US Patent 7812402 Semiconductor device

In the upper surface of a p− substrate, an n-type impurity region is formed. In the upper surface of the n-type impurity region, a p-well is formed. Also in the upper surface of the n-type impurity region, a p+-type source region and a p+-type drain region are formed. In the upper surface of the p-well, an n+-type drain region and an n+-type source region are formed. In the p− substrate, an n+ buried layer having an impurity concentration higher than that of the n-type impurity region is formed. The n+ buried layer is formed in contact with the bottom surface of the n-type impurity region at a greater depth than the n-type impurity region.