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 (μ 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 μ FE was derived from the transfer characteristics at low drain voltages ( VD ) , 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 VD region. Si TFTs exhibit non-negligible variation of drain current ( ID ) in the saturation region and their models require extra parameters to describe it. In contrast, the a-IGZO TFTs exhibit flat ID - VD 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 μ 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.