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This paper investigates the performance of diode temperature sensors when operated at ultra high temperatures (above 250degC). A low leakage silicon on insulator (SOI) diode was designed and fabricated in a 1 mum CMOS process and suspended within a dielectric membrane for efficient thermal insulation. The diode can be used for accurate temperature monitoring in a variety of sensors such as microcalorimeters, IR detectors, or thermal flow sensors. A CMOS compatible micro-heater was integrated with the diode for local heating. It was found that the diode forward voltage exhibited a linear dependence on temperature as long as the reverse saturation current remained below the forward driving current. We have proven experimentally that the maximum temperature can be as high as 550degC. Long term continuous operation at high temperatures (400degC) showed good stability of the voltage drop. Furthermore, we carried out a detailed theoretical analysis to determine the maximum operating temperature and explain the presence of nonlinearity factors at ultra high temperatures.