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Based on the physical model of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) and the extracted density of states described in Part I, a quantitative investigation of mechanisms on the negative bias illumination stress (NBIS)-induced threshold voltage VT instability of a-IGZO TFTs is presented. It is found that the shallow donor state-creation model explains the NBIS time evolution of the electrical characteristics very well. Furthermore, the semi-empirical rule of the NBIS-induced ΔVT is proposed and demonstrated based on the shallow donor state-creation model. The proposed approach can be used to optimize the fabrication process and to explore high-performance thin-film materials for mass-production-level amorphous oxide semiconductor TFTs to be innovatively used in the near future.