Mechanism of electrically tunable bulk acoustic wave resonator: Piezoelectric effect or acoustoelectric effect?

This article presents a systematic investigation of electrically tunable bulk acoustic wave (BAW) resonators composed of piezoelectric and semiconductor composite layers, analyzed through the framework of multiphysics coupling. By explicitly separating the contributions of the piezoelectric (PE) and acoustoelectric (AE) effects to frequency tuning, we demonstrate that the narrow tuning ranges reported in prior studies are mainly dominated by the PE effect, while the AE effect—although theoretically capable of providing broader tuning—remains underexploited. Our study identifies three critical factors constraining AE-based tuning: the insufficient width of the space charge region, carrier drift velocity saturation, and acoustic–electrical impedance mismatch. These factors collectively restrict the effective involvement of carriers in frequency tuning, leading to suboptimal tuning performance. To overcome these limitations, we propose a novel layered BAW design that strategically strengthens the PE–semiconductor interaction. By optimizing the n-ZnO semiconductor layer thickness and doping concentration, the design mitigates carrier velocity saturation and improves impedance matching, thereby fully leveraging the AE effect for frequency tuning. Analysis confirms that this structure achieves a frequency tuning range of 2.2% at resonance and 1.9% at anti-resonance, corresponding to an electromechanical coupling coefficient (k2) of 4.6%. This tuning performance reaches 0.48k2, approaching the theoretical AE tuning upper limit of 0.5k2. The findings establish clear design principles for electrically tunable BAW resonators, highlight the dominant mechanisms underlying PE and AE-induced tuning, and provide a compact solution for future reconfigurable RF front ends.