The optical and electrical properties of the amorphous indium gallium zinc oxide thin-film transistor ($alpha$-IGZO TFT) were studied. When the $alpha$-IGZO TFT was illuminated at a wavelength of 660 nm, the off-state drain current slightly increased, while below 550 nm it increased significantly. The $alpha$-IGZO TFT was found to be extremely sensitive, with deep-level defects at approximately 2.25 eV near the midgap. After UV light illumination, a slight change occurred on the surface of the $alpha$-IGZO films, such as in terms of the oxygen 1s spectra, resistivity, and carrier concentrations. It is believed that these results will provide information regarding the photo-induced behaviors in the $alpha$-IGZO films.