The present disclosure relates to semiconductor structures and, more particularly, to heterojunction bipolar transistors and methods of manufacture. The structure includes: a collector in a semiconductor substrate; a subcollector in the semiconductor substrate; an intrinsic base over the subcollector; an extrinsic base adjacent to the intrinsic base; an emitter over the intrinsic base; and an isolation structure between the extrinsic base and the emitter and which overlaps the subcollector.