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

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