A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). As semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, disclosed is a transistor whose active layer is formed using an amorphous oxide containing indium (In), gallium (Ga) and zinc (Zn) and having an electron carrier concentration of lower than 1018/cm3 (see Patent Document 1).
A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of change in electric characteristics, although the transistor including in oxide semiconductor can be operated at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, a BT test under light is performed, so that the threshold voltage of the transistor fluctuates. On the other hand, Patent Documents 2 and 3 each disclose a technique of preventing charge trapping at the interface of an oxide semiconductor layer with the use of an interfacial stability layer, which is provided on at least one of the top surface and the bottom surface of the oxide semiconductor layer, in order to suppress the shift of the threshold voltage of the transistor including an oxide semiconductor.