Air quality evaluation by monolithic InP-based resistive sensors

After a brief summary on environmental pollution and commonly used materials for NO2 and O3 sensing, simple InP-based resistive sensors are studied. The gas sensitive device is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance of the layer, measured between ohmic contacts, increases in the presence of oxidising gases, the most important variations occurring in NO2 and ozone-containing atmospheres. A complete study performed in laboratory at a moderate operating temperature of 80°C on the sensor exposed to nitrogen dioxide enabled to obtain a precise NO2 calibration curve. In an outdoor application, only NO2, ozone and humidity seem to act on the sensor resistance, confirming laboratory experiments. One of these devices was used to monitor air pollution in an urban atmosphere. The resistance changes were compared with the readings of a commercial analysing equipment. The results clearly show that the sensor resistance variation follow the total O3 and NO2 concentrations variation in air. Different operating methods are discussed for these type of sensors, depending on what of these two pollutants has to be measured.