This paper presents a low-cost thermal-conductivity-based humidity sensor implemented using a 0.6-mu m CMOS process, where suspended p-n junction diodes are used as the humidity-sensitive elements. The measurement method uses the difference between the thermal conductivities of air and water vapor at high temperatures by comparing the output voltages of two hea ted and thermally isolated diodes; one of which is exposed to the environment and has a humidity-dependent thermal conductance, while the other is sealed and has a fixed thermal conductance. Thermal isolation is obtained by a simple front-end bulk silicon etching process in a TMAH solution, while the diodes are protected by the electrochemical etch-stop technique. The suspended diodes are connected to an on-chip circuit using polysilicon interconnect layers in order to increase their thermal resistance to be able to heat them with less power. Due to the high electrical resistance and positive temperature coefficient of resistance of the polysilicon, temperature sensitivities of the diodes are reduced to - 1.3 mV/K at a 100-mu A bias level. The diodes and the readout circuit are monolithically integrated using a standard 0.6-mu m CMOS process. Characterization results show that humidity sensitivity of the sensor is 14.3, 26, and 46.9 mV/%RH for 20 degrees C, 30 degrees C, and 40 degrees C, respectively, with a nonlinearity less than 0.3%. Hysteresis of the sensor is less than 1%. The chip measures 1.65 mm x 1.90 mm, operates from a 5-V supply, and dissipates only 1.38-mW power.