In a thyristor to which a semiconductor device according to the present invention is applied, at least one p-n junction is exposed at one surface of a semiconductor substrate, a polysilicon field plate is formed, via an insulating film, over the p-n junction of the semiconductor substrate and a gate region formed therein, so that the exposed portion of the p-n junction is covered by the field plate, and two specific regions of the field plate are electrically connected to the gate region and cathode region to form a gate-cathode resistance between the gate region and the cathode region. The resistance of the polysilicon field plate, inserted between the gate region and the cathode region, can be determined by the length of the polysilicon field plate therebetween and/or the concentration of an impurity in the polysilicon field plate therebetween.