AP‐n‐pAlGaAs/InGaAs/GaAs collector‐up heterojunction bipolar transistor (C‐up HBT), grown by three‐stage molecular beam epitaxy, has been fabricated, and its dc and high‐frequency performances have been evaluated. The use of a graded InxGa1−xAs (x=0.0–0.09) is shown to improve the common‐emitter current gain (&bgr;) and to greatly reduce the base transit time (&tgr;b) for theP‐n‐pC‐up HBTs. A maximum current gain (&bgr;) of 150 was measured for a 16×17 &mgr;m2device. From S‐parameter measurements, a best unity‐gain cutoff frequencyfT=43 GHz at a collector current of −10 mA was achieved using a 5×10 &mgr;m2collector area. The results show that theP‐n‐pC‐up HBTs may be useful for future planar integrated circuit applications. ©1995 American Institute of Physics.