patterning a deposited insulating film on a bottom silicon substrate of a SOI (Silicon On Insulator) semiconductor substrate to form an insulating film pattern;

forming an epitaxial layer on the bottom silicon substrate and the insulating film pattern;

forming a hard mask pattern over the epitaxial layer;

etching the epitaxial layer with the hard mask pattern as an etching mask to form a device isolation trench and a recess;

forming an oxide film layer for the filling the trench and the recess;

removing the hard mask pattern to perform an ion-implanting process on the active region;

removing the oxide film layer filled in the recess; and

forming a gate over the recess.