We studied the influence of the backchannel passivation layer (BPL) on the ambient stability of amorphous indium-zinc-tin-oxide thin-film transistors (a-IZTO TFTs), in which atomic layer deposited (ALD) Al 2 O 3 films and plasma-enhanced chemical vapor deposited (PECVD) SiO 2 films were separately used to be the channel passivation layers. It was observed that the BPL deposition process strongly affects device performance and stability. From the results of the extracted activation energy (E act ), the Al 2 O 3 passivation layer can reduce the trap density in localized tail states, which improves the mobility of a-IZTO TFTs. Compared with the SiO 2 passivation layer, the Al 2 O 3 passivation process effectively suppresses H injection into the a-IZTO channel layer underneath with secondary ion mass spectrometer analysis. In addition, it is found that the a-IZTO TFT with the Al 2 O 3 passivation layer can enhance resistance against negative bias illumination stress (NBIS), making it reliable for realistic operation in flat panel displays.