A method for fabricating a semiconductor device comprises forming a fin in a layer of III-V compound semiconductor material on a silicon-on-insulator substrate; forming a semiconductor extension on the fin, the semiconductor extension comprising a III-V compound semiconductor material that is different from a material forming the fin in the III-V compound semiconductor layer; forming a dummy gate structure and a spacer across and perpendicular to the fin; forming a source/drain layer on a top surface of the substrate adjacent to the dummy gate structure; planarizing the source/drain layer; removing the dummy gate structure to expose a portion of the semiconductor extension on the fin; removing the exposed portion of the semiconductor extension; etching the semiconductor extension to undercut the spacer; and forming a replacement gate structure in place of the removed dummy gate structure and removed exposed portion of the semiconductor extension.