Semiconductor devices and fabrication methods are provided, in which metal transistor gates are provided for MOS transistors. Metal nitride is formed above a gate dielectric. A lanthaide series metal is implanted into the metal screen layer above the gate dielectric. The lanthaide metal is contained in the screen layer or at the interface between the screen metal layer and the gate dielectric. This process provides adjustment of the gate electrode work function, thereby tuning the threshold voltage of the resulting PMOS or NMOS transistors.