Toward Adequate Operation of Amorphous Oxide Thin-Film Transistors for Low-Concentration Gas Detection

We suggest the use of a thin-film transistor (TFT) composed of amorphous InGaZnO (a-IGZO) as a channel and a sensing layer for low-concentration NO₂ gas detection. Although amorphous oxide layers have a restricted surface area when reacting with NO₂ gas, such TFT sensors have incomparable advantages in the aspects of electrical stability, large-scale uniformity, and the possibility of miniaturization. The a-IGZO thin films do not possess typical reactive sites and grain boundaries, so that the variation in drain current of the TFTs strictly originates from oxidation reaction between channel surface and NO₂ gas. Especially, the sensing data obtained from the variation rate of drain current makes it possible to monitor efficiently and quickly the variation of the NO₂ concentration. Interestingly, we found that enhancement-mode TFT (EM-TFT) allows discrimination of the drain current variation rate at NO₂ concentrations ≤10 ppm, whereas a depletion-mode TFT is adequate for discriminating NO₂ concentrations ≥10 ppm. This discrepancy is attributed to the ratio of charge carriers contributing to gas capture with respect to total carriers. This capacity for the excellent detection of low-concentration NO₂ gas can be realized through (i) three-terminal TFT gas sensors using amorphous oxide, (ii) measurement of the drain current variation rate for high selectivity, and (iii) an EM mode driven by tuning the electrical conductivity of channel layers.