Comprehensive studies on the stabilities of a-In-Ga-Zn-O based thin film transistor by constant current stress

Stability under constant current stress, along with hysteresis characteristics, was studied for a-In-Ga-Zn-O thin-film transistors (TFTs) in several atmospheres and at several temperatures. Unannealed TFTs showed rather large instability; i.e., large hysteresis in transfer curves (ΔVG > 0.8 V) and large positive threshold voltage shift (ΔVth > 10 V for 50 h tests at 5 µA) with deterioration of subthreshold voltage swing was observed. The instability for the unannealed TFT had a strong dependence on the stress atmosphere and the stress temperature, which suggests that trap states generated by the stress test is related to oxygen vacancy formed by breaking weak chemical bonds. Wet annealing improved stability; the hysteresis disappeared and the ΔVth was reduced to < 2 V. The improvement is considered to be related to the reduction of weak chemical bonds by wet annealing with the strong oxidation power of water molecules.