A semiconductor device includes a substrate having an active region extending in a first direction; a gate structure disposed on the substrate, intersecting the active region, and extending in a second direction; channel layers disposed on the active region to be spaced apart from each other in a third direction, perpendicular to an upper surface of the substrate, and to be surrounded by the gate structure; source/drain regions disposed on both sides of the gate structure and connected to the channel layers; air gap regions located between the source/drain regions and the active region and spaced apart from each other in the third direction; and semiconductor layers alternately disposed with the air gap regions in the third direction and defining the air gap regions, wherein lower ends of the source/drain regions are located on a level lower than an uppermost air gap region.