A semiconductor device includes a substrate, a plurality of word line structures disposed over the substrate to be spaced apart from each other in a first direction perpendicular to a surface of the substrate. Each of the plurality of word line structures extends in a second direction parallel to the surface of the substrate. In addition, the semiconductor device includes a switching layer disposed over the substrate to contact side surfaces of the plurality of word line structures, and bit line structures disposed over the substrate to extend in the first direction and to contact a surface of the switching layer. The switching layer is configured to perform a threshold switching operation, and has a variable programmable threshold voltage.