p-GaAs Base Regrowth for GaN HBTs and BJTs

Low resistance ohmic contacts are difficult to form to p-type GaN and AlGaN due to the unavailability of growth methods for highly p-doped GaN and AlGaN. A p-type carbon-doped GaAs regrowth on p-GaN prior to ohmic metallization has been shown in previous work to improve contact resistance to p-GaN [13]. Applying the regrowth method to the p-base regions of npn structured bipolar transistors, AlGaN/GaN heterojunction bipolar transistors and GaN bipolar junction transistors have been demonstrated. GaN/AlGaN epilayers were grown with a molecular beam epitaxy system. Highly carbon-doped p-GaAs (10 20 cm −3 ) was regrown on the devices (∼500 A) in the base contact region by metal organic chemical vapor deposition after emitter mesa etching. Emitter and base mesa structures were formed by Inductively Coupled Plasma etching under low damage conditions with a Cl 2 /Ar chemistry. SiO 2 was used for emitter sidewall formation to reduce leakage current to the emitter, as well as for a mask for GaAs base regrowth. Very high current densities were obtained for common base operation in both device types. The devices were operable at 250 °C.