A semiconductor device includes a gate stack including gate electrodes stacked vertically on a substrate. Channel holes penetrate through the gate stack to extend vertically to the substrate. Each of the channel holes includes a channel region. First channel pads are each disposed at an end of a respective channel hole opposite the substrate. Each of the first channel pads includes at least one first conductivity-type impurity. Second channel pads are each disposed at an end of a respective channel hole opposite the substrate. Each of the second channel pads includes at least one second conductivity-type impurity.