1.6 A GaN Schottky Rectifiers on Bulk GaN Substrates

 

J.W. Johnson1, B. Lou1, F. Ren1, D. Palmer2, S.J. Pearton3, S.S. Park4, Y.J. Park4 and J.-I. Chyi5

 

1 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 USA

 

2 MCNC, Research Triangle Park, NC 27709 USA

 

3 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 USA

 

4 Samsung Advanced Institute of Technology, PO Box 111, Suwon 440-600, South Korea

 

5 Department of Electrical Engineering, National Central University, Chung-Li, 32054 Taiwan

 

 

ABSTRACT

 

            Large area bulk GaN rectifiers with implanted p+ 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 m W·cm2.

INTRODUCTION

GaN electronic device research has been largely dominated by field effect transistors (FETs).  MESFETs and HEMTs for high frequency, high power applications have been developed to exploit the attractive material properties of the III-nitrides.  A device which has received considerably less attention is the power rectifier.  Wide bandgaps 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 importance is beginning to take shape.

Mechanical and Si-based switches are presently used to control electric current flow across utilities transmission 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, computers, digital controls, or other sensitive electronic equipment.  An outage of less than one cycle, or a voltage sag of 25% for two cycles can cause a microprocessor 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 potentially 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 bandgap 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 thermal conductivity if bulk wafers are available.  A key 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 approach is the poor thermal conductivity of sapphire (k=0.5 W/cm·K) and the limited epilayer thicknesses employed.  In this regard, better substrate choices would be either SiC or GaN itself, since the latter has approximately the same thermal conductivity as Si.  It should be noted that the typically cited value of thermal conductivity for GaN (1.3 W/cm·K) 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/cmּK 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.

EXPERIMENTAL

The free-standing substrates were described in previously.  P-type guard rings (30 μm diameter) were formed by selective area Mg+ implantation at 50 keV, 5 ´ 1014 cm-2.  The implant was followed by an 1100°C, 30 second 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 Figure 1.

RESULTS AND DISCUSSION

            A thermal package was designed for the large area vertical rectifier.  The diode was mounted on an FR-4 board with ½ oz copper on each side.  The copper was overplated with 0.5 μm Ni and ~1 μm Au.  The diode was adhered to the board with H2OE silver-loaded epoxy 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 Figure 2.  The packaged device is shown in Figure 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 wideband current probe connected to a 500 MHz Agilent Infiniium 50662 oscilloscope.  The reverse characteristics were taken from DC measurements using an HP4145B.  Both the forward and reverse characteristics are shown together in Figure 4.  As mentioned previously in this section, the reverse breakdown voltage of the large area device was small (~6V).  However, pulsed forward current of 1.65 A was demonstrated at VF = 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 Figure 4.  For GaN-based rectifiers 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 m W·cm-2.

 

SUMMARY

            A ~20 mm2 GaN bulk rectifier produced 1.65 A of pulsed forward current at 6 V, the largest on-state current ever reported for a GaN rectifier.  Future work should focus on lowering the background doping level in the GaN. Existing material exhibits a negative temperature coefficient for VB, but this is expected to reverse sign in low defect substrates.  The viability of GaN rectifiers in most applications depends on making very large area devices with high VB, while retaining low VF and RON.

ACKNOWLEDGMENTS

            The work at UF is partially supported by NSF grant DMR 0101438.

 

 

 

 

 

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Figure Captions

 

Figure 1.         Schematic of device cross-section.

 

Figure 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.

 

Figure 3.         Photograph of large are diode package.  The approximate dimensions of the entire package are 1 cm x 3.5 cm.

 

Figure 4.         Current-voltage characteristics of packaged GaN diode measured in pulsed voltage mode (10% duty cycle).