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We have developed a temperature-sensor of pn junction diode and a microheater combined with this temperature-sensor. This pn junction diode temperature sensor can cover very wide temperature-range of about -200 °C to 500 °C with very high sensitivity and accuracy like a NTC thermistor. We have also developed a new absolute humidity sensor using this pn diode temperature sensor combined with a microheater formed by MEMS technique. We have found from simulation of the thermal conductivity λ of humid air at different temperatures that thermal conductivity difference Δ λ at different humid-air temperatures has a linear dependence on molar fraction of H2O vapor. Δ λ is determined from experimental data on temperatures Ta, Tb and Tc at point A (microheater point) and B (displaced point from A) on the micro-air-bridge, and at humid air, respectively. In the measurement Ta on the micro-air-bridge is periodically kept at Th and Ti above Tc by feedback control.