A semiconductor device, in which a cell array region and an extension region are arranged along a first direction, and in which contact regions and through regions are alternately arranged along the first direction in the extension region, including: a mold structure including a plurality of first insulating patterns and a plurality of gate electrodes, which are alternately stacked on a first substrate; a channel structure penetrating the mold structure in the cell array region to intersect the plurality of gate electrodes; respective gate contacts that are on the mold structure in the contact regions and are connected to each of the gate electrodes; and a plurality of second insulating patterns, the second insulating patterns being stacked alternately with the first insulating patterns in the mold structure in the through regions, the plurality of second insulating patterns including a different material from the plurality of first insulating patterns.