Semiconductor device and method of manufacturing the semiconductor device

A method of manufacturing a semiconductor device comprising:

• forming a first amorphous silicon film on a semiconductor substrate;
• implanting an impurity to the first amorphous silicon film in an nMOS region of the semiconductor substrate;
• forming a second amorphous silicon film on top of the first amorphous silicon film;
• patterning the first and second amorphous silicon films to form a first gate electrode in the nMOS region and a second gate electrode in a pMOS region;
• polycrystallizing the first and second amorphous silicon films by performing a thermal process after the forming of the first and second gate electrodes,
• wherein grains generated by the polycrystallizing in the first amorphous silicon film of the first gate electrode have a greater particle diameter than grains generated by the polycrystallizing in the second amorphous silicon film of the first gate electrode and grains generated by the polycrystallizing in the first amorphous silicon film of the second gate electrode.