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Medlin Research Group
MIS Sensor Development
Real-time hydrogen detection is very important for various industrial applications, including in the headspace of electrical transformers. Solid-state metal-insulator-semiconductor (MIS) sensors are highly sensitive to H2 gas, and are therefore excellent candidates for this purpose. These sensors are prepared by depositing a film of catalytic metal onto a thin layer of gate insulator material that has been grown on a semiconductor substrate. The basic H2 sensing mechanism for MIS devices can be described as follows. First, gas-phase H2 dissociates on the surface of the catalytic metal to form H atoms. Then, these H atoms rapidly diffuse through the metal film to the metal-insulator interface, where they are preferentially trapped in stabilized adsorption sites. The layer of interfacial hydrogen created by this process exists in a dipole layer, generating an additional voltage drop across the MIS sensor that can be measured as either a shift in the capacitance-voltage curve of a capacitor, or in the current-voltage characteristic of a transistor or diode.
 Several attempts have been made to apply solid-state sensor technology to monitor transformer fault gases over the last decade due to the unique sensitivity of MIS devices to hydrogen. These initial attempts were mostly unsuccessful because of the complexity of the fault gas environment. The presence of carbon monoxide, oxygen, and small hydrocarbons in the sensing environment was found to cause major deflections in sensor response, creating difficulties in efforts to interpret response trends. To address these challenges, we have attempted to approach sensor design based on a more fundamental understanding of MIS device chemistry. Early investigations have helped to characterize how various chemical species affect the ability of MIS sensors to detect hydrogen. We have used our understanding of fault gas surface chemistry to design sensors with greatly enhanced H2 selectivities that show great promise for application. This has been accomplished through incorporation of membrane films and devices that screen out the effects of other gases and only allow access of hydrogen to active surface sites.
Research Personnel:
Stephen Marshall
Other areas of research in the Medlin Research Group:
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