High pressure hydrogen annealing (HPHA) process was employed to introduce hydrogen into the amorphous indium gallium zinc oxide (a-IGZO) channel material more effectively to improve the electrical performance of a-IGZO thin film transistors (TFTs). For the comparative study, a-IGZO TFTs annealed in N<SUB>2</SUB> ambiance was also fabricated and their electrical performances were compared with those annealed in high pressure hydrogen ambiance. The electrical characteristics of the as-fabricated devices include the field effect mobility (μ<SUB>eff</SUB>) of 0.01 cm<SUP>2</SUP>/Vs, subthreshold slope (S) of 2.31 V/decade, and on/off current ratio of 1.42×10<SUP>5</SUP>. High pressure hydrogen annealing was carried out by controlling hydrogen gas pressure from 1 to 15 atm at 250<SUP>o</SUP>C for 20 min. For the control devices, rapid thermal annealing process in N<SUB>2</SUB> ambiance was applied to the as-deposited TFTs at 300<SUP>o</SUP>C for 10, 15, and 20 min. Both the post-annealing processes improved the performances of the TFTs significantly when they are compared with those of the as-fabricated devices. For the N<SUB>2</SUB>-annealed devices, the best performances were obtained from the devices annealed for 15 min. These devices exhibited the S of 1.22 V/decade, the μ<SUB>eff</SUB> of 1.03 cm<SUP>2</SUP>/Vs, and the on/off current ratio of 1.87×10<SUP>8</SUP>. Meanwhile, it was found that the performance of the devices annealed in hydrogen ambiance strongly depended upon the hydrogen gas pressure. The field effect mobility increased with the hydrogen pressure. The maximum μ<SUB>eff</SUB> of 10.33 cm<SUP>2</SUP>/Vs was obtained from the devices annealed at 15 atm. However the best on/off current ratio and the lowest S were obtained from the devices annealed at 5 atm. When the devices were annealed at hydrogen pressure of 10 atm, the carrier density increased a lot, but the field effect mobility did not change much. For the devices annealed under the hydrogen pressure of 15 atm, the current density did not change much, but the field effect mobility was improved around two times when the device performances are compared with those annealed at the hydrogen pressure of 10 atm. From these result, it can be concluded that the hydrogen incorporated in the IGZO channel material effectively increased the carrier concentration leading to the increased current density. When the devices are annealed under the higher hydrogen pressure such as 15 atm, the hydrogen in the IGZO channel effectively passivate the defects in the material so that the field effect mobility was dramatically improved. In summary, by introducing hydrogen in the annealing process, the field effect mobility was order of magnitude improved when it is compared with that of the devices annealed in N<SUB>2</SUB> ambiance. More detailed experimental procedures, material characteristics, and device performances will be presented. This work was supported by AFOSR grant monitored by Dr. Gregg Jessen and WCU program funded by MEST.