Letter: Mechanisms for on/off currents in dual-gate a-Si:H thin-film transistors using indium-tin-oxide top-gate electrodes

— Two types of dual-gate a-Si:H TFTs were made with transparent indium-tin-oxide (ITO) top-gate electrodes of different lengths to investigate the static characteristics of these devices. By changing the length of the ITO top gate, we found that the variations in the on-currents of these dual-gate TFTs with dual-gate driving are due to the high resistance of the parasitic intrinsic a-Si:H regions between the back electron channel and the source/drain contact. In the off-state of the dual-gate-driven TFTs, the Poole-Frenkel effect is also enhanced due to back-channel hole accumulation in the vicinity of the source/drain contact. Furthermore, we observed for the first time that under illumination the dual-gate-driven a-Si:H TFTs exhibit extremely low photo-leakage currents, much lower than that of single-gate-driven TFTs in a certain range (reverse subthreshold region) of negative gate voltages. The high on/off current ratio under backside illumination makes dual-gate TFTs suitable devices for use as switching elements in liquid-crystal displays (LCDs) or for other applications.