Abstract: An improved method for forming a recessed thermal SiO.sub.2 isolation region in a monocrystalline silicon semiconductor body having a major surface lying in a (100) plane as defined by the Miller indices by forming an etch resistant and oxidation resistant masking layer on the major surface of the body, forming at least one rectilinear annular opening in the masking layer, the opening being oriented with the sides parallel to the [100] directions on the major surface, removing a portion of the exposed body by anisotropic chemical etching, and oxidizing the resultant exposed portions of the body until the surface of the resultant SiO.sub.2 and major surface are substantially coplanar.A semiconductor device including a silicon substrate of a first conductivity, the major surface being in the (100) plane, an epitaxial silicon layer on the substrate, a lateral PN junction in the substrate, at least one annular rectangular shaped recessed SiO.sub.