A semiconductor device, includes: a first conductive structure on a substrate; a second conductive structure on the first conductive structure; and a first memory cell structure between the first conductive structure and the second conductive structure, wherein the first memory cell structure includes: a switching material pattern on the first conductive structure; a data storage material pattern on the switching material pattern; and an upper conductive pattern on the data storage material pattern, wherein a first width of a lower region of the data storage material pattern is less than a first width of the switching material pattern, and wherein a first width of the upper conductive pattern is less than a width of an upper region of the data storage material pattern.