A vertical semiconductor device includes a plurality of channel connection patterns, a lower insulation layer, a supporting layer, a stacked structure, and a channel structure. The channel connection patterns, on which the lower insulation layer is formed, contact a substrate. The supporting layer is formed on the lower insulation layer to be spaced apart from the channel connection patterns, and includes polysilicon doped with impurities. The stacked structure is formed on the supporting layer, and includes insulation layers and gate electrodes to form a memory cell string. The channel structure passes through the stacked structure, the supporting layer and the lower insulation layer, and includes a charge storage structure and a channel which contacts the channel connection patterns. The charge storage structure and the channel face the gate electrodes and the supporting layer. The supporting layer serves as a gate of a gate induced drain leakage (GIDL) transistor.