Semiconductor device and method for fabricating the same

A semiconductor device, comprising:

• a first gate insulating film formed on a first nMOS transistor region in a semiconductor substrate and containing nitrogen;
• a second gate insulating film formed on a first pMOS transistor region in the semiconductor substrate and containing nitrogen;
• a third gate insulating film formed on a second nMOS transistor region in the semiconductor substrate and containing nitrogen;
• and
• a fourth gate insulating film formed on a second pMOS transistor region in the semiconductor substrate and containing nitrogen,
• wherein each of the third gate insulating film and the fourth gate insulating film has a thickness smaller than that of each of the first gate insulating film and the second gate insulating film,
• the first gate insulating film has a nitrogen concentration peak at the interface between the first gate insulating film and the semiconductor substrate, and
• each of the second, third and fourth gate insulating films has a nitrogen concentration peak only near an associated one of gate electrodes formed on the second, third and fourth gate insulating films, respectively.