The stability of the a-IZTO thin film transistor annealed at 350 °C was explored under both positive bias stress (PBS) and negative bias stress (NBS). The tests were performed at drain voltage (VDS) of ±10 V with stress time of up to 1200 s. As shown in Figure 5, the transfer characteristics curve shifted to the positive direction, and thus threshold voltage also changed to the positive direction with the increase of bias stress time. This voltage shift of transfer characteristics was attributed to electron trapping at the gate/insulator interface in n-type TFTs . The threshold voltage shift under PBS is about +1.9 V while the NBS is about +3.1 V. Higher bias instability of a-IZTO thin film transistor would be expected to decrease further by applying passivation layer to prevent humidity .