A method of manufacturing a semiconductor device includes providing a semiconductor substrate. The semiconductor substrate includes a dummy gate structure formed thereon and an offset spacer formed on a sidewall of the dummy gate structure. The method further includes removing the dummy gate structure to form a gate trench, forming a high-k dielectric layer on the bottom and the sidewall of the gate trench, and forming a cover layer over the high-k dielectric layer. The cover layer has a thickness that is greater at the corners of the bottom of the gate trench than in the middle region of the bottom of the gate trench.