A semiconductor device includes a substrate having a channel region, a gate insulation layer on the channel region, a gate electrode on the gate insulation layer, and source and drain regions in recesses in the substrate on both sides of the channel region, respectively. The source and drain regions include a lower main layer whose bottom surface is located at level above the bottom of a recess and lower than that of the bottom surface of the gate insulation layer, and a top surface no higher than the level of the bottom surface of the gate insulation layer, and an upper main layer contacting the lower main layer and whose top surface extends to a level higher than that of the bottom surface of the gate insulation layer, and in which the lower layer has a Ge content higher than that of the upper layer.