A semiconductor device includes first, second and isolation regions; a first insulating film and gate electrode formed over the first region; a second insulating film and gate electrode formed over the second region; a first sidewall formed on a side of the first gate electrode and a second sidewall formed on a side of the second gate electrode; first source and drain regions formed adjacent opposite sides of the first gate electrode; second source region adjacent to the one side of the first gate electrode and overlapping the first source region, an impurity concentration of the second source region being different from an impurity of the first source region; a second drain region overlapping the first drain region and overlapping the first gate electrode; and a metal silicide formed on the first source region and the first drain region.