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 . <EOS>