the vertical geometry with full backside ohmic contact,
higher forward current conduction has been achieved in
these Schottky rectifiers. Ip et al. has reported a record for-
ward current value of 1.72 A at 6.28 V under pulsed condi-
tions . By interconnecting the output of many rectifiers,
forward current as high as 160 A has been achieved by
using a free-standing GaN substrate . However, due
to the relatively high n-doping of the substrates (?1017),
an epitaxially grown GaN layer or edge termination
scheme is needed in order to achieve a reverse breakdown
voltage higher than 600 V.
In this paper, we report the high reverse breakdown
voltage (>600 V) Schottky rectifiers fabricated directly on
bulk n-GaN substrates with no edge termination scheme.
The devices also exhibited excellent forward current con-
duction and ultrafast reverse recovery characteristics and
look promising in high power, high frequency applications.
The free-standing GaN substrates were provided by
Kyma Technologies, Inc. They were 10 · 10 mm2in dimen-
sion and ?460 lm thick. The capacitance–voltage (C–V)
measurements showed an unintentional n-doping level of
?7 · 1015cm?3. The Ga-side was polished with a typical
dislocation density of ?5 · 106cm?2, which was deter-
mined via etch pit density measurements.
Full area ohmic contacts of Ti (50 nm)/Al (100 nm) were
deposited on N-side by DC magnetron sputtering in Ar
ambient, followed by rapid thermal annealing at 750 ?C
in N2atmosphere for 30 s. Then, the Pt (250 nm) Schottky
contacts with diameters of 50, 150 and 300 lm were depos-
ited on the Ga-side by DC magnetron sputtering, patterned
by liftoff and then annealed in N2atmosphere at 500 ?C for
10 s to improve adhesion. Low-field current–voltage (I–V)
measurements was performed using a Keithley 6517
electrometer with its built-in power supply. High-field
I–V measurements were carried out using a Tektronix
471 curve-tracer. The reverse recovery characteristics were
studied using an Astable circuit by switching the applied
voltage from 9 to ?9 V to the series of the device and a
resistor. The voltage on the serial resistor was analyzed
by Tektronix TDS 744 A oscilloscope to derive the reverse
recovery characteristics of the Schottky rectifiers. All mea-
surements were performed at room temperature.
3. Results and discussion
Fig. 1 shows the room temperature low field I–V charac-
teristics of the 50 lm diameter rectifier. By fitting the curve
into the thermionic emission over a barrier,
Fig. 1. Low field I–V characteristics of the 50 lm diameter rectifier.
Comparison of semiconductor material properties at room temperature
Dielectric constant, e
Breakdown field, Ec(MV/cm)
Electron mobility, l (cm2/V s)
Maximum velocity, Vs(107cm/s)
Thermal conductivity, k (W/cm K)
S=4p2(relative to Si)
c(relative to Si)
aJFOM: Johnson’s figure of merit, a measure of the ultimate high fre-
quency capability of the material.
bBFOM: Baliga’s figure of merit, a measure of minimizing conduction
losses in power FET’s.
cCFOM: Combined figure of merit for high temperature/high power/
high frequency applications.
Y. Zhou et al. / Solid-State Electronics 50 (2006) 1744–1747
JF¼ Js? exp
Js¼ A?? T2? exp
where JFis the forward current density, Jsis the saturation
current density, e is electron charge, V is the applied volt-
age, n is the ideality factor, k is the Boltzman constant, T
is the temperature, A*is the Richardson constant for n-
GaN (assumed to be 26.4 A cm?2K?2), and Ubis the bar-
rier height, the Schottky barrier height was found to be
?1.0 eV and ideality factor to be ?1.13, indicating a rea-
sonably good contact. Defining VFas the bias voltage at
which the forward current density is 100 A/cm2, a typical
VFwas found to be 1.2 V for the 50 lm diameter rectifiers
with the Ronto be 2.2 mX cm2. However, the variation in
Ron was found between 1.2 mX cm2and 3.3 mX cm2
among different diodes.
Fig. 2 shows the room temperature high field I–V char-
acteristics of the 150 lm and 300 lm diameter rectifiers. At
the bias of 10 V, the forward currents reached 0.5 A and
0.8 A for 150 lm and 300 lm diameter rectifiers, corre-
sponding to a current density of 2830 A/cm2and 1130
A/cm2, respectively. There are a number of reports of mesa
and lateral GaN Schottky rectifiers fabricated on hetero-
epitaxial layers on sapphire substrates. Compared to bulk
GaN substrates, the major disadvantage of sapphire sub-
strates is the poor thermal conductivity (0.5 W/cm K)
which restricts the high current conduction. The typically
cited value (1.3 W/cm K) of thermal conductivity for
GaN is actually a lower limit. Recent experimental study
has shown that a higher thermal conductivity of 2.3
W/cm K was measured on a high quality, low dislocation
density bulk GaN substrate . Furthermore, the bulk
GaN substrates allow fabrication of vertical geometry
device with full backside ohmic contact, which will essen-
tially enable much higher current conduction than lateral
rectifiers fabricated on insulating substrates .
Fig. 3 shows the reverse breakdown voltages for bulk
GaN rectifiers of different diameters. The VBwas defined
as the voltage at which the reverse current reached 1 mA.
Our best results have been achieved with small diameter
rectifiers, including VBof 630 V with 50 lm diameter recti-
fiers and 600 V with 150 lm diameter rectifiers. For larger
diameter (300 lm) rectifiers, the good device yield dropped
significantly, with best VBdecreased to 260 V. It is believed
that a deceasing VBwith increasing contact diameter is due
to the higher probability of having defects within the device
active area, especially at the contact periphery, which lead
to premature breakdown [18,19]. Our data comply with
this general trend, however, the data was too scattered to
extract a linear relation as found by other researchers
. The high breakdown voltage of our devices was chiefly
attributed to the low electron concentration of the bulk
GaN substrate since the theoretical model on avalanche
breakdown in GaN predicts that 
where NDis the doping concentration of the bulk. For the
50 lm diameter rectifiers, the figure-of-merit (VB)2/Ronhad
a value of 180 MW cm?2.
Fig. 4 shows the reverse recovery characteristics of the
50 lm diameter rectifier. The reverse recovery time was
found to be less than 20 ns when switching from forward
bias to reverse bias. Similar reverse recovery characteristics
were found with 150 lm and 300 lm diameter devices. The
shorter carrier lifetime and smaller drift region length of
wide bandgap semiconductor devices have been attributed
to the improvement in turnoff energy loss, maximum
reverse recovery current and turnoff time over Si-based
devices . GaN rectifier is expected to have better reverse
recovery performance than 6H-SiC and promising in high
Fig. 2. High field I–V characteristics of the 150 lm and 300 lm diameter
Fig. 3. Reverse breakdown voltages for bulk GaN rectifiers of different
Y. Zhou et al. / Solid-State Electronics 50 (2006) 1744–1747
We have fabricated high VBSchottky rectifiers directly
on bulk n-GaN substrate without edge termination
scheme. The bulk GaN substrates with low n-doping level
greatly simplified the fabrication process and improved the
device performance in turns of forward current conduction
and reverse recovery characteristics compared to devices
fabricated on GaN epitaxial layers on sapphire substrate.
Future work should focus on further reducing backside
ohmic contact resistance and applying edge termination
schemes to further improve VB. The device exhibits prom-
ising characteristics for high power high frequency
The authors would like to thank Dr. O. Wayne Koger at
US Army and Dr. Fred Clarke at the US Army Space and
Missile Defense Command. The project was funded by
Missile Defense Agency (MDA) and managed by US Army
Space and Missile Defense Command under contract num-
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Y. Zhou et al. / Solid-State Electronics 50 (2006) 1744–1747