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Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are widely used as controlling devices for picture pixels in liquid crystal displays. In addition to flat panel display applications, a significant research effort focuses on the extension of this technology to circuitry on flexible substrates to build flexible sensor systems. This study investigates the effect of anneal time on the performance of the a-Si:H TFTs on Polyethylene naphthalate (PEN). Off-current is reduced by two orders of magnitude for 48-hours annealed TFT, and the sub-threshold slope become steeper with longer annealing. For positive gate-bias-stress, ΔVt values are positive and exhibit a power-law time dependence (PLTD). The 48-hour annealed TFTs, however, display a turnaround phenomenon (TP) at longer stress times. For negative gate-bias-stress, TFTs annealed for ≥ 24 hours possess a smaller positive ΔVt. They do not follow a PLTD and the TP is observed at longer stress times. The observed ΔVt is explained in terms of the shift in the electron and hole transfer characteristics.