Today, a large fraction of gas sensor industry has been monopolised by Metal Oxide Semiconductor (MOS) based chemoresistive gas sensors because of their simplity, ease of fabrication, reproducible response and thermal stablilty. In these type of sensors, the sensor resistance changes upon exposure to the analyte gas and attains a saturation. While, the resistance drops to the baseline when the test gas is removed. This happens because of charge transfer from MOS to the surface state near the fermi level of the semiconductor and vice versa. This reversible reaction is technologically utilized in detecting such hazrdous and toxic gases.
In our recent work, (Analysis of Unusual and Instantaneous Overshoot of Response Transients in Gas Sensors”; Sreya Suresh, Kusuma Urs., Anupama T. Vasudevan, Sharath Sriram, Vinayak Kamble*, Physica Status Solidi : Rapid Research Letters) it is shown that, the sensor response transients exhibit unexpected overshoot for an n-type MOS for hydrogen gas. The work systematically explores the role of diffusion of the test gas into the sensor material, ie. Zinc Oxide microrods. Often such overshoots, though weak in nature, are encountered in many reports. However, no efforts are made to study or explain their origin in the sensor response. This is perhaps the first of its kind experimental study revealing the mechanism of overshoot in response of gas sensors.
The sensor researcher community is moving towards smaller structures for enhanced sensitivities, but here the authors have found very high sensitivity (about 1000% change in current) towards typically few hundred ppm of hydrogen gas. This has been attributed to single crystal like nature which is devoid of barriers of grain boundaries in the ZnO microrods. The observed unusual overshoot behavior has been attributed to the sub-surface diffusion of atomic hydrogen with a typical coefficient of diffusivity, 𝐷 being 10−15 𝑚2/ 𝑠 at 250 oC, and its reaction with lattice oxygen defects in subsurface of ZnO crystal leading to trapping of induced charge carriers.