The method involves providing a semiconductor substrate comprising first and second regions in which different conductive metal-oxide semiconductor (MOS) transistors are to be formed. A gate dielectric layer above the semiconductor substrate sequentially forming a first metallic conductive layer and a second metallic conductive layer on and above the gate dielectric layer; covering the second region with a mask, and performing ion plantation of a first material into the first metallic conductive layer of the first region. Removing the second metallic conductive layer of the first region and forming a first gate electrode of the first region and a second gate electrode of the second region by patterning the gate dielectric layer and the first metallic conductive layer of the first region, and the gate dielectric layer, the first metallic conductive layer, and the second metallic conductive layer of the second region. The first and second regions of the semiconductor substrate having different work functions because the gate electrodes of the first and second regions have different thicknesses and at least one of the first and second gate electrodes include impurities.