An electronic device comprises a semiconductor substrate including majority carrier dopants of a first conductivity type, a semiconductor surface layer including majority carrier dopants of a second conductivity type, field oxide that extends on the semiconductor surface layer, and an isolation structure. The isolation structure includes a trench that extends through the semiconductor surface layer and into one of the semiconductor substrate and a buried layer of the semiconductor substrate, and polysilicon including majority carrier dopants of the second conductivity type, the polysilicon fills the trench to a side of the semiconductor surface layer.