The invention provides a semiconductor device, including a buried oxide layer disposed on a substrate. A semiconductor layer having a first conduction type is disposed on the buried oxide layer. A first well region having the first conduction type is disposed in the semiconductor layer. A second well and a third well having a second conduction type are disposed to opposite sides of the first well region. The second well and the third well are separated from the first well region. A first anode doped region is disposed in the second well. A second anode doped region and a third anode doped region having the first conduction type are disposed in the second well. The second anode doped region is positioned directly on the third anode doped region. A first cathode doped region is coupled to the third well.