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Silicon diodes can be used for accurate temperature monitoring up to higher temperatures in a variety of sensors such as micro-machined resistive and calorimetric gas sensors, thermal flow sensors, exhausts, etc. This paper investigates the performance of a diode temperature sensor when operated at ultra high temperatures (up to 780Â°C ). A low leakage silicon on insulator (SOI) diode was designed and fabricated in a 1.0 Â¿m CMOS (complementary metal oxide semiconductor) process. The diodes were suspended within a dielectric membrane [formed by post CMOS deep reactive ion etching (DRIE)] for efficient thermal insulation. A CMOS compatible micro-heater was integrated with the diode on the dielectric membrane 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 show experimentally that the maximum temperature up to which the linearity of diode's forward voltage output is maintained can be as high as 550Â°C . Long term continuous operation at high temperatures (400Â°C and 500Â°C ) showed good stability of the diode voltage drop. Finally, we present a detailed theoretical analysis that helps to determine the maximum operating temperature for the diode and also explains the presence of nonlinearity factors in diode voltage output at ultra high temperatures.