Short communication
1.6 A GaN Schottky rectifiers on bulk GaN substrates
J.W. Johnson a, B. Lou a, F. Ren a,*, D. Palmer b, S.J. Pearton c, S.S. Park d,Y.J. Park d, J.-I. Chyi e
a Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USAb MCNC, Research Triangle Park, NC 27709, USA
c Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USAd Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea
e Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan
Received 19 August 2001; received in revised form 27 October 2001; accepted 6 November 2001
Abstract
Large area bulk GaN rectifiers with implanted pþ guard rings were fabricated using additional dielectric overlap
passivation. The devices were packaged to avoid self-heating at large operating currents. A forward current of 1.65 A
was achieved in pulsed voltage mode, a record for GaN rectifiers. The on-state resistance was 3.7 mX cm2. � 2002
Elsevier Science Ltd. All rights reserved.
Keywords: GaN; HVPE; Schottky; Rectifier; Thermal package; Bulk substrate
1. Introduction
GaN electronic device research has been largely
dominated by field effect transistors (FETs). MESFETs
and HEMTs for high frequency, high power applica-
tions have been developed to exploit the attractive ma-
terial properties of the III-nitrides. A device which has
received considerably less attention is the power rectifier.
Wide band gaps allow III-nitrides to sustain extremely
high critical electric fields, leading to large blocking
voltages. Applications for these devices are numerous,
and their potential technological and commercial im-
portance is beginning to take shape.
Mechanical and Si-based switches are presently used
to control electric current flow across utilities transmis-
sion and distribution lines. Opening or closing these
switches can lead to large power sags and switching
transients delivered to the load. Such transients may be
detrimental, for instance, to major computing centers,
motor drives, digital controls, or other sensitive elec-
tronic equipment. An outage of less than one cycle, or a
voltage sag of 25% for two cycles can cause a micro-
processor to malfunction. As a result of these potential
fluctuations, the electric power grid must be operated at
capacities well below its rated value, leading to reduced
energy efficiency. A system for eliminating power sags
and switching transients would dramatically improve
power quality [1–4]. Solid state devices, if available, are
expected to show ‘‘clean’’ switching and could poten-
tially eliminate line transients and allow more efficient
operation of the grid. In addition, typical power devices
are required to operate at elevated temperatures due to
the power dissipation associated with switching large
currents and voltages. In this respect, wide band gap
switches are attractive due to their increased tolerance to
temperatures above the limits of silicon. Reduction of
bulky, expensive cooling equipment should be possible,
leading to decreased system complexity and cost. Other
end uses include electronic motor controls, lighting,
heating, and air-conditioning.
The GaN material system has a high critical field,
good saturation electron velocity and reasonable ther-
mal conductivity if bulk wafers are available. A key
Solid-State Electronics 46 (2002) 911–913
*Corresponding author. Tel.: +1-352-392-4757; fax: +1-352-
392-9513.
E-mail address: [email protected] (F. Ren).
0038-1101/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.
PII: S0038-1101 (01 )00339-2
component of the inverter modules required for many of
the previously mentioned applications is the simple
rectifier. There have been a number of reports of mesa
and lateral geometry GaN and AlGaN Schottky and p-i-
n rectifiers fabricated on heteroepitaxial layers on Al2O3
substrates [5–12]. A major disadvantage of this ap-
proach is the poor thermal conductivity of sapphire
(j ¼ 0:5 W/cmK) and the limited epilayer thicknesses
employed. In this regard, better substrate choices would
be either SiC or GaN itself, since the latter has ap-
proximately the same thermal conductivity as Si. It
should be noted that the typically cited value of thermal
conductivity for GaN (1.3 W/cmK) is effectively a lower
limit, as suggested by recent studies. It has been shown
that both defect density and carrier concentration can
significantly affect the thermal conductivity [13]. Values
near 2 W/cmK have been experimentally demonstrated
for GaN. The availability of bulk GaN substrates would
allow fabrication of vertical geometry rectifiers capable
of much higher current conduction than lateral rectifiers
fabricated on insulating substrates.
2. Experimental
The free-standing substrates were described previ-
ously [14,15]. P-type guard rings (30 lm diameter) were
formed by selective area Mgþ implantation at 50 keV,
5� 1014 cm�2. The implant was followed by an 1100 �C,30 s anneal to remove residual lattice damage. Schottky
contacts of e-beam evaporated Pt/Ti/Au with diameters
of �5 mm for large-area devices were placed on the front
(Ga-face) surface. For the large area devices the contact
was extended over a PECVD SiO2 passivation layer. A
schematic cross-section of the large area GaN rectifiers
is shown in Fig. 1.
3. Results and discussion
A thermal package was designed for the large area
vertical rectifier. The diode was mounted on an FR-4
board with 1/2 oz copper on each side. The copper was
overplated with 0.5 lm Ni and �1 lm Au. The diode
was adhered to the board with H2OE silver-loaded ep-
oxy from EpoTek. The topside Schottky metal was
connected to the pad with 1� 5 mil gold ribbon, also
mounted with EpoTek H2OE. A schematic of the
package design is given in Fig. 2. The packaged device is
shown in Fig. 3. Forward I–V characteristics of the
packaged device were measured by applying a square
wave voltage pulse (0 V–VF) to the Schottky contact and
monitoring the current using a wide band current probe
connected to a 500 MHz Agilent Infiniium 50662 oscil-
loscope. The reverse characteristics were taken from DC
measurements using an HP4145B. Both the forward and
reverse characteristics are shown together in Fig. 4. The
reverse breakdown voltage of the large area device was
small (�6 V). However, pulsed forward current of 1.65
A was demonstrated at VF ¼ 6 V. This is the highest
forward current ever obtained from a GaN rectifier.
Despite the small breakdown voltage, clear rectification
behavior is evident from Fig. 4. For GaN-based rectifi-
Fig. 1. Schematic representation of device cross-section.
Fig. 2. CAD design of thermal package for large area GaN
rectifier. The package is necessary to avoid significant self-
heating in the high current device.
Fig. 3. Photograph of large area diode package. The approxi-
mate dimensions of the entire package area 1� 3.5 cm2.
912 J.W. Johnson et al. / Solid-State Electronics 46 (2002) 911–913
ers to become useful in the commercial power grid, they
will not only be required to block large voltages, but also
conduct significant forward currents. Previous small
area GaN rectifiers have achieved impressive reverse
characteristics, but forward characteristics have always
been reported as current density. This device represents
a large step toward the achievement of practical on-state
current levels. In addition, the on-state resistance was
3.7 mX cm2.
4. Summary
A �20 mm2 GaN bulk rectifier produced 1.65 A of
pulsed forward current at 6 V, the largest on-state cur-
rent ever reported for a GaN rectifier. Future work
should focus on lowering the background doping level in
the GaN. Existing material exhibits a negative temper-
ature coefficient for VB, but this is expected to reverse
sign in low defect substrates. The viability of GaN rec-
tifiers in most applications depends on making very
large area devices with high VB, while retaining low VFand RON.
Acknowledgements
The work at UF is partially supported by NSF grant
DMR 0101438.
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Fig. 4. I–V characteristics of packaged GaN diode measured in
pulsed voltage mode (10% duty cycle).
J.W. Johnson et al. / Solid-State Electronics 46 (2002) 911–913 913
