A semiconductor device comprises a semiconductor substrate doped with dopants of a first type and a vertical transistor composed of one or more transistor cells. Each transistor cell has a first region formed in the substrate and doped with dopants of a second type, and the first regions form first pn-junctions with the surrounding substrate. At least a first well region is formed in the substrate and doped with dopants of a second type to form a second pn-junction with the substrate. The first well region is electrically connected to the first regions of the vertical transistor via a semiconductor switch. The semiconductor device comprises a detection circuit, which is integrated in the substrate and configured to detect whether the first pn-junctions are reverse biased. The switch is opened when the first pn-junctions are reverse biased and the switch is closed when the first pn-junctions are not reverse biased.