We have fabricated the transparent bottom gate thin-film transistors (TFTs) using Al and Sn-doped zinc indium oxide (AT-ZIO) as an active layer. The AT-ZIO active layer was deposited by RF magnetron sputtering at room temperature, and the AT-ZIO TFT showed a field effect mobility of 15.6 $ \hbox{cm}^{2}/\hbox{Vs}$ even before annealing. The mobility increased with increasing the $\hbox{In}_{2}\hbox{O}_{3}$ content and postannealing temperature up to 250 $^{\circ}\hbox{C}$. The AT-ZIO TFT exhibited a field effect mobility of 30.2 $\hbox{cm}^{2}/\hbox{Vs}$, a subthreshold swing of 0.17 V/dec, and an on/off current ratio of more than $10^{9}$ .