Metal oxide semiconductor material responsivity

Noble metal additives such as Pt, Pd, or Ag, are often added to metal oxide semiconductor sensor materials to enhance sensor response to a particular gas or class of gases [35]. Other metals, namely Be, Mg, Ca, Sr, Ba, have been used with tin oxide to enhance performance of an alcohol selective sensor [36], for example. Commercially produced metal oxide sensors may customarily use one or more of the above catalysts. 

Various methods have been used to fabricate tin dioxide metal oxide semiconductor gas sensors. Riviere et al (2003) used screen-printing to produce a tin dioxide film. The composition of the printed ink was varied. The materials showed a significant response to CO gas - typically, a resistance change of 1 log (Ohm i) a QQ concentration of 300 ppm, at an operating temperature... 

Various materials and fabrication methods have been investigated to produce metal oxide semiconductor CO2 gas sensors. The most widely-used materials based on tin dioxide have largely been found to be insuffident for the task. Typically, poor response behaviour to CO2 and high cross-sensitivity are major problems for tin dioxide-based materials. Examples are work conducted by Patel et al. (1994) and Hoefer et al. (1994). The sensors in the study by Hoefer et al showed a marked response in the concentration range of 1000-10000 ppm CO2 at an operating temperature of 270°C. The most significant responses were above CO2 concentrations of 5000 ppm but. 

A well-known practical example of homoepitaxy is the deposition, by CVD vapor phase epitaxy, of a single crystal Si film on a Si wafer. This is one of the primary steps in the manufacture of bipolar transistors and some metal-oxide semiconductor (MOS) transistors. Explain why an epitaxial Si film on Si is deposited while the Si substrate alone could well suffice as the material for the required electronic and mechanical response ... 

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