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 (12) is epitaxially grown on a heavily doped n- type semiconductor substrate (11), and a heavily doped n-type impurity region (13), a lightly doped p- type deep guard ring (15) and a heavily doped p- type shallow impurity region (14) are formed in said lightly doped semiconductor layer (12) in such a manner that a diode (20) has a major p-n junction (a) between the heavily doped n- type impurity region (13) and the heavily doped p-type shallow impurity region (14) and other p-n junction (b) between the lightly doped n- type semiconductor layer (12) and the lightly doped p- type guard ring (15), wherein the other p-n junction (b) is wider in area than the major p-n junction (a) so that the breakdown voltage is adjustable without increase of parasitic capacitance dominated by the other p-n junction (b).