A semiconductor device includes a first substrate structure including a first substrate, gate electrodes stacked on the first substrate, and extended by different lengths to provide contact regions, cell contact plugs connected to the gate electrodes in the contact regions, and first bonding pads disposed on the cell contact plugs to be electrically connected to the cell contact plugs, respectively, and a second substrate structure, connected to the first substrate structure on the first substrate structure, and including a second substrate, circuit elements disposed on the second substrate, and a second bonding pad bonded to the first bonding pads, wherein, the contact regions include first regions having a first width and second regions, of which at least a portion overlaps the first bonding pads, and which have a second width greater than the first width, and the second width is greater than a width of the at least one first bonding pad.