Insulated gate power semiconductor devices

A vertical power transistor trench-gate semiconductor device having a trench network extending into a semiconductor body and surrounding a plurality of closed transistor cells wherein the trench network comprises segment trench regions adjacent sides of the transistor cells and intersection trench regions adjacent corners of the transistor cells, wherein each transistor cell has a source region and a drain region which are vertically separated by a channel-accommodating body region adjacent a segment trench region at each side of the transistor cell, and wherein each segment trench region contains gate material separated from the semiconductor body by insulating material at the vertical sides and at the bottom of the segment trench region wherein the intersection trench regions each include insulating material which extends from the bottom of the intersection trench region with a thickness which is greater than the thickness of the insulating material at the bottom of the segment trench regions, gate material being provided above the insulating material in the intersection trench regions and bridging the gate material in the segment trench regions, wherein the greater thickness of the insulating material extending from the bottom of the intersection trench regions is effective to increase the drain-source reverse breakdown voltage of the device