A semiconductor device includes a field regions in a substrate to define active regions, gate trenches including active trenches disposed across the active region and field trenches in the field regions, and word lines that fill the gate trenches and extend in a first direction. The word lines include active gate electrodes occupying the active trenches, and field gate electrodes occupying the field trenches. The bottom surface of each field gate electrode, which is disposed between active regions that are adjacent to each other and have one word line therebetween, is disposed at a higher level than the bottom surfaces of the active gate electrodes.