A semiconductor device includes a substrate having a first region and a second region, insulating patterns in the substrate in the second region that define active patterns of the substrate, gate electrodes spaced apart from each other and stacked on an upper surface of the substrate and extending in a first direction, first separation regions extending in the first direction and in contact with the active patterns, second separation regions extending between the first separation regions in the first direction, and channel structures penetrating through the gate electrodes in the first region. At least one of the second separation regions is in contact with the substrate below the insulating patterns.