This study investigates the light-color-dependent bias stress effect on an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT). The color of incident photons with energies lower than the optical band gap of IGZO (3.2 eV) was varied from blue to infrared. Regardless of the bias polarity, light is regarded as a promoter for bias-stress-induced instability. The light response of the a-IGZO TFT is both color-and bias-polarity-dependent. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3584088] All rights reserved.