High-voltage operation with high current gain of pnp AlGaN∕GaN heterojunction bipolar transistors with thin n-type GaN base

A pnp AlGaN∕GaN heterojunction bipolar transistor (HBT) with a thin n-GaN base shows high-voltage operation with high current gain in the common-emitter configuration at room temperature. The device structure was grown by metalorganic vapor phase epitaxy on a sapphire substrate. The emitter area is 30μm×50μm. The HBT can operate at high voltage of 70V with the maximum current gain of 40 at the collector current of 10mA. The maximum output power density is 172kW∕cm2. Transport characteristics in the HBT were also investigated. At small collector current, the current gain is dominated by the recombination current at the emitter-base heterojunction. At moderate collector current, the calculated minority hole diffusion length well agreed with that determined from electron beam induced current measurements, indicating the current gain is dominated by the minority carrier diffusion. At large collector current, a high injection effect was observed in the current gain characteristics.