A semiconductor device is provided that includes a transistor in a semiconductor body having a main surface. The transistor includes a source region, a drain region, a body region, a drift zone, and a gate electrode at the body region. The body region and the drift zone are disposed along a first direction between the source region and the drain region. The first direction is parallel to the main surface. The semiconductor device further includes a field plate disposed in field plate trenches extending along the first direction in the drift zone, and a field dielectric layer between the field plate and the drift zone. A thickness of the field dielectric layer gradually increases along the first direction from a portion adjacent to the source region to a portion adjacent to the drain region.