Theoretical and experimental study of shear mode bulk acoustic wave transformer based on c-axis zigzag ScAlN multilayer for rectenna application

The wireless power transfer with rectennas for sensors has attracted much attention in recent years. Although a charge pump is commonly used for voltage transformers in rectennas for RF voltage amplification, it is large, has low efficiency, and has poor impedance matching. Here, we propose small bulk acoustic wave piezoelectric transformers based on c-axis zigzag polarization-inverted ScAlN multilayers. The capacitive impedance Z(C) of an n-layer c-axis zigzag multilayer resonator is n times larger than that of a single-layer resonator. A piezoelectric transformer can, therefore, be obtained by the combination of a c-axis tilted monolayer and a c-axis zigzag multilayer. Glancing angle-sputtering deposition enabled c-axis highly oriented zigzag multilayer growth. The transformers exhibited an experimental voltage gain approaching +10dB in the 0.5GHz range. The experimental results agreed well with the theoretical model calculated using the electromechanical transmission line model, including the effect of the polarization-inverted structure. Furthermore, a simple formula expressing the voltage gain and simple equivalent circuit model for the transformer was derived, which is precise near the resonant frequency. In contrast to the finite element method model, this model described by lumped components makes it possible to simulate easily the entire performance of a rectenna, including an antenna and a rectifier, using a common circuit simulator.