A field-effect semiconductor device is provided. The field-effect semiconductor device includes a semiconductor body with a first surface defining a vertical direction. In a vertical cross-section the field-effect semiconductor device further includes a vertical trench extending from the first surface into the semiconductor body and comprising a field electrode, a cavity at least partly surrounded by the field electrode, and an insulation structure substantially surrounding at least the field electrode. An interface between the insulation structure and the surrounding semiconductor body is under tensile stress and the cavity is filled or unfilled so as to counteract the tensile stress.