Amorphous oxide semiconductors have become the industry standard for display backplanes, but their limited mobility necessitates complex low-temperature polycrystalline oxide (LTPO) stacks, increasing cost and reducing yield. To realize a practical oxide-only backplane that can serve as both drive and switching transistors, a channel combining high mobility, low off current, and robust stability is required. Here, we report zinc-free crystalline indium-gallium oxide (IGO) thin-film transistors (TFTs) that crystallize below 400 °C with preferential (222) orientation. By tuning the In:Ga ratio, an amorphous-to-crystalline transition is achieved, enhancing mobility to 83.3 cm² V^-1 s^-1 while maintaining an off-current of ≈10^-13 A. The optimized crystalline IGO TFTs (In:Ga = 12:3) exhibit the smallest threshold-voltage shift (<0.5 V) under bias stress and reproducible electrical characteristics across multiple devices. Furthermore, we provide experimental investigation of the observed stability by analyzing the trap density of states (TDOS) near both the valence-band maximum (VBM) and conduction-band minimum (CBM) using a combined optoelectronic approach based on photoexcited charge collection spectroscopy (PECCS) and photoresponse capacitance-voltage (C-V). The complementary analysis reveals that crystalline IGO stabilizes oxygen-vacancy-related states closer to the conduction band, with the extracted TDOS evolution showing self-consistent correlation with the measured bias-stress-induced electrical behavior. Circuit-level validation with a complementary inverter confirms stable gain and noise margins. These results establish crystalline IGO as a viable single-material oxide channel combining high mobility, robust bias stability, and simplified processing for next-generation oxide-only backplanes.