Semiconductor device having a protective circuit

A semiconductor device comprising a semiconductor substrate having a substrate region of a first conductivity type or a second conductivity type opposite to said first conductivity type, a first well region of said first conductivity type formed on a surface region of said semiconductor substrate and having a first impurity concentration, said first well region receiving therein at least one MOS transistor, a guard ring of said first conductivity type disposed on a surface region of said semiconductor substrate and within a second well region, said at least one MOS transistor having source/drain regions of said second conductivity type and surrounded by said first well region, and a diffused region formed on the substrate region and disposed between said source/drain regions of said at least one MOS transistor and said guard ring, said diffused region being of said first conductivity type implemented by a portion of said substrate and having a second impurity concentration lower than said first concentration.