High performance pnp AlGaN∕GaN heterojunction bipolar transistors on GaN substrates

We fabricated pnp AlGaN∕GaN heterojunction bipolar transistors (HBTs) with various emitter areas on GaN substrates and investigated their common-emitter current-voltage and high-power characteristics at room temperature. The HBTs with an emitter area of 30×50μm2 exhibited a high performance: a maximum current gain of 85 at a collector current of 30mA and a maximum collector current density of 7.3kA∕cm2 at a collector-emitter voltage of 30V, which corresponds to the maximum power dissipation density of 219kW∕cm2. The current gain and the collector current density increased compared to those on sapphire substrates. For the HBT with the emitter area of 270×450μm2, the current gain was still as high as 47 and the maximum collector current reached as high as 1A, and this single HBT showed a high-power dissipation of 30W. This high performance of the HBTs is ascribed to the low dislocation density and relatively high thermal conductivity of the GaN substrate. Although the emitter crowding and self-heating effects remain, the obtained values are highest among pnp nitride-based HBTs.