Bipolar transistors with multilayer collectors

A semiconductor device and fabrication method are described for manufacturing a heterojunction bipolar transistor by forming a silicon collector region in a substrate which includes a lower collector layer, a dopant diffusion barrier layer, and an upper collector layer, where the formation of the dopant diffusion barrier layer reduces diffusion of dopants from the lower collector layer into the upper collector layer during one or more subsequent manufacturing steps which are used to form a trench isolation region in the substrate along with a heterogeneous base region and a silicon emitter region.