The present disclosure provides a semiconductor device that includes a semiconductor substrate, and a transistor formed in the substrate. The transistor has a gate structure that includes an interfacial layer formed on the substrate, a high-k dielectric layer formed on the interfacial layer, a capping layer formed on the high-k dielectric layer, the capping layer including a silicon oxide, silicon oxynitride, silicon nitride, or combinations thereof, and a polysilicon layer formed on the capping layer.