Simon Ahlers

Response mechanism of SiC-based MOS field-effect gas sensors

SiC-based field-effect gas sensors with catalytic platinum electrodes (Pt-MOSiC) have been subjected to a series of gas response measurements. Structural analysis of the Pt electrodes revealed that the gas sensitivity and the selectivity of Pt-MOSiC gas sensors depend on two major parameters: the porosity and the catalytic activity of the Pt electrodes. Pt-MOSiC gas sensors with thick, dense Pt electrodes only exhibit hydrogen sensitivity, whereas Pt-MOSiCs with thin porous gates exhibit a broad range of gas sensitivities similar to resistive metal-oxide gas sensors. A model is put forward that explains the nonhydrogen gas response of Pt-MOSiC devices and of conventional polycrystalline metal-oxide materials on a common basis.

Temperature- and Field-Effect-Modulation Techniques for Thin-Film Metal Oxide Gas Sensors

Temperature- and field-effect modulation experiments have been performed on thin-film metal oxide materials deposited onto micromachined heater elements. Both stimuli have been found to induce a sequence of reactions with increasingly longer time constants: short-term changes in gas sensing properties and longer-term changes in the clean-air baseline level. Whereas the short-term changes are attractive for shaping cross sensitivity profiles in thin-film gas sensor arrays, the longer-term changes shed light on drift- and contact-related phenomena in metal oxide sensing materials. We propose that these latter phenomena are supported by charge-carrier-driven equilibration processes in the surface oxygen-ion and/or in the vacancy defect densities.

Materials and Materials Issues Relating to High-Performance Gas Sensing Technologies

High-performance gas sensing technologies like photo-acoustic gas detection and ion mobility spectrometry rely on microcomponents, which require extended operation at high temperatures. In this context silicon-on-insulator wafers as well as wide-bandgap materials such as silicon carbide and metal oxide semiconductors are of interest. We show that such materials can be combined to perform a variety of mechanical, thermal, electrical and gassensing functionalities. Materials and technology development issues relating to such materials are outlined.