Semiconductor device and method of forming semiconductor device having non-uniformly doped well

A field effect transistor device, comprising:

• a semiconductor substrate:
• a well region of a first conduction type formed in said semiconductor substrate, said semiconductor substrate having source and drain regions each having a second conduction type opposite to that of said first conduction type;
• and
• a gate electrode with an underlying insulation layer, formed between said source and drain regions,
• wherein said well region has at least two portions each of which extends at least between said source and drain regions, the at least two portions including (1) a first portion, which is contiguous to said underlying insulation layer and said source and drain regions and situated under said gate electrode at a lesser depth than under said source and drain regions, and has a first net concentration of first conductive type impurities, and (2) a second portion having a substantially flat bottom which has a second net concentration higher than said first net concentration of said first conductive type impurities.