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

Abstract

Thin-film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under positive bias stress with different gate and drain voltages (VGS and VDS, 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 VDS 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 VGS and VDS fields determine which mechanism dominates, providing an analytical insight for the design of stable devices for driving transistors in AMOLED backplanes.