A semiconductor structure corresponds to a first diode and a second diode connected in series. A first well region is on a first deep well region. Two second well regions are at two sides of the first well region respectively. A first doping region and a second doping region are on the first well region. A first isolation region is between the first doping region and the second doping region. A third well region is on a second deep well region. Two fourth well regions are at two sides of the third well region respectively. A third doping region and a fourth doping region are on the third well region. A second isolation region is between the third doping region and the fourth doping region. The second doping region and third doping region are connected. The second deep well region is separated from the first deep well region.