A semiconductor device is includes a substrate, a gate positioned on the substrate, and a drain region and a source region formed at two respective sides of the gate in the substrate. The drain region includes a first doped region having a first conductivity type, a second doped region having a second conductivity type, and a third doped region. The first conductivity type and the second conductivity type are complementary to each other. The semiconductor device further includes a first well region formed under the first doped region, a second well region formed under the second doped region, and a third well region formed under the third doped region. The first well region, the second well region, and the third well region all include the first conductivity type. A concentration of the second well region is different from a concentration of the third well region.