forming a gate insulating film on a semiconductor substrate layer of a first conductivity type;

forming a gate electrode by depositing a first polycrystalline silicon on the gate insulating film, introducing impurities to the first polycrystalline silicon, and thereafter patterning the first polycrystalline silicon;

forming a first impurity diffusion layer for a low impurity concentration drain of a second conductivity type within the semiconductor layer of the first conductivity type;

forming a second impurity diffusion layer for a high impurity concentration drain of the second conductivity type adjacent to the first impurity diffusion layer, the second impurity diffusion layer having a higher impurity concentration than the first impurity diffusion layer;

forming an interlayer insulating film on the semiconductor substrate layer;

forming a drain extension region having a high thermal conductivity on a surface of the first impurity diffusion layer;

forming a contact hole through the interlayer insulating film and up to the second impurity diffusion layer; and

depositing a wiring metal layer into the contact hole to form therein a drain electrode that is electrically connected to the second impurity diffusion layer and that is disposed apart from and not in contact with the drain extension region.