A method of manufacturing a semiconductor device including a transistor comprises forming field plate trenches in a main surface of a semiconductor substrate, a drift zone being defined between adjacent field plate trenches, forming a field dielectric layer in the field plate trenches, thereafter, forming gate trenches in the main surface of the semiconductor substrate, a channel region being defined between adjacent gate trenches, and forming a conductive material in at least some of the field plate trenches and in at least some of the gate trenches. The method further comprising forming a source region and forming a drain region in the main surface of the semiconductor substrate.