Air quality evaluation by monolithic InP-based resistive sensors

After a brief summary on environmental pollution and commonly used material s for NO2 and O-3 sensing, simple InP-based resistive sensors are studied. The gas sensitive device is a thin n-type InP epitaxial layer grown on a se mi-insulating InP substrate. The electrical resistance of the layer, measur ed between ohmic contacts, increases in the presence of oxidising gases, th e most important variations occurring in NO2 and ozone-containing atmospher es. A complete study performed in laboratory at a moderate operating temper ature of 80 degreesC on the sensor exposed to nitrogen dioxide enabled to o btain a precise NO2 calibration curve. In an outdoor application, only NO2, ozone and humidity seem to act on the sensor resistance, confirming labora tory experiments. One of these devices was used to monitor air pollution in an urban atmosphere. The resistance changes were compared with the reading s of a commercial analysing equipment. The results clearly show that the se nsor resistance variation follow the total O-3 and NO2 concentrations varia tion in air. Different operating methods are discussed for these type of se nsors, depending on what of these two pollutants has to be measured. (C) 20 01 Elsevier Science B.V. All rights reserved.