A semiconductor device includes a substrate including a lower horizontal layer and an upper horizontal layer and having a cell array region and a connection region, an electrode structure including electrodes, which are stacked above the substrate, and which extend from the cell array region to the connection region, a vertical channel structure on the cell array region that penetrates the electrode structure and is connected to the substrate, and a separation structure on the connection region that penetrates the electrode structure. The lower horizontal layer has a first top surface in contact with a first portion of the separation structure, and a second top surface in contact with a second portion of the separation structure, and an inflection point at which a height of the lower horizontal layer is abruptly changed between the first top surface and the second top surface.