HIGH-TEMPERATURE SENSORS BASED ON METAL-INSULATOR-SILICON CARBIDE DEVICES

High temperature gas sensors based on catalytic metal-insulator-silico n carbide (MISiC) devices are developed both as capacitors and Schottk y diodes. A maximum operation temperature of 1000 degrees C is obtaine d for capacitors based on 4H-SiC, and all sensors work routinely for s everal weeks at 600 degrees C. Reducing gases like hydrocarbons and hy drogen lower the flat band voltage of the capacitor and the barrier he ight of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 degrees C total combustion occurs on the sensor surface and the signa l is high for fuel in excess and low for air in excess. At temperature s around 400 degrees C the response is more linear. The high temperatu re operation causes interdiffusion of the metal and insulator layers i n these devices; and this interdiffusion has been studied. At sufficie ntly high temperatures the inversion capacitance shows different level s for hydrogen free and hydrogen containing ambients, which is suggest ed to be due to a reversible hydrogen annealing effect at the insulato r-silicon carbide interface.