Enhancement of base conductivity via the piezoelectric effect in AlGaN/GaN HBTs

Abstract

Large polarization effects in nitride-based heterostructures provide opportunities for controllably introducing negative charge, equivalent to acceptor-doping, in AlGaN/GaN HBT structures. This paper reviews evidence for polarization-based doping effects in nitride-based materials, and discusses potential applications of these effects for the improvement of p-contacts and p-type base conductivity. It is shown that in conventional HBTs grown with Ga-terminated faces in the emitter-up configuration, the polarization-induced charges detract from the acceptor doping of the base. In contrast, the polarization effects add to effective doping in the base in structures that are grown and processed as collector-up devices, or in devices that are grown emitter-up and utilize the transferred substrate approach.