Semiconductor device and method of manufacturing the same

A semiconductor device comprising:

• a first semiconductor region formed in a semiconductor substrate and having a first conductivity type due to first-conductivity-type active impurities contained in the first semiconductor region, said first-conductivity-type active impurities contained in the first semiconductor region being configured to have a concentration profile that decreases toward the surface of the semiconductor substrate and includes a part wherein said concentration profile decreases more than 10% per 3 nm;
• a second semiconductor region formed between the first semiconductor region and the surface of the semiconductor substrate and having a second conductivity type due to second-conductivity-type active impurities contained in the second semiconductor region, the second semiconductor region also containing first-conductivity-type active impurities whose minimum concentration is substantially zero or smaller than a quarter of a concentration of the second-conductivity-type active impurities contained in the second semiconductor region, said second-conductivity-type active impurities being configured to have a concentration at the bottom of the second semiconductor region being smaller than half a maximum concentration of the first-conductivity-type impurities contained in a depletion layer of the first semiconductor region formed during the operation of the semiconductor device, and configured to a maximum concentration being smaller than twice of a minimum concentration of the second-conductivity-type active impurities between the surface of the semiconductor substrate and the bottom of the depletion layer of the first semiconductor region during the operation of the semiconductor device;
• wherein the bottom of said second semiconductor region is at a depth of about 20-30 nm from the surface of said semiconductor substrate;
• an insulating film formed on the surface of the semiconductor substrate over the second semiconductor region;
• a conductor formed on the insulating film, and having work function close to mid-gap of the semiconductor substrate;
• a third semiconductor region of the second conductivity type formed in contact with the second semiconductor region;
• and
• a fourth semiconductor region of the second conductivity type formed in contact with the second semiconductor region.