A method of manufacturing a semiconductor device, the semiconductor device including a MOS transistor, a source electrode and a drain electrode on the MOS transistor each include a first carbon doped silicon layer including carbon at a first carbon concentration and phosphorus at a first phosphorus concentration and a second carbon doped silicon layer over the first silicon carbide layer, which includes phosphorus at a second phosphorus concentration higher than the first phosphorus concentration, and which includes carbon at a second carbon concentration less than or equal to the first carbon concentration.