We investigated the effect of In and Zn component ratios on an amorphous indium-gallium-zinc-oxide (a-IGZO) layer in oxide thin-film transistors (TFTs). Different types of stress including negative bias, positive bias, negative bias illumination stress, positive bias illumination stress, and instability of a-IGZO TFTs were compared. Owing to the difference in the composition ratio of In: Ga: Zn (5: 1: 4, 6: 1: 3, 7: 1: 2), the respective devices had threshold voltages of 7, 5, and 4.5 V. The high content of In in a-IGZO leads to a higher concentration of oxygen vacancy, which results in sensitivity toward illumination stress. The above results indicate that the threshold voltage shift is primarily due to the trapping of electrons in the traps located in the interfaces or dielectric layers. We analyzed the IGZO thin film properties using UV-visible, X-ray diffraction, and X-ray photoelectron spectroscopy measurements.