The triple-targets magnetron co-sputtering system with three targets of In$_{2}$O$_{3}$, Ga$_{2}$O $_{3}$, and Zn was used to deposit amorphous indium gallium zinc oxide (a-IGZO) films. The deposited a-IGZO films were used as the channel layers of the transparent thin film transistors (TFTs). The In$_{2}$O$_{3}$ RF power of 50 W, Ga$_{2}$O$_{3}$ RF power of 25 W and Zn DC power of 10$~$ W were independently tuned to obtain the optimal composition $(\hbox{In:Ga:Zn} = 3.5{:}1{:}2.7)$ of the a-IGZO films. The effective field-effect mobility, on-to-off current ratio, and subthreshold swing of the optimal a-IGZO TFTs were 62.3 cm$^{2}$/V $\cdot$s, 6.5$\,\times\,$ 10$^{6}$ , and 0.23 V/decade, respectively. Using the SiO $_{\rm x}$ passivation and thermal-annealing treatment, the effective field-effect mobility, on-to-off current ratio, and subthreshold swing of the optimal a-IGZO TFTs were further improved to 68.5 cm$^{2}$/V $\cdot$s, 7.0$\times$ 10$^{6}$ , and 0.22 V/decade, respectively. In addition, stable performances were also noted.