A method of manufacturing a semiconductor device includes forming a gate structure on a substrate; forming a sacrificial spacer may be formed on a sidewall of the gate substrate; implanting first impurities into portions of the substrate by a first ion implantation process using the gate structure and the sacrificial spacer as ion implantation masks to form source and drain regions; removing the sacrificial spacer; and implanting second impurities and carbon atoms into portions of the substrate by a second ion implantation process using the gate structure as an ion implantation mask to form source and drain extension regions and carbon doping regions, respectively.