Processing and Device Performance of GaN Power Rectifiers
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Department of Chemical Engineering University of Florida, Gainesville, FL 32611 USA (2) Department of Materials Science and Engineering University of Florida, Gainesville, FL 32611 USA (3) Sandia National Laboratories, Albuquerque, NM 87185 USA (4) Department of Electrical Engineering National Central University, Chung-Li, 32054 Taiwan (5) Department of Physics, National Central University, Chung-Li 32054 Taiwan (6) Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 USA (7) Consultant, Stevenson Ranch, CA 91381
ABSTRACT Mesa and planar geometry GaN Schottky rectifiers were fabricated on 3-12µm thick epitaxial layers. In planar diodes utilizing resistive GaN, a reverse breakdown voltage of 3.1 kV was achieved in structures containing p-guard rings and employing extension of the Schottky contact edge over an oxide layer. In devices without edge termination, the reverse breakdown voltage was 2.3 kV. Mesa diodes fabricated on conducting GaN had breakdown voltages in the range 200-400 V, with on-state resistances as low as 6m ΩÂFP-2. INTRODUCTION The AlGaN materials system is attractive from the viewpoint of fabricating unipolar power devices because of its large bandgap and relatively high electron mobility. [1-6] An example is the use of Schottky diodes as high-voltage rectifiers in power switching applications. [2-4,6] These diodes will have lower blocking voltages than p-i-n rectifiers, but have advantages in terms of switching speed and lower forward voltage drop. Edge termination techniques such as field rings or field plates, bevels or surface ion implantation are relatively well-developed for Si and SiC and maximize the high voltage blocking capability by avoiding sharp field distributions within the device. [7] However in the few GaN Schottky diode rectifiers reported to date [2,3], there has been little effort made on developing edge termination techniques. Proper design of the edge termination is critical both for obtaining a high breakdown voltage and reducing the on-state voltage drop and switching time. In this paper we report on the effect of various edge termination techniques on the reverse breakdown voltage, VB, of planar GaN Schottky diodes which deplete in the lateral direction. A maximum VB of 3.1 kV at 25oC was achieved with optimized edge termination, which is a record for GaN devices. We also examined the temperature
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dependence of VB in mesa diodes and found a negative temperature coefficient of this parameter in these structures. EXPERIMENTAL The GaN was grown on c-plane Al2O3 substrates by Metal Organic Chemical Vapor Deposition using trimethylgallium and ammonia as the precursors. For verticallydepleting devices, the structure consisted of a 1µm n+ (3x1018 cm-3, Si-doped) contact layer, followed by undoped (n=2.5x1016 cm-3) blocking layers which ranged from 311µm thick. These samples were formed into mesa diodes using Inductively Coupled Plasma etching with Cl2/Ar discharges (300 W source power, 40 W rf chuck power). The dc self-bias during etching was –85 V. To remove
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