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Graded-base AlGaAs/InGaAs collector-up heterojunction bipolar transistors (C-up HBTs) were successfully fabricated using a novel selective area regrowth process to reduce the base resistance and their dc and microwave performances were evaluated. The base is compositionally graded to provide a quasi-built-in field which decreases the base transit time for high-frequency response and increases the base transport factor at low-temperature operation. A unity-gain cutoff frequency fT=55 GHz and a maximum frequency of oscillation fmax=74 GHz for the C-up n-p-n HBT, and an fT=48 GHz and an fmax= 39 GHz for the C-up p-n-p HBT were obtained for devices with a 5-μm×10-μm collector area. The nonself-aligned C-up HBT's reported here show great promise for future high-speed C-up complementary bipolar IC's.