A diode which includes a first region formed in a polycrystalline silicon layer formed on a substrate. The diode has a predetermined width W and is one of an intrinsic region and a region including impurities at a low concentration therein, a second region and a third region including P-type impurities and N-type impurities therein respectively and both being oppositely arranged from each other with the first region therebetween in the polycrystalline silicon layer. Electrodes are electrically connected to the second region and the third region respectively, and further the film characteristic of the polycrystalline silicon layer and the predetermined width W thereof are determined in such a manner as to fulfill the following equation:W.sub.D .ltoreq.W.ltoreq.LL represents a carrier diffusion length and W.sub.D represents a width of the depletion layer created in the polycrystalline silicon layer when the voltage corresponding to the withstand voltage required by the polycrystalline diode as mentioned above, is applied thereto.