High-Temperature SiC MOSFET Gas Sensors
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High-Temperature SiC MOSFET Gas Sensors Kevin Matocha, Vinayak Tilak, Peter Sandvik and Jesse Tucker Semiconductor Technology Laboratory, GE Global Research Center One Research Circle Niskayuna, NY 12309 USA email: [email protected] ABSTRACT Due to tightening restrictions on combustion exhaust emissions, low-cost sensors are desired for monitoring NOx production in high-temperature exhaust streams. This paper reports the characterization of Silicon Carbide MOSFET NO sensors for use in combustion exhaust monitoring. SiC depletion-mode MOSFETs were fabricated using a thermally-grown silicon dioxide gate dielectric and a Pt catalytic metal gate electrode. SiC MOSFET gas sensors were characterized at temperatures as high as 525ºC in an ambient of synthetic air and NO (50-200 ppm) for 30 hours with no degradation.
INTRODUCTION Combustion exhaust emission regulations are gradually tightening to control the emission of NOx and particulate matter, both of which degrade surface air quality. These stringent requirements require enhanced control of combustion processes in transportation and power generation applications. In automotive applications in particular, low-cost NOx sensors are desired which can operate in the hot exhaust gas streams at both the entrance and exhaust of the catalyst system. While zirconia-based NOx sensors are commercially available, their present high cost limits their feasibility for use in mass-production applications. One alternate low-cost gas sensor technology is the use of wide-bandgap semiconductor sensors. Previous work has examined silicon carbide (SiC) Schottky diodes[1,2], metalinsulator-SiC (MISiC) capacitors[3-5] and SiC MOSFETs[6] for use in high-temperature sensing applications, however little has been reported on the sensor response to NO[4]. Schottky diode sensors can suffer from degradation when the Schottky metal alloys with the SiC surface at high temperature[2]. The MISiC structure prevents alloying of the catalyst and the semiconductor but is somewhat difficult to implement in practice, since it requires the measurement of the device capacitance. Characterization of the SiC MOSFET structure is simplified since the drain current
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provides the sensor response. This work presents the characterization of SiC MOSFETs for hightemperature NO sensing. EXPERIMENTAL 4H-SiC depletion-mode MOSFETs were fabricated using an epitaxially-grown n-type channel layer on a p-type buffer layer. After mesa isolation, source/drain implantation and fieldoxide deposition, the gate dielectric was formed using thermal oxidation. Ohmic contacts were fabricated using Nickel with rapid-thermal annealing at 1050ºC. The catalytic gate metal was electron-beam evaporated using 40 nm of Platinum and annealed at 600C for 10 minutes in dry Nitrogen. Devices were then packaged using high-temperature ceramic materials able to withstand the harsh test environment. The gas testing system consists of a set of computer-controlled mass-flow controllers with a manifold for mixing N2, O2 and NO. The g
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