Effect of Zn Composition Ratio in InZnO TFT CO₂ Gas Sensor

Abstract

CO₂ sensor device using thin film transistor (TFT) structure is attracting attention from the viewpoint of gas sensitivity and low power consumption because CO₂ monitoring everywhere is required in various fields. Oxide semiconductors are used as its channel material, however, the correlation between electrical properties of oxide thin film and gas sensitivity in TFT-type gas sensor has not been clarified, thus far. In this study, we fabricated InZnO TFTs with different composition ratios of In₂O₃ and ZnO, and investigated the influence of their electrical properties on CO₂ sensitivity. In terms of the TFT properties, InZnO with a Zn content of 31% showed the best field-effect mobility. This is because the addition of Zn generated more oxygen vacancy and increased carrier density. On the other hand, further increasing Zn content, the electrical properties deteriorated compared to non-doped In₂O₃ and the maximum activation energy increased. For CO₂ sensing, the sensitivity improved as the Zn composition ratio decreased, and thus the non-doped In₂O₃ TFT showed the highest performance. We believe that adsorption of gas molecules becomes more effective when an oxide semiconductor film has both smaller activation energy and moderately high carrier density.