The instabilities of indium-zinc oxide thin film transistors under bias and/or illumination stress are studied in this paper.Firstly,illumination experiments are performed,which indicates the variations of current-voltage characteristics and electrical parameters (such as threshold voltage and sub-threshold swing) are dominated by the stress-induced ionized oxygen vacancies and acceptor-like states.The dependence of degradation on light wavelength is also investigated.More negative shift of threshold voltage and greater sub-threshold swing are observed with the decrease of light wavelength.Subsequently,a negative bias illumination stress experiment is carried out.The degradation of the device is aggravated due to the decrease of recombination effects between ionized oxygen vacancies and free carriers.Moreover,the contributions of ionized oxygen vacancies and acceptor-like states are separated by using the mid-gap method.In addition,ionized oxygen vacancies are partially recombined at room temperature and fully recombined at high temperature.Finally,low-frequency noise is measured before and after negative bias illumination stress.Experimental results show few variations of the oxide trapped charges are generated during stress,which is consistent with the proposed mechanism.