There are a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and has been used for transparent electrodes required in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are, for example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide is already known (for example, see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
Examples of metal oxides include not only an oxide of a single metal element but also an oxide of a plurality of metal elements (multi-component oxides). For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material as multi-component oxides including In, Ga, and Zn (e.g., see Non-Patent Documents 2 to 4 and the like).
In addition, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used for a channel formation region of a thin film transistor (e.g., see Patent Document 5, Non-Patent Documents 5 and 6, and the like).
In order to achieve high speed operation of a transistor or the like, miniaturization of the transistor is needed. For example, in Patent Document 6, a thin film transistor including an oxide semiconductor used for a channel layer with a thickness of about 10 nm or smaller is disclosed. In Non-Patent Document 7, a thin film transistor including an oxide semiconductor whose channel length is 2 μm to 100 μm is disclosed.