A semiconductor device capable of high-voltage operation includes a semiconductor substrate having a first conductivity type. A first well doped region is formed in a portion of the semiconductor substrate. The first well doped region has a second conductivity type. A first doped region is formed on the first well doped region, having the second conductivity type. A second doped region is formed on the first well doped region and is separated from the first doped region, having the second conductivity type. A first gate structure is formed over the first well doped region and is adjacent to the first doped region. A second gate structure is formed beside the first gate structure and is close to the second doped region. A third gate structure is formed overlapping a portion of the first gate structure and a first portion of the second gate structure.