A semiconductor device includes a first well in a substrate, a gate structure on the first well, a second well below the gate structure in the first well, a third well in a first side of the gate structure and in the first well to be adjacent to the second well, the third well having a conductivity type different from that of the second well, a fourth well overlapped with the third well, a fifth well in a second side of the gate structure and in the second well, a sixth well below the gate structure and in the second well, the sixth well being adjacent to the fifth well and having an impurity concentration higher than the impurity concentration of the second well, and a first device isolation layer overlapped with the second well and disposed farther away from the gate structure than the fifth well.