1. A Bulk Acoustic Wave (BAW) resonator, comprising : a bottom electrode; a top electrode structure; and a multilayer transduction structure sandwiched between the bottom electrode and the top electrode structure, wherein : the multilayer transduction structure is composed of a plurality of transduction layers; at least one of the plurality of transduction layers is formed of a first ferroelectric material, whose polarization varies with an electric field across the first ferroelectric material; each of the plurality of transduction layers includes a transduction border (BO) portion positioned at a periphery of a corresponding transduction layer and a transduction central portion surrounded by the transduction BO portion, wherein a combination of the transduction BO portion of each of the plurality of transduction layers forms a transduction BO section of the multilayer transduction structure, and a combination of the transduction central portion of each of the plurality of transduction layers forms a transduction central section of the multilayer transduction structure; and the transduction BO section has a first electromechanical coupling coefficient, and the transduction central section has a second electromechanical coupling coefficient, which is larger than the first electromechanical coupling coefficient of the transduction BO section, wherein the transduction central section is configured to provide a resonance of the BAW resonator.
