Abstract: Disclosed is a method for fabricating a transistor of a semiconductor device, the method comprising the steps of: providing a semiconductor; forming a gate electrode; performing a low-density ion implantation process with respect to the substrate, thereby forming an LDD ion implantation layer; forming an insulation spacer on a sidewall of the gate electrode; forming a diffusion barrier; performing a high-density ion implantation process with respect to the substrate, thereby forming a source/drain; performing a first thermal treatment process with respect to a resultant structure, so as to activate impurities in the source/drain, and simultaneously causing a diffusion velocity of the impurities in the source/drain to be reduced by the diffusion barrier; and forming a salicide layer.