Analysis of parasitic barriers formed at SiGe/Si heterojunctions due to PN junction displacement

The effects of displacement of a pn junction from its corresponding SiGe/Si heterojunction have been investigated using a simple analytical model. The phenomenon is of interest for understanding the degradation in the performance of SiGe/Si heterojunction bipolar transistors when there is boron outdiffusion from the base that can produce pn junction displacement at both the emitter and collector-base junctions. This analysis describes the formation of parasitic barriers in the conduction band and their dependence on the device structure, pn junction displacement and bias. The barrier is found to be significantly larger when the heterojunction is displaced into the p-side of the pn junction.