According to example embodiments, a semiconductor device includes horizontal patterns stacked on a substrate. The horizontal patterns define an opening through the horizontal patterns. A first core pattern is in the opening. A second core pattern is in the opening on the first core pattern. A first active pattern is between the first core pattern and the horizontal patterns. A second active pattern containing a first element is between the second core pattern and the horizontal patterns. The second active pattern contains the first element at a higher concentration than a concentration of the first element in the second core pattern.