Silicon based micromachining technology enables the realization of high performance micro electromechanical systems (MEMS) including a range of physical and environmental sensors. Pressure sensors are used for a wide range of monitoring and control applications, e.g. environmental, industrial, aircraft, automotive. Monitoring of vehicle tyre pressures offers benefits such as improved safety, fuel economy, and tyre life. Micromachined pressure sensors are used at present, but require further research to improve their performance in terms of size, power consumption and manufacturing cost. This thesis has reviews pressure sensor technology and new developments in this area. A comparison of existing and potential future sensing mechanisms has been undertaken and identified as silicon piezoresistors. The focus of the research is motivated by the recently discovered enhanced piezoresistive effect in silicon nanowires where sensitivity can be increased by decreasing the dimension of nanowire. This thesis investigates the piezoresistive effect in p-type <110> silicon nanowires, fabricated using top down approach. It is found that the piezoresistive effect increases when the nanowire width is reduced below 400nm. Compared with micrometre sized piezoresistors, silicon nanowires have produced up to 100% enhancement. In addition, measurements indicate that the temperature coefficient of resistance (TCR) of silicon nanowire has improved with up to 40% decrease in TCR. The improvement in these two areas will be beneficial for the development of new MEMS pressure sensors. COMSOL is employed to simulate the piezoresistance effect in p-type <110> silicon for a range of doping concentrations. Simulation results demonstrate a similar trend to experimental results and publication data and show that the piezoresistance effect decreases as the doping concentration increases.

A novel method of improving the selectivity of metal oxide gas sensors has been developed by using catalytically active molecular sieve materials. They have been successfully introduced into a proprietary sensor array and a commercially available electronic nose. The cracking patterns of various organic compound groups including alkanes, aromatics and flavours over transition metal exchanged zeolites (ZSM-5, zeolite Y, and zeolite p) have been measured using a zeolite bed/GC/MS experimental set-up within a temperature range of 200 C to 400 C. The findings have been successfully translated for the use of the zeolites as filter technology on chromium titanium oxide (CTO) sensors for the purposes of selective gas sensing. Studies have been carried out regarding the effects of metal loading, zeolite type, material fabrication techniques and operating temperature with regards to catalytic activity and selectivity. Variations in activity due to alkane chain length have been related to the ability of the molecule to enter the zeolite cavity, the quantity of supported metal complexes and their oxidation state in the zeolite pores. The composite sensors utilising the novel zeolite materials have been used in a custom built sensor rig that houses 3 dual electrode sensors and can measure real time responses of these sensors to an introduced headspace generated from organic liquids. The response data have been utilised in a statistical software package (SPSS 12.0) to rationalise sensor discriminatory behaviour to various compound groups. The zeolite coated CTO sensors have also been tested in a commercial electronic nose array which has provided enhanced discrimination as compared to a standard CTO sensor array for a number of active and potential commercial applications mainly involving complex flavour compound mixtures.

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