In this study, a low-thermal-budget microwave irradiation (MWI) technique was applied as a post-deposition annealing (PDA) process to lower the trap densities that exist in transparent amorphous oxide semiconductor thin film transistors (TAOS TFTs). As channel layers of TAOS TFTs, two types of indium gallium zinc oxide (IGZO) with different compositions as well as aluminum zinc tin oxide (AZTO) and zinc oxide (ZnO) thin films were deposited with various thicknesses through radio frequency (RF) magnetron sputtering at 25°C. Cost-effective and energy-efficient MWI was conducted to enhance the electrical performance of transistors by removing traps and defects. The electrical characteristics of IGZO (1:1:1 and 4:2:3)-, ZnO-, and AZTO-based TFTs treated by MWI were evaluated by measuring the transfer curves. In particular, the relation between the interface trap density (Dit) and bulk trap density (Nt) of microwave-irradiated TFTs was quantitatively evaluated by the subthreshold swing (SS) variation based on channel thickness. The results indicated that of the four types of channel layers, the performance of IGZO (4:2:3) TFTs was the best and that of AZTO TFTs was the worst, in terms of electrical properties such as on/off current ratio, mobility SS, and trap density. In particular, it was demonstrated that the trap density of MWI-treated TAOS TFTs was much lower than that of conventional furnace annealing (CFA)-treated devices. Despite the short annealing duration of a few minutes, the MWI more effectively reduced the trap sites than did the furnace treatment, and significantly enhanced the electrical properties of the TAOS TFTs. It is expected that high-performance TAOS TFTs can be fabricated by applying MWI, which is a highly efficient and low-thermal-budget annealing method, to the PDA process and can thus reduce trap density.