A semiconductor device may include active fins spaced apart from each other by a recess therebetween, each of the active fins protruding from an upper surface of a substrate, an isolation structure including a liner on a lower surface and a sidewall of a lower portion of the recess and a blocking pattern on the liner, the blocking pattern filling a remaining portion of the lower portion of the recess and including a nitride, a carbide or polysilicon, a gate electrode structure on the active fins and the isolation structure, and a source/drain layer on a portion of each of the active fins adjacent to the gate electrode structure.