A semiconductor device includes a semiconductor substrate having a channel region interposed between a first active region and a second active region, and a gate structure formed on the channel region. A first dual-layer source/drain region is on the first active region and a second dual-layer source/drain region is on the second active region. The first and second dual-layer source/drain regions include stacked layers formed of different semiconductor materials. A first extension region is embedded in the first active region and a second extension region is embedded in the second active region.