Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors Part I: Long-Channel Devices

Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numerical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent analysis based on experiments, numerical simulations, and analytical modeling is provided.