Local-Degradation-Induced Threshold Voltage Shift in Turned-OFF Amorphous InGaZnO Thin Film Transistors Under AC Drain Bias Stress

Local degradation caused by drain bias (VDS) stressing is recently considered as a key issue in amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). In this letter, we investigate the instability of turned-OFF a-IGZO TFTs under ac VDS stressing. The negative threshold voltage shift, which was well fitted by a stretched exponential function, was accelerated with increasing duty cycle despite the same effective stress time. A capacitance measurement reveals that a higher duty cycle induced more donor states near the drain, implying that the stretched-exponential time dependence cannot be fully explained by trapping mechanism. Temperature-dependent τ followed the Arrhenius relation, whereas β showed unusual temperature dependence in contrast to that under dc VDS stressing. These findings suggest an additional origin such as a stress release effect under an ac VDS stress other than hopping/tunneling mechanism.