We investigated the effect of photon irradiation with various energies on the gate bias instability of indium-gallium-zinc oxide transistors. The illumination of red and green light on the transistor caused positive threshold voltage (V-th) shifts of 0.23 V and 0.18 V. respectively, while it did not affect the V-th value in blue light after a positive bias stress. However, the stability of transistors was deteriorated with increasing photon energy after a negative bias stress: negative V-th shifts for red (-0.23 V) and blue light (-3.7 V). This difference can be explained by the compensation effect of the electron carrier trapping and the creation of meta-stable donors via photon excitation. (C) 2011 Elsevier Ltd. All rights reserved.