PURPOSE: To realize the safety of a protective diode by a method wherein a highly resistive low-impurity-concentration region of one conductivity type and one pair of high-impurity-concentration regions, of the other conductivity type, which form a diode junction to it so as to sandwich it from both sides are formed and the high- impurity-concentration regions are connected to a semiconductor device whose overvoltage is to be protected.

CONSTITUTION: The central part of a polycrystalline silicon film 20 is used as a p-type low-impurity-concentration region 21; parts on both ends are used as n-type high- impurity-concentration regions 22, 23. The polycrystalline silicon film 20 for a protective diode is arranged and installed simultaneously with a polycrystalline silicon film for a gate 3; after that, an interlayer insulating film 7 composed of phosphosilicate glass or the like is applied to the whole surface; windows are opened in required parts; prescribed interconnections are executed by using an aluminum interconnection film 8; a source terminal S, a drain terminal D and a gate terminal G for a field-effect transistor 10 are led out. At this time, the high-impurityconcentration region 23 of the polycrystalline silicon film 20 for the protective diode is connected to the source terminal S via an extended part 3a at an interconnection film 3.