A method for forming a semiconductor structure is provided, which comprises the following steps. A gate is formed by a method comprising the following steps. A gate dielectric layer is formed on a substrate. A gate electrode is formed on the gate dielectric layer. A nitride spacer is formed on a sidewall of the gate electrode. A phosphorus containing dielectric layer is formed on the gate. The phosphorus containing dielectric layer has a varied phosphorus dopant density distribution profile. The phosphorus containing dielectric layer comprises a phosphorus dopant density region on an upper surface of the gate and having a triangle-like shape.