A process of forming a field effect transistor (FET) of a type of high electron mobility transistor (HEMT) reducing damages caused in a semiconductor layer is disclosed. The process carries out steps of: (a) depositing an insulating film on a semiconductor stack; (b) depositing a conductive film on the insulating film; (c) forming an opening in the conductive film and the insulating film by a dry-etching using ions of reactive gas to expose a surface of the semiconductor stack; and (d) forming a gate electrode to be in contact with the surface of the semiconductor stack through the opening, the gate electrode filling the opening in the conductive film and the insulating film.