Patent ID: 7056797

Claim:
A method of manufacturing a semiconductor device comprising: implanting an impurity of a first conductive type through a substrate surface located above first and second drain formation regions of a semiconductor substrate of a second conductive type, wherein the implantation is a single implantation; diffusing the implanted impurity in the substrate by providing a first gate insulation film on the semiconductor substrate by applying a heat treatment, so as to form a first drain region partially under the first gate insulation film and a second drain region adjacent to and above the first drain region, wherein said first drain region is formed in the first drain formation region and has a lower impurity concentration than the second drain region, which is formed in the second drain formation region, wherein the first and second drain regions are formed by a single step of implanting the impurity and a single step of forming the first gate insulation by applying heat treatment; providing a second gate insulation film on the semiconductor substrate except where the first gate insulation film is disposed; providing a gate electrode that spans from the first gate insulation film to the second gate insulation film; providing a source region of the first conductive type disposed proximally to one end of said gate electrode; and providing a third drain region of the first conductive type disposed distally from the other end of said gate electrode and disposed in said second drain region wherein the third drain region is surrounded by the second drain region.