A semiconductor power switch having an array of basic cells in which peripheral regions in the active drain region extend beside the perimeter of the base-drain junction, the peripheral regions being of higher dopant density than the rest of the second drain layer. Intermediate regions in the centre of the active drain region are provided of lighter dopant density than the rest of the second drain layer. This provides an improved compromise between the on-state resistance and the breakdown voltage by enlarging the current conduction path at in its active drain region. On the outer side of each edge cell of the array, the gate electrode extends over and beyond at least part of the perimeters of the base-source junction and the base-drain junction towards the adjacent edge of the die. Moreover, on the outer side of each edge cell, the second drain layer includes a region of reduced dopant density that extends beyond the gate electrode right to the adjacent edge of the die.