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Zinc oxide (ZnO) has drawn much attention because of their interesting semiconducting properties as a transparent device to visible lights with a wide band-gap energy, 3.37eV. In fabricating ZnO devices, the easy sputtering process can facilitate the formation of thin film as a TFT on a plastic flexible substrate with a rather high mobility and a high on/off ratio over 105. But the stability problems such as reproducibility or hysteresis became crucial for the practical application together with the recipe of enhancing the mobility. Voltage-current characteristics of ZnO devices showed different behaviors under the floating gate or well-defined gate voltages. When measuring the current, the voltages were swept from zero to a positive maximum then to a negative maximum and again to zero. The current level for the sweeping-up stages was smaller than the case for the sweeping-down stages. The hysteretic voltage-current characteristics were significant, attributed to the existence of traps in the gate side and the channel side close to an insulator-semiconductor interface. In the case of the floating gate configuration, the charges in the insulator side traps kept so the polarization of the dielectric (gate insulator) is determined by the initial charges (electron) in gate insulator side traps. The dielectric polarization can influence on the gate electrode indirectly. The gate effect increased as the increase of drain voltage (Vds) can induce the migration of the initial charges on a semiconductor. Based on the assumption of constant charges the gate bias will change, giving the positive curvature of Vds characteristics different from the conventional FETs. The changes of the gate effects can be verified by measuring variation of capacitances on a dielectric layer, sweeping drain voltages. The voltage-current characteristics as sweeping gate voltage showed a reversed hysteresis different from the output characteristics, which can be explained by threshold voltage variation as Vgs increase. The pulsed bias was observed to decrease the hysteretic curves, which can be explained by the reduced trap charging. The specific conduction mechanism will be given in the presentation together with the experimental proves. |