Field effect transistor

A MIS type field effect transistor including gate dielectrics having a rare-earth metal oxynitride layer with a high dielectric constant, which can maintain good interface characteristics, can be provided. A field effect transistor according to one aspect of this invention includes a gate dielectric having a substantially crystalline rare-earth metal oxynitride layer containing one or more metals selected from rare-earth metals, oxygen, and nitrogen. The rare-earth metal oxynitride layer contacts a predetermined region of a Si semiconductor substrate, and the nitrogen exists at the interface between the rare-earth metal oxynitride layer and the Si semiconductor substrate, and in the bulk of the rare-earth metal oxynitride. The transistor further includes a gate electrode formed on the gate dielectrics and source and drain regions, one being formed at one side of the gate electrode and the other being formed at the other side of the gate electrode in the Si semiconductor substrate.