We fabricated Pnp AlGaNGaN heterojunction bipolar transistors (HBTs) with various base widths WB and investigated their common-emitter current-voltage characteristics at room temperature to clarify their carrier transport mechanisms. The current gain Β increased as WB decreased. The maximum current gain Βmax was 40 in a HBT with a WB of 30 nm. HBTs with different base widths exhibited almost the same tendency for Β to increase with increasing the collector current IC, indicating that the carrier transport mechanism is the same in all the n-GaN base layers. With a low IC, recombination in the emitter-base depletion region is the dominant carrier transport mechanism. Β was less affected when IC was high, and the carrier transport was dominated by the minority hole diffusion in the neutral base layer. The minority hole diffusion length obtained from the HBT characteristics agrees well with previous results obtained with electron beam induced current measurements, also indicating that Βmax was determined by the minority hole diffusion length in the n-GaN base layer.
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