High power operation of Pnp AlGaN/GaN heterojunction bipolar transistors

We fabricated a Pnp AlGaN/GaN heterojunction bipolar transistor and investigated its common - emitter current - voltage characteristics at room temperature. The device structures were grown by metalorganic vapor phase epitaxy on the sapphire substrates. The buffer layer was a newly developed Al₂O₃/AlN/AlON/ Al₂O₃, resulting in the dislocation density of 6 × 10⁸ cm^-2 in MOVPE-grown GaN layer. This relatively low dislocation density led to the high voltage operation in the devices, corresponding to the breakdown field of 2.4 MV/cm. AlGaN/(Al)GaN superlattices were applied to the emitter and subcollector to increase the hole concentrations in these layers. An n-type GaN base width was 80 nm. The sheet resistivity and the specific contact resistance were 900 Ω/square and 2.6 × 10^-5 Ω-cm² for a 80 nm base, respectively. The base sheet resistivity of Pnp AlGaN/GaN HBT was two orders of magnitude smaller than that of Npn AlGaN/GaN HBTs. The maximum current gain was 8 at the collector current of 11.5 mA for the 30 μm × 50 μm device. It operated at the collector current of 20 mA at the collector - emitter voltage of 65 V with a current gain of 5. The corresponding current density and power density were 1.3 kA/cm² and 84.5 kW/cm². High power operation was achieved by using the relatively low dislocation density GaN and low resistance superlattices.