A multi-layered gate electrode of a crystalline TFT is constructed as a clad structure formed by deposition of a first gate electrode, a second gate electrode and a third gate electrode, to thereby to enhance the thermal resistance of the gate electrode. Additionally, an n-channel TFT is formed by selective doping to form a low-concentration impunty region which adjoins a channel forming region, and a sub-region overlapped by the gate electrode and a sub-region not overlapped by the gate electrode, to also mitigate a high electric field near the drain of the TFT and to simultaneously prevent the OFF current of the TFT from increasing.