providing a semiconductor substrate;

implanting a dopant into the substrate;

forming an epitaxial layer on the substrate;

causing the dopant in the substrate to diffuse upward into the epitaxial layer so as to form a buried layer;

forming a pair of trenches in said epitaxial layer, each of said trenches having a bottom located in the epitaxial layer above an interface between the epitaxial layer and the substrate, the trenches defining a mesa between said trenches, said mesa being located over said buried layer;

substantially filling said trenches with a dielectric material;

after forming and substantially filling said trenches, introducing a dopant into the mesa; and

causing said dopant in said mesa to diffuse downward and said buried layer to diffuse upward such that said dopant in said mesa merges with said buried layer to form said isolation diffusion, said isolation diffusion extending across said mesa so as to abut at least a portion of a sidewall of each of said trenches.