A method for fabricating a semiconductor device includes providing a substrate including a cell region and a core/peripheral region around the cell region, forming a gate insulating film on the substrate of the core/peripheral region, forming a first conductive film of a first conductive type on the gate insulating film, forming a diffusion blocking film within the first conductive film, the diffusion blocking film being spaced apart from the gate insulating film in a vertical direction, after forming the diffusion blocking film, forming an impurity pattern including impurities within the first conductive film, diffusing the impurities through a heat treatment process to form a second conductive film of a second conductive type and forming a metal gate electrode on the second conductive film, wherein the diffusion blocking film includes helium (He) and/or argon (Ar).