Semiconductor integrated circuit device and method of manufacturing thereof

A semiconductor integrated circuit device wherein plural field effect transistors having different threshold values are integrated on one chip by forming plural gate electrodes of silicon-germanium mixed crystals having different germanium contents. By varying the germanium content of the gate electrode material, a work function with respect to the channel region can be varied, so a semiconductor integrated circuit device wherein plural field effect transistors having different threshold voltage values are integrated on one chip can be manufactured.