Y3+ (0, 6, 12, or 20 mol%)-doped In2O3 (YInO; YIO) thin films were fabricated by the sol–gel spin-coating technique, and they were used as the active layer of thin-film transistor (TFT) devices. The YIO-TFTs operate in the n-channel enhancement mode and exhibit a well-defined pinch-off and saturation region. The Y3+ (12 mol%)-doped In2O3 TFT possesses the optimal performance, and its field-effect mobility in the saturated regime, threshold voltage, on–off ratio, and S factor are 0.95 cm2 V−1 s−1, 6.74 V, 1.55 × 105, and 2.37 V decade−1, respectively. The yttrium ion can act as the carrier suppressor to reduce the carrier concentration of In2O3 thin film because of its lower electronegativity (1.22) and standard electrode potential (−2.372 V). The carrier concentrations and conductivities of In2O3 thin films decrease from 2.5 × 1014 to 3.8 × 1011 cm−3, and 14.3 to 1.5 × 10−4 S m−1, respectively, with the increase of Y3+ doping concentrations from 0 to 12 mol%. In addition, the Y3+ (12 mol%)-doped In2O3 thin film also possesses the minimal surface roughness (4.19 nm) and lowest trap states (1.07 × 1013). Therefore, by Y3+ doping the electrical properties of In2O3 thin films can be improved to match the basic requirement of the TFT devices.