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

Abstract

Local degradation caused by drain bias ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {\mathbf {DS}}}$ </tex-math></inline-formula> ) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {\mathbf {DS}}}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau $ </tex-math></inline-formula> followed the Arrhenius relation, whereas <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> showed unusual temperature dependence in contrast to that under dc <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {\mathbf {DS}}}$ </tex-math></inline-formula> stressing. These findings suggest an additional origin such as a stress release effect under an ac <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {\mathbf {DS}}}$ </tex-math></inline-formula> stress other than hopping/tunneling mechanism.