A semiconductor device includes a stack structure on a substrate, the stack structure including alternating gate electrodes and insulating layers stacked along a first direction, a vertical opening through the stack structure along the first direction, the vertical opening including a channel structure having a semiconductor layer on an inner sidewall of the vertical opening, and a variable resistive material on the semiconductor layer, a vacancy concentration in the variable resistive material varies along its width to have a higher concentration closer to a center of the channel structure than to the semiconductor layer, and an impurity region on the substrate, the semiconductor layer contacting the impurity region at a bottom of the channel structure.