▪ MOS gas sensors are less expensive to produce and used widely for CO, propane, and other hazardous gas detection. 1. When semiconductors (e.g. tin dioxide) are heated in air at high temperature, oxygen is adsorbed on the sensing surface by capturing free electrons. 2. The reduction in free electrons increases the resistance of the sensor. 3. In the presence of reducing gases, the surface density ofadsorbed oxygen decreases as it reacts with the reducing gases. 4. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor.