A method of forming a heterojunction bipolar transistor by depositing a first stack comprising an polysilicon layer and a sacrificial layer on a mono-crystalline silicon substrate surface; patterning that stack to form a trench extending to the substrate; depositing a silicon layer over the resultant structure; depositing a silicon-germanium-carbon layer over the resultant structure; selectively removing the silicon-germanium-carbon layer from the sidewalls of the trench; depositing a boron-doped silicon-germanium-carbon layer over the resultant structure; depositing a further silicon-germanium-carbon layer over the resultant structure; depositing a boron-doped further silicon layer over the resultant structure; forming dielectric spacers on the trench sidewalls; filling the trench with emitter material; exposing polysilicon regions outside the trench side walls by selectively removing the sacrificial layer; implanting boron impurities into the exposed polysilicon regions to define base implants; and exposing the resultant structure to a thermal budget for annealing the boron impurities.