A power MOS transistor formed of an array of source cells and drain cells on an IC chip substrate has a plurality of substrate contact cells, each formed external to the source cells, having respective substrate potential-setting electrodes to which an externally supplied substrate bias voltage can be applied, enabling the substrate potential to be set independently of the source potential of the transistor. It thereby becomes possible to modify the threshold voltage of the transistor or maintain a constant potential difference between the substrate potential and that of a gate input signal. Since the requirement for a substrate contact region within each source cell is eliminated, and the number of substrate contact cells can be fewer than that of the source cells, the chip area occupied by the transistor can be reduced by comparison with a prior art configuration providing such a substrate potential control capability.