SEMICONDUCTOR DEVICE

A transistor device includes a semiconductor substrate having a first major surface and one or more transistor cells. Each transistor cell includes a columnar trench formed in the substrate, a columnar field plate arranged in the columnar trench, and a mesa arranged around the columnar trench. The columnar trench includes a field dielectric, a base, and a side wall. The side wall extends from the base to the first major surface. The field dielectric lines the base and side wall. A first thickness of the field dielectric at a first distance from the base is smaller than a second thickness of the field dielectric at a second distance from the base, the first distance being greater than the second distance. A first perimeter of the columnar field plate at the first distance is greater than a second perimeter of the columnar field plate at the second distance.