A semiconductor device includes a substrate, a gate formed over the substrate, a gate spacer provided against first and second sidewalls of the gate, and a source/drain region formed in the substrate proximate to the gate spacer. The source/drain region includes first and second epitaxial layers including Ge, wherein the second epitaxial layer which is formed over an interfacial layer between the first epitaxial layer and the substrate has a higher germanium concentration than that of the first epitaxial layer.