The invention provides a heterojunction bipolar transistor which has a low reistance SiGe base and is high in current gain and cutoff frequency even at low temperatures near the liquid nitrogen temperature. The transistor fabrication process comprises forming an n-type collector layer on a silicon substrate and a dielectric film on the collector layer, forming a base electrode of p.sup.+ -type polysilicon having an opening on the dielectric film, isotropically etching the dielectric film on the collector layer by using the opening of the base electrode to form a window, forming an external base layer of p.sup.+ -type silicon on the collector layer exposed by the window, selectively etching the external base layer to form an aperture in a central region, forming a p-type SiGe intrinsic base layer in the aperture of the external base layer and then forming an n.sup.+ -type emitter on the intrinsic base layer.