Simple Analytical Model of On Operation of Amorphous In-Ga-Zn-O Thin-Film Transistors

The on operation of an amorphous In-Ga-Zn-O thin-film transistor (a-IGZO TFT) was studied employing a gated-four-probe (GFP) structure TFT to build a simple analytical model based on bias-dependent field-effect mobility (mu(FE)). The electrical characteristics of the a-IGZO GFP TFT revealed that the contact resistances were negligible compared with the channel resistance. The bias-dependent mu(FE) was derived from the transfer characteristics at low drain voltages (V-D), and approximately represented by a power function of the bias voltage, like that for hydrogenated amorphous Si (a-Si: H) TFTs. The mobility model reproduced both the current-voltage characteristics and the potential distribution in the channel, including the high V-D region. Si TFTs exhibit non-negligible variation of drain current (I-D) in the saturation region and their models require extra parameters to describe it. In contrast, the a-IGZO TFTs exhibit flat I-D-V-D characteristics in the saturation region, and their model does not require an extra parameter. Due to these features, the model of the a-IGZO TFTs is simpler than those of a-Si: H TFTs. The temperature dependence of the TFT characteristics indicated that the bias dependence of mu(FE) cannot be explained just by the exponential subgap traps. The dependence should be understood by introducing the carrier-density dependent mobility of a-IGZO films.