MOS transistor for withstanding a high voltage

Claim 1

A MOS transistor comprising: a well region of a first conductivity type formed in a semiconductor substrate; a drain region formed in the well region to form a substantially flat junction with said well region, said drain region having a periphery with a curved portion and having a conductivity type opposite to the first conductivity type; an oxide film coverng the drain region and the well region, said oxide film having a thinner portion located on a main portion of the drain region and expanding into a thicker portion partially covering the curved portion of the periphery of the drain region; and a drift region formed in the well region under the thicker portion of the oxide film to form a junction with the well region and having the same conductivity type as the drain region and having an impurity concentration lower than the drain region, said drift region partially surrounding the drain region at the curved portion of the periphery of the drain region and forming a junction with the curved portion of the periphery of the drain region to cover said curved portion, such that the junction between the well region and the drain region is substantially flat.