Abstract: A transistor power switch device comprising an array of vertical transistor elements for carrying current between the first and second faces of a semiconductor body and a vertical avalanche diode electrically in parallel with the array of vertical transistors. The array of transistor elements includes at the first face an array of source regions of a first semiconductor type, at least one p region of a second semiconductor type opposite to the first type interposed between the source regions and the second face, at least one control electrode for switchably controlling flow of the current through the p region, and a conductive layer contacting the source regions and insulated from the control electrode.