Surface voltage sustaining structure for semiconductor devices

A surface voltage sustaining structure for a semiconductor device comprising a first semiconductor region of a second conductivity type formed on a second semiconductor region of a first conductivity type, said first semiconductor region being doped at a higher density of impurities than said second semiconductor region, said surface voltage sustaining structure comprising a third semiconductor region formed around said first semiconductor region, said third semiconductor region having an average doping density of the second conductivity type that decreases from a value of about N.sub.B W.sub.pp gradually with increasing of a distance to said first semiconductor region, said third semiconductor region being depleted under a reverse voltage close to a breakdown voltage applied to a junction formed by said first and second semiconductor regions, where N.sub.B is the doping density of said second semiconductor region, and where W.sub.pp is the depletion width of an abrupt parallel plane junction made by said second semiconductor region, and where said average doping density of the second conductivity type of said third region refers to the average density of effectively ionized impurities of the second conductivity type in a surface area having a lateral dimension smaller than W.sub.pp.