Study on the response and recovery properties of semiconductor gas sensors using a high-speed gas-switching system

A high-speed gas-switching system, in which a low-dead volume chamber (0.6 cm3) was connected to a gas flow apparatus equipped with a high-speed gas-switching valve operative at a rate of 30 ms, was designed to investigate the real response and recovery properties of semiconductor gas sensors. The developed system allowed rapid replacement of the gas atmosphere in the chamber where a gas sensor device was placed within 0.3 s. It was revealed that the response speed of the sensor device based on a SnO2 porous film (pore size at maximum population: 37 nm) was remarkably fast, reaching a response time of less than 1 s for H2 and CO detection at 250 and 350 °C. This suggests that the diffusion and surface reaction of H2 and CO are quite fast in the porous film. On the other hand, the recovery speed was not comparably fast and the resistance of the device did not recover to the original state within 20 s after switching the gas atmosphere in the chamber from the sample gases to air. This is possibly due to the slow desorption of the H2O and CO2 that were formed by the surface reaction of H2 and CO, respectively with the adsorbed oxygen on SnO2.