A thin film transistor (TFT) formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. TFTs using amorphous silicon have low electric field mobility, but can respond to increase in size of glass substrates. On the other hand, TFTs using polycrystalline silicon have high electric field mobility, but need a crystallization step such as laser annealing and are not always adaptable to increase in size of glass substrates.
In contrast, a technique in which a TFT is formed using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element or the like in an image display device.
A TFT in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have higher electric field mobility than a TFT using amorphous silicon. An oxide semiconductor layer can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process of the TFT using an oxide semiconductor layer is simpler than that of the TFT using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.