Semiconductor devices having local oxide isolation

A semiconductor monolithic integrated circuit comprising a monocrystalline semiconductor substrate of a first conductivity type, a second-conductivity-type epitaxial semiconductor layer of a given thickness on a surface of said substrate, at least one isolation zone laterally dividing the epitaxial layer into a plurality of islands, at least one PN junction isolating each of said islands from the substrate, and at least one circuit element within one of the islands, said isolation zone including a region of silicon oxide extending from the epitaxial layer surface down into said epitaxial layer but spaced from said substrate such that said silicon oxide extends through only a part of the thickness of said epitaxial layer, and a region of said first conductivity type more heavily doped than said epitaxial layer and extending from said silicon oxide layer into contact with said substrate to complete the isolation of the islands.