Vertical Transistor Device Structure with Cylindrical-Shaped Field Plates

A vertical power transistor device includes a semiconductor layer of a first conductivity type, with a plurality of dielectric regions disposed in the semiconductor layer. The dielectric regions extend in a vertical direction from a top surface of the semiconductor layer downward. Each dielectric region has a rounded-square cross-section in a horizontal plane perpendicular to the vertical direction. Adjacent ones of the dielectric regions are laterally separated by a narrow region of the semiconductor layer. Each dielectric region has a cylindrical field plate member centrally disposed therein. The cylindrical field plate member extends in the vertical direction from the top surface downward to near a bottom of the dielectric region. The dielectric region laterally separates the cylindrical field plate member from the narrow region. A source region is disposed at the top surface, and a drain region is disposed at the bottom, of the semiconductor layer.