Semiconductor device having metal nitride gate doped with a low work function

A method for fabricating a semiconductor device, the method comprising:

• forming a trench in a substrate;
• forming a gate dielectric layer covering sidewalls and a bottom surface of the trench;
• forming a first gate electrode gap-filling a bottom portion of the trench over the gate dielectric layer;
• forming a second gate electrode including a metal nitride which is the same as the first gate electrode over the first gate electrode;
• forming a buffer layer covering a top surface of the second gate electrode and the gate dielectric layer exposed over the second gate electrode;
• doping the second gate electrode with a low work function adjusting element;
• and
• forming a capping layer over the buffer layer to gap-fill the trench.