A method for isolating a semiconductor device is disclosed including the steps of sequentially growing a plurality of material layers on a semiconductor substrate, etching the material layers down to a predetermined depth of the substrate to thereby define an active region, forming a semi-insulating film on the exposed semiconductor substrate in order to planarize the step-difference of the active region and the isolation region, and then, forming an ohmic metal layer on a space where the semi-insulating film is regrown.