In this paper, we describe the fabrication of thin film transistors (TFTs) with amorphous indium-tin-zinc-oxide (ITZO) as the active material. A transparent ITZO channel layer was formed under an optimized oxygen partial pressure (OPP (%) = O2/(Ar + O2)) and subsequent annealing process. The electrical properties exhibited by this device include field-effect mobility (μ(eff)), sub-threshold swing (SS), and on/off current ratio (I(ON/OFF)) values of 28.97 cm2/V x s, 0.2 V/decade, and 2.64 x 10(7), respectively. The average transmittance values for each OPP condition in the visible range were greater than 80%. The positive gate bias stress resulted in a positive threshold voltage (V(th)) shift in the transfer curves and degraded the parameters μ(eff) and SS. These phenomena originated from electron trapping from the ITZO channel layer into the oxide/ITZO interface trap sites.