Simulation analyses of carrier dynamics in npn-type GaN-HBTs with different-hole-concentration p-base layers

The device operations of npn-type GaN-based heterojunction bipolar transistors (HBTs) with different-hole-concentration p-base layers were analyzed via the device simulation. The HBTs with a low-hole-concentration (3.0 × 1017 cm−3) p-base layer exhibited anomalous current behavior. The carrier dynamics analyses indicated that enormous hole currents generated via the following steps. First, even at a low base current injection, the depletion layer occupied most of the p-base layer and caused a kind of the punch-through phenomenon. Then, with the increase in the base current injection, large hole currents generated just around the underneath of the emitter layer. Furthermore, the above enormous hole currents were found to be enhanced with the increase in the energy bandgap offset between the emitter and the base layers.