Patent ID: 8592920

Claim:
A semiconductor device comprising: a drain region of a field-effect transistor formed over a semiconductor substrate; a channel region of the field-effect transistor formed over the drain region; a source region of the field-effect transistor formed over the channel region; a trench reaching the drain region from an upper surface of the source region; a first insulating film formed in the trench and disposed at a lower part of the trench; a first gate electrode of the field-effect transistor formed over the first insulating film in the trench and disposed at the lower part of the trench; a second insulating film formed in the trench and disposed at a upper part of the trench; and a second gate electrode of the field-effect transistor formed over the second insulating film in the trench and disposed at the upper part of the trench, wherein the first gate electrode includes a first poly silicon film, wherein the second gate electrode includes a second poly silicon film, and wherein an impurity concentration of the first poly silicon film is lower than an impurity concentration of the second poly silicon film.