Semiconductor device having element isolation region

A method for manufacturing a semiconductor device, comprising the steps of:

• forming a buried diffusion layer of a second conductivity type in the surface layer of a prescribed area of a substrate of a first conductivity type;
• forming a first high-concentration isolation diffusion layer of the first conductivity type in the substrate so as to surround the buried diffusion layer;
• forming an epitaxial layer of the second conductivity type on the substrate, the buried diffusion layer, and the first high-concentration isolation diffusion layer;
• forming a low-concentration isolation diffusion layer of the first conductivity type in the epitaxial layer on the first high-concentration isolation diffusion layer, the low-concentration isolation diffusion layer isolating the epitaxial layer into a plurality of island regions;
• forming a second high-concentration isolation diffusion layer of the first conductivity type in the junction area between the first high-concentration isolation diffusion layer and the low-concentration isolation diffusion layer;
• and
• forming a high-concentration diffusion layer of the first conductivity type in direct physical contact with an upper layer of said buried diffusion layer.