Highly Sensitive Wireless NO₂ Gas Sensing System

There is a need for reliable and accurate air quality monitoring systems in order to secure human health from the air pollutants such as nitrogen dioxide (NO2). The development of low-cost and modern technology-compatible sensing systems is essential. In this work, a highly sensitive and selective indium gallium zinc oxide (IGZO) thin film transistor has been used to design an efficient and wireless NO2 sensing system. The sensing system uses the IGZO thin film transistor (TFT) as a current source for the current-starved ring oscillator (CSRO) where exposed NO2 concentration determines the oscillation frequency. The presented NO2 gas sensing system exhibited excellent sensitivity of 5.39 MHz/ppm with the lowest detection limit (LOD) of 50 ppb at room temperature. To the best of the authors' knowledge, the achieved sensitivity is the best-reported performance for frequency-based NO2 sensors. The sinusoidal output of the sensor obviates the need for costly peripheral signal conditioning circuits and allows direct integration with wireless systems. The 13.56 MHz RFID antenna connected to the output of the sensor is used to show wireless sensing compatibility and highlights the excellent candidacy of the proposed work for the growing Internet of Things (IoT) and untethered sensor applications.