According to example embodiments, there is provided a semiconductor device including a substrate and an isolation layer structure. The substrate includes an active region having an upper active pattern and a lower active pattern on the upper active pattern. The active region has a first aspect ratio larger than about 13:1 and a second aspect ratio smaller than about 13:1. The first aspect ratio is defined as a ratio of a sum of heights of the upper active pattern and the lower active pattern with respect to a width of the upper active pattern. The second aspect ratio is defined as a ratio of the sum of the heights of the upper active pattern and the lower active pattern with respect to a width of the lower active pattern. The isolation layer structure is adjacent to the active region.