The present disclosure relates to semiconductor structures and, more particularly, to a heterojunction bipolar transistor having an emitter base junction with a silicon-oxygen lattice interface and methods of manufacture. The device includes: a collector region buried in a substrate; shallow trench isolation regions, which isolate the collector region buried in the substrate; a base region on the substrate and over the collector region; an emitter region composed of a single crystalline of semiconductor material and located over with the base region; and an oxide interface at a junction of the emitter region and the base region.