A memory device includes a plurality of word lines extending along a first direction and spaced apart from each other along a second direction that is perpendicular to the first direction; a plurality of bit lines extending along the second direction and spaced apart from each other in the first direction, the plurality of bit lines being spaced apart from the plurality of word lines in a third direction that is perpendicular to both the first and second directions; and a plurality of memory cells being respectively arranged between the corresponding word and bit lines. Each of the memory cells includes a selection device layer, and a variable resistance layer, wherein the selection device layer includes a chalcogenide switching material having a composition according to a particular chemical formula.