Methods of fabricating a semiconductor device is disclosed. An illustrated method comprises: providing a substrate including an active region and a non-active region; forming a first gate electrode including a dielectric layer pattern, a first conducting layer pattern, and an insulating layer pattern; growing a thermal oxide layer on the substrate and the first gate electrode; forming a nitride layer over the substrate and the thermal oxide layer; and removing the nitride layer and the thermal oxide layer using an etch back process to form spacers on sidewalls of the first gate electrode. By including the thermal oxide layer, the spacers ensure insulation capability.