Semiconductor device and method for manufacturing the same

A semiconductor device that is a MOS transistor, said semiconductor device comprising:

• a semiconductor substrate;
• a semiconductor layer of a first conductivity type formed above said semiconductor substrate;
• and
• a gate electrode formed above said semiconductor layer such that a gate insulating film is placed between said gate electrode and said semiconductor layer,
• wherein said semiconductor layer has:
• a body region of the first conductivity type;
• a source region of a second conductivity type formed in said body region to be exposed at a surface of said semiconductor layer;
• a drain offset region of the second conductivity type;
• a drain contact region of the second conductivity type formed in said drain offset region to be exposed at the surface of said semiconductor layer;
• a drain buffer region of the second conductivity type formed between said drain offset region and said body region to adjoin said drain offset region and said body region, and
• an impurity region of the first conductivity type formed immediately under said drain buffer region;
• wherein said gate insulating film includes:
• a thin film part positioned at a side of said source region;
• and
• a thick film part positioned at a side of said drain contact region and having a tapered section with an end connected to said thin film part, and
• wherein said drain buffer region is in contact with said end of said thick film part so as to cover a bottom of said tapered section, and has a depth that is less than a depth of said drain offset region;
• and
• wherein said drain offset region is in contact with a bottom of said thick film part so as to cover said bottom.