By thermal oxidation a field oxide layer is formed that lines first and second trenches that extend from a main surface into a semiconductor layer. After the thermal oxidation, field electrodes and trench gate electrodes of power transistor cells are formed in the first and second trenches. A protection cover including a silicon nitride layer is formed that covers a cell area with the first and second trenches. With the protection cover covering the cell area, planar gate electrodes of lateral transistors are formed in a support area of the semiconductor layer.