We fabricated high-performance thin-film transistors (TFTs) with an amorphous-Al–Sn–Zn–In–O (a-AT-ZIO) channel deposited by cosputtering using a dual Al–Zn–O and In–Sn–O target. The fabricated AT-ZIO TFTs, which feature a bottom-gate and bottom-contact configuration, exhibited a high field-effect mobility of 31.9 $ \hbox{cm}^{2}/\hbox{V}\cdot\hbox{s}$, an excellent subthreshold gate swing of 0.07 V/decade, and a high $I_{{\rm on}/{\rm off}}$ ratio of $≫\hbox{10}^{9}$, even below the process temperature of 250 $^{\circ}\hbox{C}$. In addition, we demonstrated that the temperature and bias-induced stability of the bottom-gate TFT structure can significantly be improved by adopting a suitable passivation layer of atomic-layer-deposition-derived $\hbox{Al}_{2} \hbox{O}_{3}$ thin film.