A semiconductor device includes a fin structure on a substrate and extending in a first direction, a gate electrode crossing over the fin structure, source/drain regions on the fin structure at opposite sides of the gate electrode, and a barrier layer between the fin structure and each of the source/drain regions. The fin structure includes a material having a lattice constant different from that of the substrate, the fin structure, the source/drain regions, and the barrier layer include germanium, and a germanium concentration in the barrier layer is greater than that in the fin structure and less than a maximum germanium concentration in each of the source/drain regions.