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The evolution with time of interface trap density and bulk density of states in amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs), for negative-bias-under-illumination-stress (NBIS), is traced. Based on the combined analysis of TFT current-voltage and capacitance-voltage characteristics, position of Fermi energy, flat band voltage, interface trap density, and gap state density per unit energy are investigated as function of NBIS time and applied gate voltage. These key parameters help to identify the degradation phenomena responsible for the negative threshold voltage shift caused by NBIS. In particular, the interface trap density becomes more positive; from 0.03 × 1011/cm2 to 0.65 × 1011/cm2, while the gap trap density per unit energy also increases after NBIS, supporting defect creation in the bulk and build-up of positive charge at the gate insulator/active-layer interface as the mechanism responsible for NBIS instability.