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.