A method includes forming a hard mask over an epitaxy layer of a substrate; forming a patterned mask over the hard mask; etching the hard mask and the epitaxy layer to form a trench in the epitaxy layer, in which a remaining portion of the hard mask covers a topmost surface of the epitaxy layer, and the trench exposes a sidewall of the epitaxy layer; forming a P-well region by directing p-type ion beams into the trench along an oblique direction that is non-parallel to a normal line of the topmost surface of the epitaxy layer, in which the topmost surface of the epitaxy layer is protected from the p-type ion beams by the remaining portion of the hard mask during directing the p-type ion beams into the trench; and after directing the p-type ion beams into the trench, forming a gate structure in the trench.