Amorphous In−Ga−Zn−O (a-IGZO) has been studied as a channel layer in thin-film transistors (TFTs). To improve the bias-induced instability of a-IGZO TFTs, we introduced yttrium with high bond enthalpy by magnetron co-sputtering system. The Y-doped a-IGZO (a-IGZO:Y) films show relatively lower carrier concentration and higher Hall mobility, which is due to the suppression of oxygen vacancies caused by Y doping. The a-IGZO:Y showed a relatively higher transmittance in the visible light region compared to non-doped IGZO, which could be due to the decrease of shallow defect levels caused by oxygen vacancy in the band gap. The a-IGZO without Y doping showed dramatic changes in electrical properties as times progressed (over 240 h); however, the a-IGZO:Y showed no significant changes. The a-IGZO:Y TFTs demonstrated a more stable driving mode as exhibited in the positive gate bias stress test even though the values of VTH and SS were slightly degraded.