Self-Aligned Process For Emitter- And Base- Regrowth GaN HBTs And BJTs
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Self-Aligned Process For Emitter- And Base- Regrowth GaN HBTs And BJTs K. P. Lee 1, A. P. Zhang 2 , G. Dang 2, F. Ren 2, J. Han 3, S.N.G.Chu 4 ,W. S. Hobson 4, J. Lopata 4, C. R. Abernathy 1, S. J. Pearton 1, J. W. Lee 5 1 Department of Materials Science and Engineering University of Florida, Gainesville FL 32611 2 Department of Chemical Engineering University of Florida, Gainesville FL32611 3 Sandia National Laboratories, MS0603, PO Box 5800, Albuquerque, NM 87185 4 AT&T Bell Labs, Lucent Technologies, 600 Mountain Ave. Murray Hill, NJ 07974 5 Department of Optical Engineering, Inje University, Kimhae, Korea ABSTRACT The development of a self-aligned fabrication process for small emitter contact area (2x4 um2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance. INTRODUCTION Wide bandgap Heterojunction Bipolar Transistors (HBT) and Bipolar Junction Transistor (BJTs) have a number of advantages over heterostructure field effect transistors (HFETs), including higher power density capability, better linearity, more uniform threshold voltages and higher transconductance. In the GaN/AlGaN materials system there are several factors currently limiting the performance of HBTs (1-9) and BJTs (10-12) including the low p-type doping achieved in the base region of npn devices, the low electron lifetime in the neutral base, high leakage currents in the collector–emitter junction and high recombination rates. Some of these limitations may be alleviated with use of pnp structures,(13,14) but the microwave performance of these would be expected to be inferior to npn devices. While GaN/AlGaN HBTs and BJTs are attractive candidates for high frequency switching and communications applications, the main interest is in microwave power amplifiers in the 1-5 GHz frequency range and with operating temperature > 400 °C. To fulfill the frequency requirement, devices with small emitter-contact areas are necessary (i.e. < 10 µm2). To date, all of the reported GaN/AlGaN HBTs and GaN BJTs have had much larger active areas (1-4,7,8,10). In this paper we describe a self-aligned fabrication process for these devices that includes dielectric sidewall spacers and selected-area growth of GaAs (C) on the base region or GaN/AlGaN as a regrown emitter to decrease contact resistance. The resulting device performance is compared with large-area devices fabricated on the same wafer. EXPERIMENTAL The layer structure for HBTs was grown by metal-organic chemical vapor deposition (MOCVD) in c-plane sapphire substrates, as described previously(3). The structure consisted of a 2 µm undoped GaN buffer, a 1.44µm thick n+ (3x1018 cm-3) GaN subcollector , a 0.5 µm thick n (3x1016 cm-3) GaN collector, a 0.15µm thick p (3x 1017 cm-3) GaN base, a 0.1 µm thick n+ (n=5x1017 cm-3 ) A
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