An acoustic resonator device with low thermal impedance has a substrate and a single-crystal piezoelectric plate having a back surface attached to a top surface of the substrate via a bonding oxide (BOX) layer. An interdigital transducer (IDT) formed on the front surface of the plate has interleaved fingers disposed on a diaphragm of the plate that is formed over a cavity in the substrate. The piezoelectric plate and the BOX layer are removed from a least a portion of the surface area of the substrate to provide lower thermal resistance between the IDT and the substrate.