Appropriate choice of channel ratio in thin-film transistors for the exact determination of field-effect mobility

Abstract

Field-effect mobilities are the most important figures of merit to evaluate the feasibility of semiconductors for thin-film transistors ͑TFTs͒. They are, however, sometimes extracted from TFTs with the active semiconductor area undefined and in small channel ratios without the effect of the fringing electric field at the ends of source/drain electrodes taken into account. In this letter, it is demonstrated that the field-effect mobilities extracted from undefined nanoparticulate zinc oxide ͑ZnO͒ TFTs at the channel ratio of 2.5 are overestimated by 418%, and the choice of large channel ratios gives the real value of field-effect mobilities.Thin-film transistors ͑TFTs͒ fabricated in lowtemperature and high throughput processes are in great demand for low-cost and large-area production. 1 Organic semiconductors, including small molecules 2 and conjugated polymers, 3,4 have intensively been studied and developed for TFTs, taking the advantage of compatibility with lightweight and flexible substrates. As an alternative route, inorganic semiconductors, including crystalline oxides 5-7 and amorphous oxides, 8,9 have been attracting significant interest in the past decade. 10 These inorganic TFTs generally work in the n-channel mode and benefit from transparency to visible lights, processibility with solutions, and compatibility with lightweight and flexible substrates as well. In order to evaluate the feasibility of any semiconducting material for TFTs, the most important figure of merit is the field-effect mobility . It is frequently extracted from the transfer characteristics of TFTs in the saturation regime, according to the equationwhere C i is the gate capacitance per unit area, W and L are the channel width and length, respectively, and I ds , V gs , and V th are the drain current, the gate voltage, and the threshold voltage, respectively. The channel ratio is commonly defined by W / L. In the case of polycrystalline zinc oxide ͑ZnO͒ TFTs, 11-14 early investigations have shown of 0.1-1.2 cm 2 V −1 s −1 for a channel ratio of 6-40. However, it has been reported that the fieldeffect mobility strongly depends on the channel ratio, ranging from 1.9 to 27 cm 2 V −1 s −1 for the channel ratio from 64 to 1.4. 15-17 Figure 1͑a͒ shows a top-view image of a typical TFT in the bottom gate configuration with the active semiconductor area undefined. It is expected that, particularly in the case of small channel ratios, the effective channel width W eff is not equivalent to the geometrical channel width W 0 but is extended somewhat because of the fringing electric field at the ends of the electrodes, which can lead to the overestimation of the field-effect mobility. Actually, the mobilities reported on the undefined TFTs 12,14-17 seem much higher than those reported on the defined TFTs, 11,13 if they are compared at the same channel ratio. In this letter, therefore, nanoparticulate ZnO TFTs in the bottom gate configuration are modeled with the channel width extensions ⌬W 1,2 included in the effective channel width W eff , as s...

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