A semiconductor device, which is a high-voltage MOSFET, comprising: a semiconductor substrate having a first conductivity type; a first well doped region formed in a portion of the semiconductor substrate, having a second conductivity type that is the opposite of the first conductivity type; a first doped region, which is a source doped region, formed on the first well doped region, having the second conductivity type; a second doped region, which is a drain doped region, formed on at least a portion of the first well doped region and separated from the first doped region, having the second conductivity type; a first gate structure formed over the first well doped region and adjacent or close to the first doped region; a second gate structure formed beside and separate from the first gate structure and close to the second doped region; a third gate structure formed over at least a portion of the first gate structure and at least a first portion of the second gate structure; a fourth gate structure, wherein a top surface of the third gate structure is aligned to a top surface of the fourth gate structure; and a second well doped region formed in a portion of the first well doped region, having the first conductivity type, wherein the first doped region is formed on a portion of the second well doped region, wherein the first gate structure is formed overlapping both a portion of the second well doped region and a portion of the first well doped region, wherein: - the fourth gate structure is formed beside the third gate structure and close to the second doped region and overlaps a second portion of the second gate structure, which is different from the first portion of the second gate structure; or - the fourth gate structure covers a second portion of the second gate structure and a top surface of the first well doped region between the second gate structure and the second doped region. a semiconductor substrate having a first conductivity type; a first well doped region formed in a portion of the semiconductor substrate, having a second conductivity type that is the opposite of the first conductivity type; a first doped region, which is a source doped region, formed on the first well doped region, having the second conductivity type; a second doped region, which is a drain doped region, formed on at least a portion of the first well doped region and separated from the first doped region, having the second conductivity type; a first gate structure formed over the first well doped region and adjacent or close to the first doped region; a second gate structure formed beside and separate from the first gate structure and close to the second doped region; a third gate structure formed over at least a portion of the first gate structure and at least a first portion of the second gate structure; a fourth gate structure, wherein a top surface of the third gate structure is aligned to a top surface of the fourth gate structure; and a second well doped region formed in a portion of the first well doped region, having the first conductivity type, wherein the first doped region is formed on a portion of the second well doped region, wherein the first gate structure is formed overlapping both a portion of the second well doped region and a portion of the first well doped region, wherein: - the fourth gate structure is formed beside the third gate structure and close to the second doped region and overlaps a second portion of the second gate structure, which is different from the first portion of the second gate structure; or - the fourth gate structure covers a second portion of the second gate structure and a top surface of the first well doped region between the second gate structure and the second doped region. - the fourth gate structure is formed beside the third gate structure and close to the second doped region and overlaps a second portion of the second gate structure, which is different from the first portion of the second gate structure; or - the fourth gate structure covers a second portion of the second gate structure and a top surface of the first well doped region between the second gate structure and the second doped region.