Disclosed are a resonant device and an acoustic filter. The resonant device includes a wafer substrate, a piezoelectric layer and an interdigital electrode layer. The piezoelectric layer is located on a side of the wafer substrate and includes a piezoelectric monocrystal material, and the piezoelectric monocrystal material includes a first crystal axis, a second crystal axis and a third crystal axis perpendicular to each other. A direction of an electric field generated by the interdigital electrode layer in the piezoelectric layer is a device direction.

A surface acoustic wave resonator device comprises a substrate supporting: a gateable, electrically conducting layer; an interdigital transducer (IDT); a reflector grating that comprises a plurality of electrically separated fingers; a main ohmic contact; and a gate element. The IDT is configured to be connectable to a ground. The conducting layer is configured to be connectable to the ground via the main ohmic contact, while each of said fingers is electrically connected to a lateral side of the conducting layer. This defines a gateable channel, which extends from the fingers to the ground via the conducting layer and the main ohmic contact. The gate element is electrically insulated from the conducting layer. The gate element is configured to allow an electrical impedance of the gateable channel to be continuously tuned by applying a voltage bias to this gate element with respect to the ground, in operation of the device.