A semiconductor device includes a first active region and a second active region, which are disposed in a semiconductor substrate and have side surfaces facing each other, an isolation pattern disposed between the first and second active regions, a semiconductor extension layer disposed between the first and second active regions, a first source/drain semiconductor layer disposed on the first active region, and a second source/drain semiconductor layer disposed on the second active region. The facing side surfaces of the first and second active regions are closer to the semiconductor extension layer than the isolation pattern.