A semiconductor device includes a peripheral circuit structure including a lower substrate, a plurality of circuits formed on the lower substrate, and a plurality of wiring layers connected to the plurality of circuits, an upper substrate covering the peripheral circuit structure and including a through opening, a memory stack structure including a plurality of gate lines, a memory cell contact passing through at least one of the plurality of gate lines to contact one gate line from among the plurality of gate lines, the memory cell contact extending to the peripheral circuit structure through the through opening and being configured to be electrically connected to a first wiring layer from among the plurality of wiring layers, and a plurality of dummy channel structures passing through at least one of the plurality of gate lines to extend to the peripheral circuit structure through the through opening.