Characterization of Top-Gate Effects in Amorphous InGaZnO4 Thin-Film Transistors Using a Dual-Gate Structure

We investigate the dependence of bottom-gate transfer characteristics on top-gate voltage, which we call top-gate effects, using amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) having a dual-gate structure. We found that the positive top-gate effect varies depending on the density of electron traps at the top-channel interface, while the negative top-gate effect has a similar impact on the bottom-gate transfer characteristics irrespective of the top-channel property. OFF-current increase due to the positive top-gate effect, which is one of the undesirable behaviors for practical use, was found to be effectively suppressed by sacrificing the subthreshold performance. These behaviors were described in terms of mutual interactions between the bottom-gate and top-gate electric fields. In comparison with conventional hydrogenated amorphous silicon (a-Si: H) TFTs, a-IGZO TFTs showed more significant top-gate effects. We consider this result to be due to the intrinsic material nature of a-IGZO, i.e., high electron mobility and nonexistence of hole accumulation in a-IGZO. (C) 2012 The Japan Society of Applied Physics