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High-speed InGaP/GaAs heterojunction bipolar transistors (HBT's) with a small emitter area are described. WSi is used as the base electrode to fabricate HBT's with a narrow base contact width and a buried SiO2 structure. An HBT with an emitter area of 0.8×5 μm exhibited an fT of 105 GHz and an fmax of 120 GHz. These high values are obtained due to the reduction of C/sub BC/ by using buried SiO2 with a narrow base contact width, indicating the great potential of GaAs HBT's for high-speed and low-power circuit applications.