Study on Current Crowding in the Output Characteristics of Amorphous InGaZnO4 Thin-Film Transistors Using Dual-Gate Structures with Various Active-Layer Thicknesses

We investigate current crowding in the output characteristics of amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) using dual-gate structures with various active-layer thicknesses in comparison with that of hydrogenated amorphous silicon (a-Si:H) TFTs. As the active-layer thickness becomes more than 150 nm, current crowding is found to begin to occur for both types of TFTs. We show that the current crowding for a-IGZO TFTs is less significant than that for a-Si:H TFTs at the same active-layer thickness. The current crowding is also found to gradually decrease with increasing temperature. To gain an insight into the mechanism of current crowding, we investigate the temperature dependence of the current-voltage characteristics for an ITO/a-IGZO/ITO sandwich structure, which we assume to be equivalent to an electrode--intrinsic-channel (e-i-c) structure in the source/drain regions of a-IGZO TFTs. Results suggest that current crowding arises from space-charge-limited current in the e-i-c structure. The smaller degree of current crowding for a-IGZO TFTs makes it possible to utilize a relatively thick a-IGZO active layer, which might be desirable both for commercial applications such as photodetecting devices and for fabrication processes in mass production. (C) 2009 The Japan Society of Applied Physics