Design and development of solidly mounted bulk acoustic wave resonator (SMR)-based ammonia gas sensor

This paper demonstrates design and simulation of an acoustic wave resonator-based ammonia gas sensor which is resonating at ∼ 3.95 GHz resonant frequency. The bulk acoustic wave-based solidly mounted resonators (SMRs) are robust, simple to manufacture, miniaturized and most suitable for high sensitivity gas sensors. The designed sensor consists of multilayer stack of (Si/SiO₂/BR/Mo/ZnO/Mo/rGO) thin films. Here, reduced graphene oxide (rGO) is utilized as an ammonia gas-sensing layer and zinc oxide (ZnO) as piezoelectric layer. Bragg reflector (BR) is applied to confine the acoustic wave. The simulated sensor is based on the resonant frequency shift due to mass loading effect by adsorbed ammonia gas molecules. The amount of frequency shift depends on the concentration of ammonia gas. The ammonia gas sensor results in sensitivity (S) of 7.113 MHz pg⁻¹ with outstanding selectivity over other exposed gases to the sensor. The SMR-based ammonia gas sensor is designed using air tight gas chamber and gases are exposed to the sensor in controlled ambient. In addition, the number of BR pairs (Mo/SiO₂) is also optimized for optimal performance of sensor, using transfer matrix model. The simulated ammonia gas sensor is small in size, highly sensitive and can find applications in safety and environmental monitoring in various industries, including chemical plants, refrigeration systems and agricultural applications.