A semiconductor device includes a peripheral circuit structure including peripheral circuits on a substrate and first bonding pads electrically connected to the peripheral circuits and a cell array structure including memory cells on a semiconductor layer and second bonding pads electrically connected to the memory cells and bonded to the first bonding pads. The cell array structure includes a stacked structure including insulating layers and electrodes, an external connection pad on a surface of the semiconductor layer, a dummy pattern at a same level as the semiconductor layer relative to the substrate, and a photosensitive insulating layer on the semiconductor layer and the dummy pattern. A first thickness of a portion of the photosensitive insulating layer vertically overlapping the external connection pad is greater than a second thickness of another portion of the photosensitive insulating layer vertically overlapping the dummy pattern.