A GaAs/Si heterojunction bipolar transistor (HBT) structure is proposed having application for high‐frequency operation. The structure combines the high‐frequency capability of the GaAs/AlGaAs system with the advanced processing technology of Si. The proposed device consists of an n‐AlGaAs/p‐GaAs emitter and base layers on an n‐Si collector with improved junction characteristics at the GaAs/Si heterointerface afforded by thermal annealing. This novel device structure combines the advantages associated with a wide band‐gap AlGaAs emitter, the high electron mobility of GaAs, and the substantial reduction in device parasitics accorded the self‐aligned structure. Additionally, the proposed device offers the possibility of planar GaAs processing. With the use of a compact transistor model, calculations of the high‐speed capability of this transistor are presented. For an emitter‐base junction area of 1 μm×5 μm, optimized fmax=108 GHz and fmax=ft=89 GHz were computed for the GaAs/Si HBT, compared to 76 and 62 GHz, respectively, for equivalent GaAs/AlGaAs HBT’s.