Dynamic formation of a parasitic barrier to electron flow in SiGe HBTs operating at high current densities

Abstract The formation of a parasitic barrier to electron flow at the collector–base junction in SiGe heterojunction bipolar transistors during device operation at high current densities is analyzed. Due to the valence band discontinuity at the base–collector junction, hole injection into the collector at the onset of base pushout is blocked. Hole accumulation occurs at the collector end of the base inducing electron pile-up nearby in the collector and formation of a parasitic field and potential barrier to electron flow. This analysis takes into account the device physics of the electric field and carrier profiles in the vicinity of the collector junction and the closely related base pushout phenomenon to obtain an improved description of barrier formation and its dependence on device structure and current density. Also calculated is the corresponding electron buildup at the collector end of the quasi-neutral base that produces a degradation in base transport and device performance.