A semiconductor device includes an active region on a substrate, a gate structure on the substrate and intersecting the active region, a source/drain region on the active region on both sides of the gate structure and including silicon (Si), and a contact structure on the source/drain region. The source/drain region includes a shallow doping region doped with germanium (Ge) and is in an upper region including an upper surface of the source/drain region. A concentration of germanium (Ge) in the shallow doping region gradually decreases from the upper surface of the source/drain region toward an upper surface of the substrate in a direction that is perpendicular to an upper surface of the substrate.