This study attempts to make amorphous IZO:Hf films by sol-gel method under the one-hour annealing heat treatment at the temperature of 500 ℃, yet trying with different degrees of doping concentration, and applies on the active layer of the thin film transistors. By XRD, we are able to determine that these thin films, under the annealing treatment at 500 ℃, remain amorphous and bear the rate of optical transmittance at up to more than 85% via the visible (400 ~ 800 nm) light. Films with various degrees of doping concentration are continuous and smooth; the thickness of these films is approximately 20 nm. In terms of the roughness, the higher the doping concentration is, the rougher those surfaces of films get. The optimal roughness at 0.367 nm occurs when the concentration of Hf is measured at 5 %.This study successfully applies the amorphous IZO:Hf films in the active layer of thin film transistors. By using three different sets of concentrations of doping, we improve the device characteristics of indium zinc oxide (IZO).As the Hf is doping concentration of Hf is at 5 %, the starting voltage (Vth) is -9.2 V; the switching current ratio (Ion / Ioff) is roughly 105 ; the subthreshold swing is 1.4 V/dec, and the carrier mobility rate is 0.59 cm2/Vs. If the doping concentration of Hf is set at 10 %, the initial voltage (Vth) is 1.1 V; the switching current ratio (Ion / Ioff) is about 105; the subthreshold swing is 0.7 V/dec, and the rate of the carrier mobility was 0.42 cm2/Vs. When the doping concentration of Hf increases to 30%, the starting voltage (Vth) is unknown V; the switching current ratio (Ion/Ioff) is about 104; the subthreshold swing is 3 V/dec, and the carrier mobility rate is 0.12 cm2 / Vs.