A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The thin film transistors have been used in display devices typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a semiconductor thin film that can be used in thin film transistors. Besides, an oxide semiconductor has attracted attention as another material.
As a material for the oxide semiconductor, zinc oxide or a substance containing zinc oxide is known. 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.