Demonstration of SA TG Coplanar IGZO TFTs With Large Subthreshold Swing Using the Back-Gate Biasing Technique for AMOLED Applications

Abstract

We demonstrate that the shorter channel self-aligned top-gate (SA TG) coplanar indiumgallium- zinc oxide (IGZO) thin-film transistors (TFTs), with negative voltage applied to the back-gate, exhibit superior characteristics as driving transistors in organic light-emitting diode (OLED) pixels compared to their longer channel counterparts. The shorter channel IGZO TFTs (with a channel length (L) of 3 μm) biased with a back gate voltage of −3.5 V showed a larger subthreshold swing (SS = 0.21 V/dec) than the longer channel ones (with L = 5 μm, SS = 0.16 V/dec) with a similar threshold value (VTH = 0.7–0.8 V). A large SS is beneficial for controlling grayscale levels, especially at low gray levels, when IGZO TFTs are used as driving transistors in OLED pixels. Furthermore, the negatively back-gate-biased shorter channel SA TG coplanar IGZO TFTs exhibited significantly enhanced electrical stability compared to the longer channel ones under both positive gate bias and hot carrier stresses. The findings of this study are expected to be useful in expanding the utility of IGZO TFTs in OLED displays.