The prolonged bias stress of ZnO TFTs transistors with Al₂O₃ deposited at 100, 175, and 250°C is presented. Fully patterned bottom gated and top contacted devices serve as the test structures. The reliability study shows increasing threshold voltage shifting of 10.5, 18.6, and 27.2 % with deposition temperature with no significant change in the density of interface states for all the samples. Nevertheless, there is a dependence of the oxide trap states with stress time. The analysis of the transconductance as a function of the threshold voltage shifting indicates that oxide traps states near the interface are the dominant instability mechanism for significant stress times. The Al₂O₃ deposited at a temperature of 100 °C contains a higher concentration of oxygen compared to the other samples. This present oxygen excess could be filling oxygen vacancies present in the Al₂O₃, thereby resulting in a smaller ΔV_TH.