Semiconductor structures with electrical isolation and methods of forming a semiconductor structure with electrical isolation. A shallow trench isolation region, which contains a dielectric material, is positioned in a semiconductor substrate. A trench extendes through the shallow trench isolation region and to a trench bottom in the semiconductor substrate beneath the shallow trench isolation region. A dielectric layer at least partially fills the trench. A polycrystalline region, which is arranged in the semiconductor substrate, includes a portion that is positioned beneath the trench bottom.