monolithic integration of low - capacitance p - n junctions and low - resistance p - n junctions is provided . three epitaxial layers are used . the low - capacitance junctions are formed by the top two epitaxial layers . the low - resistance p - n junction is formed in the top epitaxial layer , and two buried structures at interfaces between the three epitaxial layers are used to provide a high doping region that extends from the low - resistance p - n junction to the substrate , thereby providing low resistance to current flow . the epitaxial layers are lightly doped as required by the low - capacitance junction design , so the buried structures are needed for the low - resistance p - n junction . the high doping region is formed by diffusion of dopants from the substrate and from the buried structures during thermal processing .