A method for forming a heterojunction bipolar transistor, the method comprising: forming a doped region in a group IV semiconductor layer of a substrate, forming an epitaxially grown III-V semiconductor body on a surface portion of the doped region, said body extending from the surface portion and protruding vertically above the doped region, wherein said doped region and said body form a first sub-collector part and a second sub-collector part, respectively, and forming an epitaxially grown III-V semiconductor layer stack on said body, the layer stack comprising a collector, a base and an emitter; wherein said doped region comprises a semiconductor fin, the fin having an upper surface defining an upper surface of the doped region, and the method further comprises: embedding the semiconductor fin in an insulating layer, opening the insulating layer to expose the upper surface of the fin, thereby forming an upper portion of an aperture, and etching back the upper surface of the fin, thereby forming a lower portion of said aperture and an etched-back surface portion of the doped region, wherein said upper portion of said aperture is wider than said lower portion of said aperture, wherein said body is formed on the etched-back surface portion of the doped region and comprises a lower portion formed in the lower portion of the aperture and a wider upper portion formed in the upper portion of the aperture. forming a doped region in a group IV semiconductor layer of a substrate, forming an epitaxially grown III-V semiconductor body on a surface portion of the doped region, said body extending from the surface portion and protruding vertically above the doped region, wherein said doped region and said body form a first sub-collector part and a second sub-collector part, respectively, and forming an epitaxially grown III-V semiconductor layer stack on said body, the layer stack comprising a collector, a base and an emitter; wherein said doped region comprises a semiconductor fin, the fin having an upper surface defining an upper surface of the doped region, and the method further comprises: embedding the semiconductor fin in an insulating layer, opening the insulating layer to expose the upper surface of the fin, thereby forming an upper portion of an aperture, and etching back the upper surface of the fin, thereby forming a lower portion of said aperture and an etched-back surface portion of the doped region, wherein said upper portion of said aperture is wider than said lower portion of said aperture, wherein said body is formed on the etched-back surface portion of the doped region and comprises a lower portion formed in the lower portion of the aperture and a wider upper portion formed in the upper portion of the aperture. embedding the semiconductor fin in an insulating layer, opening the insulating layer to expose the upper surface of the fin, thereby forming an upper portion of an aperture, and etching back the upper surface of the fin, thereby forming a lower portion of said aperture and an etched-back surface portion of the doped region, wherein said upper portion of said aperture is wider than said lower portion of said aperture, wherein said body is formed on the etched-back surface portion of the doped region and comprises a lower portion formed in the lower portion of the aperture and a wider upper portion formed in the upper portion of the aperture.