Forming method of transistor structure

The invention discloses a forming method of a transistor structure. The forming method comprises the following steps of providing a semiconductor substrate; forming an interfacial layer on the semiconductor substrate; forming a high-K gate dielectric layer on the interfacial layer; forming a cap layer on the high-K gate dielectric layer; forming a metal gate on the cap layer; after forming the cap layer, carrying out anaerobic densified treatment on the interfacial layer and the high-K gate dielectric layer. According to the forming method, the interfacial layer is firstly formed on the semiconductor substrate, the high-K gate dielectric layer is formed on the interfacial layer, the cap layer is formed on the high-K gate dielectric layer, and then the anaerobic densified treatment is carried out on the interfacial layer and the high-K gate dielectric layer, owing to formation of the cap layer, the cap layer can protect the high-K gate dielectric layer in an anaerobic densified treatment, so that oxidation of trace amounts of oxygen in the environment on the interfacial layer and the high-K gate dielectric layer can be prevented, and thus shift of threshold voltage of a transistor device which is subsequently formed, and the performance of the transistor is improved.