A method for producing a field-effect semiconductor device includes providing a semiconductor body with a first surface defining a vertical direction, defining an active area, forming a vertical trench from the first surface into the semiconductor body, forming a field dielectric layer at least on a side wall and a bottom wall of the vertical trench, depositing a conductive layer on the field dielectric layer, forming a closed cavity on the conductive layer in the vertical trench, and forming an insulated gate electrode on the closed cavity in the vertical trench.