A semiconductor structure includes a semiconductor substrate of a first conductivity type; a pre-high-voltage well (pre-HVW) in the semiconductor substrate, wherein the pre-HVW is of a second conductivity type opposite the first conductivity type; a high-voltage well (HVW) over the pre-HVW, wherein the HVW is of the second conductivity type; a field ring of the first conductivity type occupying a top portion of the HVW; and a tunnel of the first conductivity type in the pre-HVW and the HVW, and electrically connecting the field ring and the semiconductor substrate.