A semiconductor body (11) has first and second opposed major surfaces (11a and 11b). First and second main regions (13 and 14) meet the second major surface (11b) and a voltage-sustaining zone is provided between the first and second regions (13 and 14). The voltage-sustaining zone has a semiconductor region (11) of one conductivity type forming a rectifying junction (J) with a region (15) of the device such that, when the rectifying junction is reverse-biased in one mode of operation, a depletion region extends in the semiconductor region of the voltage-sustaining zone. A number of conductive regions (22) are isolated from and extend through the semiconductor region (11) in a direction transverse to the first and second major surfaces (11a and 11b) so as to be spaced apart in a direction between first and second main regions. A voltage regulator (20; 20null; 20a and 20b) is provided for setting the voltage at each conductive regions (22) so as to control the voltage distribution, and thus the electrical field profile, in the voltage- sustaining zone when the rectifying junction is reverse-biased in said one mode of operation.