In this study, high-performance indium–gallium–zinc oxide thin-film transistors (IGZO TFTs) with a dual-gate (DG) structure were manufactured using plasma treatment and rapid thermal annealing (RTA). Atomic force microscopy measurements showed that the surface roughness decreased upon increasing the O₂ ratio from 16% to 33% in the argon–oxygen plasma treatment mixture. Hall measurement results showed that both the thin-film resistivity and carrier Hall mobility of the Ar–O₂ plasma–treated IGZO thin films increased with the reduction of the carrier concentration caused by the decrease in the oxygen vacancy density; this was also verified using X-ray photoelectron spectroscopy measurements. IGZO thin films treated with Ar–O₂ plasma were used as channel layers for fabricating DG TFT devices. These DG IGZO TFT devices were subjected to RTA at 100 °C–300 °C for improving the device characteristics; the field-effect mobility, subthreshold swing, and I_ON/I_OFF current ratio of the 33% O₂ plasma–treated DG TFT devices improved to 58.8 cm²/V·s, 0.12 V/decade, and 5.46 × 10⁸, respectively. Long-term device stability reliability tests of the DG IGZO TFTs revealed that the threshold voltage was highly stable.