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Positive-bias temperature-stress-induced degradation in p-channel poly-Si thin-film transistors is investigated. Two-stage degradation behavior is observed. In the first-stage degradation, on-state current (ION) initially increases associated with a positive threshold voltage (Vth) shift and is then followed by a gradual ION decrease. In the second stage, Vth shifts to the negative, and ION significantly degrades. A comprehensive physical model is proposed to clarify the two-stage degradation behavior. The first-stage degradation is attributed to electron trapping and detrapping, whereas the second-stage degradation is dominated by positive charge generation.