MOS CAPACITOR

PROBLEM TO BE SOLVED: To provide a MOS capacitor wherein the capacitance in the usual application range is constant, capacitance increase in the same area is possible, and a gate insulation film is hard to be damaged. SOLUTION: An N-type well 20 is formed in the part except the forming part of a protective film for protection at the time of contact for an electrode 3. A diffusion region 9 is so formed in the N-type well 20 that a range region is surrounded. In this structure, capacitance can be made constant to the change of a bias voltage by the effect of the N-type well 20 formed in the greater part of the gate region. Large capacitance also can be realized in the same area. Since a semiconductor substrate 2 having a conductivity different to the diffusion region 9 exists in the step-difference part caused by a protective film 5, the influence of a bias voltage upon a fragile part of a gate insulation film 11 is reduced by the effect of voltage drop of the potential barrier part of a PN junction, so that crack generation in the fragile part can be restrained.