A method for fabricating a semiconductor device includes forming a first high-k (HK) dielectric layer over a substrate, performing a wet treatment process to the first HK dielectric layer. The wet treatment includes a dopant. The method also includes performing an annealing process to the first HK dielectric layer such that the dopant diffuses into the first HK dielectric layer to form a modified HK dielectric layer. Therefore the modified HK dielectric layer has a second dielectric constant which is different than the first dielectric constant.