A semiconductor device includes a field shield region that is doped
opposite to the conductivity of the substrate and is bounded laterally by
dielectric sidewall spacers and from below by a PN junction. For example,
in a trench-gated MOSFET the field shield region may be located beneath
the trench and may be electrically connected to the source region. When
the MOSFET is reverse-biased, depletion regions extend from the
dielectric sidewall spacers into the "drift" region, shielding the gate
oxide from high electric fields and increasing the avalanche breakdown
voltage of the device. This permits the drift region to be more heavily
doped and reduces the on-resistance of the device. It also allows the use
of a thin, 20 .ANG. gate oxide for a power MOSFET that is to be switched
with a 1V signal applied to its gate while being able to block over 30V
applied across its drain and source electrodes, for example.