A method for manufacturing a semiconductor device includes providing a substrate containing a front-end device and forming a dielectric layer on the substrate. The front-end device includes a first dummy gate in a first type metal gate transistor region, a second dummy gate in a second type metal gate transistor region, and a polysilicon gate in a polysilicon gate region. The method also includes removing a thickness of the first, second, and polysilicon gates and forming a protective layer on the polysilicon layer to protect the polysilicon layer during a CMP process, thereby improving the performance and yield of the semiconductor device.