In recent years, a thin film transistor (TFT) including an oxide semiconductor film such as InGaZnO, which is oxide of indium (In), gallium (Ga) and zinc (Zn) has been developed. It is known that the InGaZnO thin film transistor can be easily fabricated at a low temperature, and the InGaZnO thin film has a high mobility of 10 cm2/Vs or more. Therefore, realization of a back end of line (BEOL) transistor, which is embedded in multilayer interconnection, has been expected. Furthermore, application to liquid crystal panels, and the like has been expected. In addition, it is known that the channel region of an InGaZnO-TFT is not an inversion layer but a storage layer unlike a silicon (Si) metal oxide semiconductor field effect transistor (MOSFET) or a Si-TFT.
Meanwhile, the channel length is getting shorter along with recent fining of patterns, and the shortening of the channel length causes a problem of significant deterioration of the threshold voltage of a TFT. For example, a threshold voltage around 0 V may be changed to −10 V or lower.