A thin-film bulk acoustic wave (BAW) resonator, such as SBAR or FBAR, for use in RF selectivity filters operating at frequencies of the order of 1 GHz. The BAW resonator comprises a piezoelectric layer (14) having first and second surfaces on opposing sides, a first electrode (16) extending over the first surface, and a second electrode (12) extending over the second surface, the extent of the area of overlap (R1) of the first and second electrodes determining the region of excitation of the fundamental thickness extensional (TE) mode of the resonator. The insertion loss to the resonator is reduced by providing a dielectric material (18) in the same layer as the first electrode (16) and surrounding that electrode. The material constituting the dielectric material (18) has a different mass, typically between 5% and 15%, from the material comprising the first electrode (16) it surrounds. The mass of the dielectric material (18) can be lower or higher than the mass of the first electrode (16). Planarisation of the dielectric material (18) enhances the performance of the device.