A memory device is provided. The memory device includes a variable resistance layer. A selection device layer is electrically connected to the variable resistance layer. The selection device layer includes a chalcogenide switching material having a composition according to chemical formula 1 below, [GeASeBTeC](1-U)[X]U  (1) where 0.20≤A≤0.40, 0.40≤B≤0.70, 0.05≤C≤0.25, A+B+C=1, 0.0≤U≤0.20, and X is at least one selected from boron (B), carbon (C), nitrogen (N), oxygen (O), phosphorus (P), or sulfur (S).