A semiconductor device including a substrate, channels, a gate stack, and a pad separating region. The substrate has a pad region adjacent to a cell region. The channels extend in a direction crossing an upper surface of the substrate in the cell region. The gate stack includes a plurality of gate electrode layers spaced apart from each other on the substrate and enclosing the channels in the cell region. The pad separating region separates the gate stack into two or more regions in the pad region. The gate electrode layers have different lengths in the pad region.