The trench MOS transistor according to the present invention includes a drain region in a form of a trench filled with a semiconductor material. The trench has a bottom surface and side surfaces and extends vertically downward from the top surface of the covering layer into the buried layer, the bottom surface of the trench lies in the buried layer, an insulating layer lines the side surfaces of the trenches, and the semiconductor material within the trench overlies the insulating layer and contacts the buried layer at the bottom surface of the trench.