High common-emitter current gains obtained by pnp GaN bipolar junction transistors
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High common-emitter current gains obtained by pnp GaN bipolar junction transistors Kazuhide Kumakura, Toshiki Makimoto and Naoki Kobayashi NTT Basic Research Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan ABSTRACT We fabricated pnp GaN bipolar junction transistors and investigated their common-emitter and common-base current-voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The base thickness was 0.12 µm and its electron concentration was estimated to be 3 × 1017 cm-3 from the common-emitter current-voltage characteristics and the base conductivity. The common-emitter current-voltage characteristics showed very low leak current. The maximum current gains at room temperature were 50 and 69 from the common-emitter and the common-base current-voltage characteristics, respectively. INTRODUCTION Wide band-gap semiconductors, such as group III – nitrides, have attracted much attention for realizing electronic devices that can operate at high temperature and/or high power as well as under high frequency conditions. Bipolar junction transistors (BJTs) potentially have more uniform threshold voltages, higher linearity, higher current densities as well as lower phase noise than field effect transistors (FETs). Therefore, many works have been focused on Npn BJTs and heterojunction bipolar transistors (HBTs). In these works, the npn transistors have been widely developed due to the high mobility and the saturation velocity of electrons as minority carrier. For example, npn GaN BJTs with current gain β = 10 [1], Npn AlGaN/GaN HBTs with β ≤ 35 have been reported [2-5]. In these devices, a common problem is the highly resistive p-type base layer due to the relatively deep acceptor level of Mg atoms, which results in the low hole concentration below 1018 cm-3 at room temperature. Moreover, it is difficult to decrease the base thickness to increase the current gain because of the low hole concentration in the base and the severe etching damage to the p-type base. Therefore, high hole concentrations and materials that are not damaged by etching are preferable for device application. Recently, we have reported p-type InGaN with a hole concentration near 1019 cm-3 [6, 7] and have shown that the In atoms in (In)GaN are effective for improving the ohmic characteristics of the surfaces damaged by electron cyclotron resonance (ECR) plasma etching I12.2.1
[8,9]. Since these characteristics are promising for the base layer of HBTs, we applied p-type InGaN with the hole concentration of 5 × 1018 cm-3 to the base of Npn InGaN/GaN HBTs and obtained maximum current gain of 20 at a collector current of 3 mA at room temperature [10]. For the pnp configuration, the base resistance can be reduced because we can easily obtain the electron concentration above 1019cm-3 in n-type GaN. This enables us to fabricate thinner base layers to increase the current gain, even though minority carriers are holes. The pnp BJTs and HBTs are also intere
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