The Effect of Gate and Drain Fields on the Competition Between Donor-Like State Creation and Local Electron Trapping in In–Ga–Zn–O Thin Film Transistors Under Current Stress

Thin-film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under positive bias stress with different gate and drain voltages (V_GS and V_DS, respectively). The transfer characteristics with respect to stress time were examined, focusing on the threshold voltage (V_T) values obtained when the source and drain electrodes are interchanged during readout (forward and reverse V_DS sweep). The V_T values shift toward either negative or positive values during stress, while transitions from negative to positive shifts are also observed. The negative V_T shift under positive bias stress is interpreted to occur by the generation of donor-like states related to ionized oxygen vacancies. On the other hand, positive V_T shifts result from the trapping of electrons near the IGZO/gate insulator interface. The transitions from negative to positive V_T shift are believed to result from the local electron trapping mechanism that gradually takes over donor-like state creation. From the experimental results and TCAD device simulation, it is suggested that a competition occurs between donor-like state creation and electron trapping. The relative magnitudes of the V_GS and V_DS fields determine which mechanism dominates, providing an analytical insight for the design of stable devices for driving transistors in AMOLED backplanes.