A semiconductor device includes a gate stack structure. The gate stack structure includes an interfacial layer formed on a semiconductor substrate, a high-k dielectric formed on the interfacial layer, a silicide gate including a diffusive material and an impurity metal, and formed over the high-k dielectric, and a barrier metal with a barrier effect to the diffusive material, and formed between the high-k dielectric and the metal gate. The impurity metal has a barrier effect to the diffusive material so that the diffusive material in the silicide gate can be prevented from being introduced into the high-k dielectric.