| Summary: | The detection and monitoring of toxic and explosive gases is often performed using semiconductor gas sensors. The substrate forms an important part of these sensors and current designs were investigated and tested. Various new designs were developed and thick and thin-film technologies employed to fabricate substrates and complete sensors. Substrateless sensors were also analysed and alterations performed to fashion new devices. A number of ceramic materials were tested for their suitability as semiconductor gas sensor substrates. The adhesion of thin-films and thermal conductivity were found to be the most crucial properties, in addition to those typical of ceramics, such as high temperature stability. Alumina is routinely used in semiconductor gas sensors and many other substrates and its performance was compared with less commonly used materials such as beryllia and aluminium nitride. These materials have a much greater thermal conductivity than alumina, and this was shown to improve sensitivity. A semi-empirical formula was derived to enable the prediction of sensitivity loss of a semiconductor gas sensor fabricated on a substrate with high temperature gradients, compared with one where gradients are minimal. The heaters used to raise the temperature of the substrate are typically made from platinum films. The longevity of thin platinum films depends on the film thickness and substrate surface, but for a given film thickness on a given substrate, additional adhesion layers of various metals were also shown to change the films stability and hence lifetime. Various substrate geometries were investigated to optimise temperature distribution and sensitivity. Predominantly subtle effects were observed, but a significant increase of sensitivity was found with an increased surface area. Electrical circuitry used to control and monitor sensors was summarised and a new substrate developed which could be used in conjunction with switching circuitry, the main advantage being that the fabrication of the substrate was more economical than standard substrate configurations.
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