A semiconductor device includes a first buried layer and a second buried layer both have a first conductivity type and are disposed in a substrate, where the second buried layer is disposed on the first buried layer. A first well region has the first conductivity type and is disposed above the second buried layer. A second well region has a second conductivity type and is adjacent to the first well region. A deep trench isolation structure is disposed in the substrate and surrounds the first and second well regions, where the bottom surface of the deep trench isolation structure is lower than the bottom surface of the first buried layer. A source region is disposed in the second well region. A drain region is disposed in the first well region. A gate electrode is disposed on the first and second well regions.