ABSTRACT
Nitride semiconductors have the preference to the high power electronic devices due to their wide bandgap, so there have been several reports of AlGaN/GaN heterojunction bipolar transistors (HBTs) along with AlGaN/GaN heterostructure field effect transistors (HFETs). These HBTs have several advantages such as high current density, high breakdown voltage and uniform threshold voltage. These characteristics are also preferable to high power operation, so nitride HBTs have a potential for high power electronic devices. However, the reported common-emitter current gains are less than or equal to 11 [1-3] for Npn AlGaN/GaN HBTs. These low current gains are ascribed to two major problems for the p-GaN base layer. One is that p-GaN layers have lower hole concentrations at room temperature due to their deeper acceptor levels (about 160 meV) [4]. The other problem is severe etching damage induced by the HBT fabrication process. Recently, we have reported that Mg-doped InGaN layers show high hole concentrations at room temperature [5-8]. The maximum hole concentration was 2.3xl0 19 cm‘ 3 at the In mole fraction of 0.18. The corresponding resistivity decreased with increasing In mole fraction. The resistivity of Mg-doped GaN is about 10 times higher than that of Mg-doped Ino.28Gao.72N. These Mg-doped InGaN layers were applied to the contact layer of a p-type nitride layer, resulting in a low specific contact resistance of 1. lxlO' 6 Ω-cm 12 without any thermal treatment [9]. One reason for these high hole
concentrations is their lower acceptor levels in Mg-doped InGaN, compared with Mgdoped GaN. Another reason for them is that the electrical activity of Mg atoms becomes higher. Using these Mg-InGaN layers, p-InGaN/n-GaN heterojunction diodes were fabricated and we found that they showed good rectified characteristics with high breakdown voltages [10]. We have also reported that In atoms incorporated in Mgdoped GaN reduce the etching damage to improve p-type Ohmic characteristics on the etched surfaces [7,11]. Therefore, Mg-doped InGaN is a promising material for a base layer of nitride HBTs and we have applied it to the base layer of InGaN/GaN double heterojunction bipolar transistors (DHBTs). In this conference, we will report their common-emitter I-V characteristics and discuss a minority carrier diffusion length and a minority carrier lifetime in p-InGaN.
