Humidity sensing characteristics of Ga-doped zinc oxide film grown on a polycrystalline AlN thin film based on a surface acoustic wave

In this work, the effect of the Ga dopant concentration on the sensing responses of a surface acoustic wave (SAW) humidity sensor with a nanocrystalline ZnO/polycrystalline aluminum nitride (AlN)/Si-layered structure was investigated. Sol–gel derived Ga-doped ZnO was used as the sensing layer. The experimental results showed that the frequency shift of the SAW humidity sensors increased as the Ga dopant concentration increased from 0.0 to 3.0 wt% over a relative humidity (RH) range from 10 to 90% at 25 °C. The largest shift in the frequency response was at approximately 420 kHz for the sensor doped with 3.0 wt% Ga. The sensor demonstrated good short-term repeatability and low hysteresis. In addition, the SAW velocity of the sensors decreased slightly from 5105 to 5040 m/s as the Ga doping concentration increased from 0.0 to 3.0 wt% under 30% RH at 25 °C. The increase in temperature (16–37 °C) led to a reduction in the variability of the sensor transmission characteristics, such as the center frequency and the insertion loss caused by the different RH values (10–90%).