A semiconductor device includes a first conductive line on a lower structure and extending in a first horizontal direction; a second conductive line on the first conductive line and extending in a second horizontal direction, the second horizontal direction being perpendicular to the first horizontal direction; and a memory cell structure between the first conductive line and the second conductive line. The memory cell may structure include a data storage material pattern and a selector material pattern overlapping the data storage material pattern in a vertical direction. The data storage material pattern may include a phase change material layer of InαGeβSbγTeδ. In the phase change material layer of InαGeβSbγTeδ, a sum of α and β may be lower than about 30 at. %, and a sum of γ and δ may be higher than about 70 at. %.