Ternary oxide thin films in the In₂O₃-ZnO-SnO₂ system were studied for potential applications in oxide semiconductor thin film transistors (TFTs). An amorphous In-Zn-Sn-O (a-IZTO) active channel layer was deposited by RF magnetron sputtering at room temperature on an n++ Si substrate. Films from a sintered ceramic target with a nominal chemical composition of In:Zn:Sn = 40:50:10 at.% were prepared with thicknesses ranging from 15 nm to 150 nm, which was followed by annealing at 350 °C for 30 minutes in air. Subsequently, a bilayer Cu/Ti metal contact was deposited as the source/drain electrodes on the top surface through a shadow mask using an e-beam evaporator. The thickness of the active channel layer greatly influenced the characteristics of the oxide thin film transistors. The best transistor characteristics were observed from the test device with a channel thickness of 30 nm and a high on/off current ratio of approximately 108, high field effect mobility of 25 cm²/Vs, low threshold voltage of -0.1 V, and very small subthreshold swing of 0.14 V/dec.