Diode having breakdown voltage adjustable to arbitrary value without increase of parasitic capacitance and process for fabrication thereof

A lightly doped n-type semiconductor layer is epitaxially grown on a heavily doped n-type semiconductor substrate, and a heavily doped n-type impurity region, a lightly doped p-type deep guard ring and a heavily doped p-type shallow impurity region are formed in said lightly doped semiconductor layer in such a manner that a diode has a major p-n junction between the heavily doped n-type impurity region and the heavily doped p-type shallow impurity region and other p-n junction between the lightly doped n-type semiconductor layer and the lightly doped p-type guard ring, wherein the other p-n junction is wider in area than the major p-n junction so that the breakdown voltage is adjustable without increase of parasitic capacitance dominated by the other p-n junction.