The effects of high carrier densities near the base-collector (B-C) heterojunction in npn SiGe heterojunction bipolar transistors during device operation at high current densities has been investigated using a commercial numerical device simulator. Due to electron velocity saturation in the B-C space charge region and the presence of the valence band discontinuity at the B-C junction, hole accumulation occurs at the collector end of the base at the onset of base pushout at high current densities. Formation of a parasitic barrier to electron flow in the conduction band occurs at the collector junction which increases recombination in the base and the base current and produces saturation in the collector current. Together, these effects produce an abrupt degradation in the trransistor's current gain and cutoff frequency with increasing emitter junction bias. In this paper, we investigate the onset and relationship of parasitic barrier formation and base pushout, and their dependence on device structure and biasing.