A method of forming a semiconductor device includes forming an interfacial layer on a semiconductor substrate, forming a high-k dielectric on the interfacial layer, forming a barrier metal on the high-k dielectric, forming a poly-silicon layer on the barrier metal, patterning the interfacial layer, the high-k dielectric, the barrier metal and the poly-silicon to form a gate stack forming spacers, extension regions, sidewalls and source/drain regions, forming an interlayer dielectric on the gate stack, etching off a portion of the interlayer dielectric to expose the poly-silicon layer, forming an impurity metal layer, which includes an impurity metal having a barrier effect to the diffusive material, and a metal layer including a diffusive material, on the poly-silicon layer and converting the poly-Si layer into a silicide containing the impurity metal. The barrier metal includes a titanium nitride (TiN) or a tantalum nitride (TaN).