A vertical MOS semiconductor device exhibits a high breakdown voltage and low
on-resistance,
reduces the tradeoff relation between the on-resistance and the breakdown voltage,
and realizes high speed switching. The semiconductor device has a breakdown-voltage
sustaining layer, such as an n--type drift layer, and a well region,
such as a p-type well region, in the breakdown-voltage sustaining layer. The resistivity
(cm) of the breakdown-voltage layer is within a range expressed
in terms of the breakdown voltage Vbr (V). The semiconductor device also has stripe
shaped surface drain regions that extend from the well region and are surrounded
by the well region. The surface area ratio between surface drain regions and the
well region, which includes the source region, is from 0.01 to 0.2.