The electrical performance of HfO2/SiNx stacked dielectric amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) fabricated using different oxygen flow rates, channel thicknesses, annealing temperatures, and deposition powers was investigated. The results showed that when the thin films were deposited with a higher oxygen flow rate, the device saturation mobility (μsat) decreased while transfer curves shifted positively. Free carriers accumulated easily at the interfaces with higher channel thicknesses, which led to the increase in device mobility. The electrical characteristics of the a-IGZO TFTs were strongly affected by annealing temperature. This was because annealing generated free carriers and oxygen vacancies, which resulted in a negative shift in threshold voltage (Vth) and an increase in μsat. An a-IGZO TFT deposited with suitable parameters showed excellent electrical performance: a Vth value of 3.4 V, a saturation mobility of 18.1 cm2 V−1 s−1, an on/off current ratio of 108, and a subthreshold swing of 137 mV dec−1.