A semiconductor structure includes a semiconductor substrate; an n-type tub extending from a top surface of the semiconductor substrate into the semiconductor substrate, wherein the n-type tub comprises a bottom buried in the semiconductor substrate; a p-type buried layer (PBL) on a bottom of the tub, wherein the p-type buried layer is buried in the semiconductor substrate; and a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device over the PBL and within a region encircled by sides of the n-type tub.