High-k Y₂O₃ thin films were investigated as the gate dielectric for amorphous indium zinc tin oxide (IZTO) thin-film transistors (TFTs). Y₂O₃ gate dielectric was deposited by radio frequency magnetron sputtering (RF-MS) under various working pressures and annealing conditions. Amorphous IZTO TFTs with SiO₂ as the gate dielectric showed a high field-effect mobility (μFE) of 19.6 cm²/Vs, threshold voltage (Vth) of 0.75 V, on/off current ratio (Ion/Ioff) of 2.0×106, and subthreshold swing (SS) value of 1.01 V/dec. The IZTO TFT sample device fabricated with the Y₂O₃ gate dielectric showed an improved subthreshold swing value compared to that of the IZTO TFT device with SiO₂ gate dielectric. The IZTO TFT device using the Y₂O₃ gate dielectric deposited at a working pressure of 5 mtorr and annealed at 400 °C in 6 sccm O₂ for 1 hour showed a high μFE of 51.8 cm²/Vs, Vth of -0.26 V, Ion/Ioff of 6.0×10³, and SS value of 0.19 V/dec. With the application of a Y₂O₃ gate dielectric, the Vth shift improved under a positive bias stress (PBS) but was relatively unaffected by negative bias stress (NBS). These shifts were attributed to charge traps within the gate dielectric and/or interfaces between the channel and gate dielectric layer.