The formation and characteristics of a parasitic conduction band barrier located at a SiGe/Si heterojunction have been investigated using a commercial numerical simulator and a simple, three-region model of a heterojunction with a nearby p-n junction. The barrier's formation is examined as a function of the displacement of the p-n junction from the heterojunction, but also found to depend on the germanium concentration, junction doping and the applied bias. The phenomenon is of interest for understanding the performance of SiGe/Si heterojunction bipolar transistors, where the p-n junction is intentionally displaced from the heterojunction at either the emitter or collector junctions or where boron outdiffusion from the base produces p-n junction displacement. The barrier is found to scale with the germanium mole fraction and to be significantly larger when the heterojunction is displaced into the p-side of the p-n junction. Beyond some minimum separation of the junctions, the barrier height rises with junction displacement and saturates. For a given displacement, the barrier's height can be suppressed with reverse bias or enhanced by forward bias of the p-n junction. The results of the numerical simulations are compared with those from a simple analytical model as an aid in understanding the barrier's formation and characteristics. (C) 2002 Elsevier Science Ltd. All rights reserved.