The gate oxide layer of a trench MIS device includes a graduated transition region,
where the thickness of the gate oxide layer decreases gradually from a thick section
adjacent the bottom of the trench to a thin section adjacent the sidewall of the
trench. The PN junction between the body and drain regions intersects the trench
in the transition region. This structure allows for a greater margin of error in
the placement of the PN junction during the manufacture of the device, since the
intersection between the PN junction can be located anywhere in the transition
region. The MIS device also has improved breakdown characteristics.
