Figure 4. An exemplary PS-BAW GaN resonator with measured acoustoelectric amplification.

(a) A schematic of a two-port thin film BAW resonator made from epitaxially grown GaN. The resonator is normally driven using only an RF stimulus, but a DC bias applied across the resonant cavity causes a change in the performance. The resonator is seen in (b) a scanning electron micrograph (SEM) and (c) a microscope image that shows the edge of the released GaN membrane. Scale bar corresponds to 100 μm. (d–e) RF transmission measurements (S₂₁) of the resonator with indicating both nominal operation and the evolution of BAW resonator performance improvement with an increasing DC bias. (f) This improvement can be quantified by the improved insertion loss, decreased bandwidth, and consequently increased Q. (g) The measured Q is compared with the extracted mechanical quality factor, Q_m, which is free of all parasitic effects and quantizes the intrinsic mechanical quality factor of the BAW resonator. Q_m shows significant improvement with applied DC, demonstrating that there is a fundamental reduction in internal loss and consequently an improvement in the energy confinement in the BAW resonator.