In another exemplary aspect, the present disclosure is directed to a method of forming a field effect transistor (FET). The method includes providing a semiconductor substrate and a fin extending from the semiconductor substrate and through an isolation structure covering the semiconductor substrate, the fin including a first semiconductor material and having a source region, a channel region, and a drain region for the FET; forming a gate stack over the channel region; performing an oxidization process on top and sidewall surfaces of the fin in the source and drain regions; performing an etching process on the oxidized top and sidewall surfaces of the fin in the source and drain regions; repeating the performing of the oxidization process and the performing of the etching process until a width of the fin in the source and drain regions is reduced by a predetermined value; and epitaxially growing a material layer covering the fin in the source and drain regions. In an embodiment, the etching process includes wet etching. In an embodiment, the performing of the etching process also removes a top portion of the isolation structure. In an embodiment, a bottom portion of the fin is covered by a liner film, and wherein after the width of the fin in the source and drain regions is reduced by the predetermined value, a portion of the liner film is above the isolation structure. In an embodiment, the isolation structure is a shallow trench isolation (STI) feature. In an embodiment, a portion of the material layer extends below the gate stack.