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The electrical instability behaviors of a positive-gate-bias-stressed amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistor (TFT) are studied under monochromatic light illumination. It is found that as the wavelength of incident light reduces from 750 nm to 450 nm, the threshold voltage of the illuminated TFT shows a continuous negative shift, which is caused by photo-excitation of trapped electrons at the channel/dielectric interface. Meanwhile, an increase of the sub-threshold swing (SS) is observed when the illumination wavelength is below 625 nm ($2.0 eV). The SS degradation is accompanied by a simultaneous increase of the field effect mobility (l FE) of the TFT, which then decreases at even shorter wavelength beyond 540 nm ($2.3 eV). The variation of SS and l FE is explained by a physical model based on generation of singly ionized oxygen vacancies (V o þ) and double ionized oxygen vacancies (V o 2þ) within the a-IGZO active layer by high energy photons, which would form trap states near the mid-gap and the conduction band edge, respectively.