A semiconductor substrate of a first conductivity type is coated with a semiconductor layer of a second conductivity type. A buried region of the second conductivity type is formed an interface between the semiconductor substrate and the semiconductor layer. First and second wells of the first conductivity type are provided in the semiconductor layer. A second region of the second conductivity type is formed in the first well. A third region of the second conductivity type is formed in the second well. The first well, the semiconducting layer, the second well and the third region form a first lateral thyristor. The second well, the semiconductor layer, the first well and the second region form a second lateral thyristor. The buried region and semiconductor substrate form a zener diode which sets the trigger voltage for the lateral thyristors.