A technique in which a transistor is formed using a semiconductor layer formed over a substrate having an insulating surface is known. For example, a technique in which a transistor is formed over a glass substrate using a thin film including a silicon-based semiconductor material and the transistor is applied to a liquid crystal display device or the like is known.
A transistor which is used in a liquid crystal display device is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. Although transistors including amorphous silicon have low field effect mobility, a larger glass substrate can be used in the case of using amorphous silicon. Meanwhile, transistors including polycrystalline silicon have high field effect mobility; however, they need to be subjected to a crystallization step such as laser annealing and thus are not always suitable for larger glass substrates.
As another material, an oxide semiconductor attracts attention. Zinc oxide and a materials including zinc oxide are known as oxide semiconductor materials. Thin film transistors each of which is formed using an amorphous oxide (an oxide semiconductor) having an electron carrier concentration of lower than 1018/cm3 are disclosed in Patent Documents 1 to 3.