Thermal detection mechanism of SiC-Based Resistive Gas Sensors

Abstract

AbstractSilicon carbide-based resistive gas sensors from our laboratory have been previously reported to detect hydrogen at concentrations ranging from less than 1% to 100% H2 in Ar and at temperatures ranging from 50°C to 450°C. The gas sensing mechanism for these devices was not well understood, hindering further improvement in this technology. In this report, resistive devices built on a thin 3C-SiC epitaxial layer grown on 150Å thick Si layer wafer bonded to a polycrystalline SiC substrate were studied. The polycrystalline SiC substrate is insulating, allowing the formation of isolated epitaxy resistors on the 3C-SiC layer. The gas sensing devices consisted of rectangular ohmic NiCr contacts with a Au overlayer fabricated on the 3C-SiC surface. Under a constant dc bias, nominally 10V in this study, these sensors demonstrated a decrease in current of up to ~25.4 mA upon the introduction of 100% H2, relative to 100% N2 in the test gas stream. The time constant for this device, estimated as a first-order exponential decay, was ~16-22 sec, with the full response occurring after ~90-120 sec. Upon the introduction of 100% H2 to the sensing environment, the device temperature, as measured by an resistance temperature detector (RTD) in intimate thermal contact with the device, decreased from 400°C to 237°C (ΔT = ~163°C). This large decrease in device temperature was driven by increased heat transfer coefficient of H2 relative to N2. The sensitivity to CH4 in N2, CO2 in N2 and He in Ar was also tested. Sensitivities, defined as the smallest change in concentration, as low as 300 ppm H2 in N2 were achieved with devices operating at 400°C and 10 V dc. Details of the device performance and a model of the sensing mechanism will be discussed.