A film bulk acoustic resonator oscillator based humidity sensor with graphene oxide as the sensitive layer

A film bulk acoustic wave resonator (FBAR) is a type of resonator with high frequency and small dimensions, particularly suitable for use as a sensor for physical and biochemical sensing with high sensitivity. FBAR-based sensors have been extensively studied, however they commonly use discrete devices and network analyzers for evaluation, and therefore are far from being able to be used in practical applications. This paper reports the design and analysis of an FBAR-based Pierce oscillator and a field-programmable gate array (FPGA)-based frequency counter, and the use of the oscillator as a humidity sensor with the frequency counter as the measuring circuit. Graphene oxide (GO) is used as the sensitive film to improve the sensitivity. The resonant frequency of the oscillator with a GO film shows a linear decrease with an increase in relative humidity, with a sensitivity of ca. 5 kHz per % RH (relative humidity) in the range of 3% RH to 70% RH, and a higher frequency shift is induced above 70% RH. The FBAR oscillator sensor shows excellent stability and repeatability, demonstrating the feasibility and potential sensing application using the integrated FBAR chip and simple frequency counter, particularly suitable for portable electronics.