AlGaN/SiC Heterojunction Bipolar Transistors Featuring AlN/GaN Short-Period Superlattice Emitter

Growth and electrical characterization of aluminum gallium nitride (AlGaN)/SiC heterojunction bipolar transistors (HBTs) featuring AlN/GaN short-period superlattice as a quasi-AlGaN emitter are presented. The AlN/GaN superlattice emitter was grown by molecular beam epitaxy on off-axis SiC, which showed adequate structural and electronic properties as the emitter of the HBTs. We investigated the impact of Al composition in the emitter on the transport characteristics and current gain of the HBTs. Using Al composition of over 0.5, we achieved type-I band alignment in AlGaN/SiC, and suppressed the tunneling current via interface traps, resulting in an improved current gain of up to 2.7. Toward further improvement of current gain, we also investigated the effect of n-SiC spacer between n-AlGaN and p-SiC and p-SiC base width. Using 200-nm-thick n-SiC spacer and 250-nm-thick p-SiC base layer, we achieved an improved current gain of 13 owing to the reduced interface and bulk recombination.