Device modeling using a commercial numerical simulator has been employed for design of SiGe heterojunction bipolar transistors for high frequency, high power applications. In this study, the effects of the design of the epitaxial layer structure on power gain at X-band (10 GHz) were investigated. In particular, the base doping and width, the germanium concentration, profile and width, and the emitter and collector doping levels were investigated. Device layout issues such as emitter finger width and spacing and transistor self-heating effects were not considered. A Gaussian profile was assumed for the base doping based on a SIMS profile and the known boron out-diffusion during epitaxy. Device simulations were found to show that displacement of the collector p-n junction from its corresponding SiGe/Si heterojunction, such as arising from boron out-diffusion from the base, was found to contribute to the formation of a parasitic barrier in the conduction energy band that produced a degradation in power gain and device performance. A similar, but smaller parasitic barrier was also found to form at the emitter-base junction as a result of p-n junction displacement. The device performance was also investigated as a function of the displacement of the peak base doping from the center of the SiGe base. The simulation results show that the device can achieve significant power gain at X-band frequencies, but that the device performance achieved in practice is likely to be a sensitive function of the exact boron doping profile in the base and the extent, if any, of the displacement of the emitter and collector p-n junctions from the SiGe/Si heterojunctions at the ends of the base. (C) 2002 Elsevier Science Ltd. All rights reserved.