Semiconductor devices having asymmetrical structures

A semiconductor device includes a substrate including first and second active regions extending in a first direction and isolated from direct contact with each other in the first direction; a device isolation layer between the first and second active regions in the substrate; and first and second gate structures extending in a second direction on the substrate while respectively intersecting end portions of the first and second active regions. The first gate structure includes a first gate electrode. The second gate structure includes a second gate electrode. The first gate structure protrudes further toward the device isolation layer, as compared to the second gate structure, in a vertical direction that is perpendicular to the first and second directions, and a lower end of the first gate electrode is located on a lower height level than a lower end of the second gate electrode.