Reduction of Base Access Resistance in AlGaN/GaN Heterojunction Bipolar Transistors using GaInN Base Cap Layer and Selec
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Reduction of Base Access Resistance in AlGaN/GaN Heterojunction Bipolar Transistors using GaInN Base Cap Layer and Selective Epitaxial Growth Jay M. Shah1, Thomas Gessmann1, Hong Luo2, Yangang Xi2, Kaixuan Chen2, Jong Kyu Kim1, and E. Fred Schubert1 1 Electrical, Computer, and Systems Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, U.S.A. 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, U.S.A. ABSTRACT One of the major challenges affecting the performance of Npn AlGaN/GaN heterojunction bipolar transistors (HBTs) is the high base access resistance, which is comprised of the base contact resistance and the base bulk resistance. A novel concept is proposed to reduce the base access resistance in Npn AlGaN/GaN HBTs by employing polarization-enhanced contacts and selective epitaxial growth of the base and emitter. In addition, this technique reduces the exposed base surface area, which results in a lower surface recombination current. Such a structure would enable better performance of AlGaN/GaN HBTs in terms of higher current gain and a lower offset voltage. Theoretical calculations on polarization-enhanced contacts predict p-type specific contact resistance lower than 10–5 Ωcm2. Experimental results using transmission line measurement (TLM) technique yield specific contact resistances of 5.6×10–4 Ωcm2 for polarization-enhanced p-type contacts and 7.8×10–2 Ωcm2 for conventional p-type contacts. INTRODUCTION The development of GaN-based HBTs is motivated by the demand for high-power and highfrequency electronic devices capable of working at temperatures greater than 100 ºC. GaN is desirable for electronic applications due to various advantages like large bandgap, large breakdown field, high projected saturation velocity, and the ability to form heterojunctions with AlGaN and GaInN alloys. However, there are several issues, mostly related to p-type GaN, that need to be resolved before obtaining high performance GaN-based HBTs. First, it is difficult to obtain a high hole concentration, because Mg is a deep acceptor in GaN. Second, the mobility of holes in GaN is low due to high hole effective mass. Low hole concentration and mobility result in low conductivity. This results in high base bulk and contact resistance, which in turn results in lower cut-off frequency, higher collector–emitter offset voltage, and lower transconductance. In addition, the lateral base resistance causes current crowding in the base-emitter junction. In this article, we propose a novel design to reduce the base access resistance in Npn AlGaN/GaN HBTs by using polarization-enhanced contacts and selective epitaxial growth. Reduction of the base access resistance in the proposed structure can be achieved by utilizing 2-dimensional hole gas (2DHG), generated by polarization-enhanced contacts, for lateral current transport in the base. We demonstrate that the specific contact resistance, of polarization-enhanced contacts, is lower than that of
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