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Bias-temperature-stress (BTS) induced electrical instability of the RF sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated. Both positive and negative BTS were applied and found to primarily cause a positive and negative voltage shift in transfer (I DS-V GS) characteristics, respectively. The time evolution of bulk-state density (N BS) and characteristic temperature of the conduction-band-tail-states (TG) are extracted. Since both values showed only minor changes after BTS, the results imply that observed shift in TFT I DS-V GS curves were primarily due to channel charge injection/trapping rather than defect states creation. We also demonstrated the validity of using stretch-exponential equation to model both positive and negative BTS induced threshold voltage shift (DeltaV th) of the a-IGZO TFTs. Stress voltage and temperature dependence of DeltaV th evolution are described.