Experiment and theoretical analysis of relative humidity sensor based on film bulk acoustic-wave resonator

Relative humidity (RH) was measured with ZnO based film bulk acoustic-wave resonator (FBAR). The resonant frequency of the FBAR decreased linearly in a two-stage manner as the RH increased in the environment. For low RH (RH < 50%), a frequency downshift of 2.2 kHz per 1% RH change was observed. For high RH (RH > 50%), a frequency downshift of 8.5 kHz per 1% RH change was obtained. It was demonstrated that the two-stage response of the FBAR can be interpreted using the power law theory for semiconductor gas sensors and the mass loading effect.