Threshold Voltage Instability and Dielectric Breakdown in Flexible Bottom-Gate Ti/Al2O3/IGZO Thin-Film Transistors

This paper explores the instability of threshold voltage (VTH) and dielectric breakdown in flexible n-type thin-film transistors (TFTs) featuring a bottom-gate Ti/Al2O3/IGZO structure. Measurements were performed in dark conditions at 25°C on a set of devices with the same channel width (W= 280 μm) and different channel lengths (L) ranging from 100 μm down to 5 μm. Regarding theVTHinstability, both hysteresis (applyingVGSfrom -1 V to 5 V and vice versa) and positive gate bias stress (applyingVGS= 5 V for 1000 s) data show a positiveVTHshift (ΔVTH) ascribed to electron trapping at the channel/oxide interface or within the bulk oxide. Our study also demonstrates that in some devices the stress-induced ΔVTHis fully recovered by applying aVGS= -2 V for 1000 s as a result of releasing charges previously trapped during stress. Moreover, measurements highlight an increasing trend of both hysteresis- and stress-induced ΔVTHfor shorter channel devices. Consistently, time-dependent dielectric breakdown data (applyingVG= 11 V) indicate a faster process in smaller devices. Then, overall results show lower reliability in more scaled devices, thus proving that a key challenge for the investigated technology is to realize reliable short-channel TFTs.