Thin-film transistors (TFTs) with a top-gate coplanar structure were fabricated using a free-gallium amorphous indium-aluminum-zinc-oxide film as the active layer and a polymethyl methacrylate film as the dielectric layer based on a low-cost solution process method,and the influence of the Al content in the active layer on the device performance was investigated.The results indicated that Al atoms acted as a carrier suppressor in the InZnO films. Therefore,the carrier concentration in the InZnO film decreased with increasing Al content and hence, the threshold voltage of the device moved forward positively, the off-state current of TFTs decreased, and the on/off current ratio of the device improved. Moreover, the hysteresis stability of the threshold voltage improved via the optimization of the trap density in the interface between the active and insulator layers by adjusting the Al content of the active layer. Overall, optimum device performance was achieved at an Al content of 30%.