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The performance and stability of thin-film transistors with zinc oxide as the channel layer are investigated using gate bias stress. It is found that the effective channel mobility, ON/OFF ratio, and subthreshold slope of the devices that incorporate SiN are superior to those with SiO2 as the dielectric. The application of positive and negative stress results in the device transfer characteristics shifting in positive and negative directions, respectively. The devices also demonstrate a logarithmic time-dependent threshold voltage shift suggestive of charge trapping within the band gap and the band tails responsible for the deterioration of device parameters. It is postulated that this device instability is partly a consequence of the lattice mismatch at the channel/insulator interface. All stressed devices recover to near-original characteristics after a short period at room temperature without the need for any thermal or bias annealing.