A semiconductor device includes a semiconductor substrate including a group III element and a group V element, and a gate structure on the semiconductor substrate. The semiconductor substrate includes a first region which contacts a bottom surface of the gate 5 structure and a second region which is disposed under the first region. The concentration of the group III element in the first region is lower than that of the group V element in the first region, and the concentration of the group III element in the second region is substantially equal to that of the group V element in the second region.