An insulated gate semiconductor device comprising an insulator substrate having provided thereon a source and a drain region; a channel region being incorporated between said source and said drain regions, said channel region comprising a polycrystalline, a single crystal, or a semi-amorphous semiconductor material; and a region provided under said channel region, said region comprising an amorphous material containing the same material as that of the channel region as the principal component, or said region comprising a material having a band gap larger than said channel region.A process for fabricating the device is also disclosed.