US Patent 7288816 Semiconductor device

According to a semiconductor device of an embodiment of the present invention, a P-type buried diffusion layer is formed across a substrate and an epitaxial layer. An N-type buried diffusion layer is formed in the P-type buried diffusion layer. An overvoltage protective PN junction region is formed below an element formation region. A breakdown voltage of the PN junction region is lower than a source-drain breakdown voltage. This structure prevents a breakdown current from concentratedly flowing into the PN junction region and protects the semiconductor device from overvoltage.