A semiconductor device includes an active pattern extending on a substrate in a first direction, divided into a plurality of regions by a separation region, and having a first edge portion exposed toward the separation region; first, second and third channel layers vertically separated and sequentially disposed on the active pattern; a first gate electrode extending in a second direction, intersecting the active pattern, and surrounding the first, second and third channel layers; source/drain regions disposed on the active pattern, on at least one side of the first gate electrode, and contacting the first, second and third channel layers; a semiconductor structure including first semiconductor layers and second semiconductor layers alternately stacked on the active pattern, and having a second edge portion exposed toward the separation region; and a blocking layer covering at least one of an upper surface, side surfaces, or the second edge portion, of the semiconductor structure.