  We demonstrate high-frequency (> 3 GHz), high quality factor radio frequency
(RF) resonators in unreleased thin film gallium nitride (GaN) on sapphire and
silicon carbide substrates by exploiting acoustic guided mode (Lamb wave)
resonances. The associated energy trapping, due to mass loading from the gold
electrodes, allows us to efficiently excite these resonances from a 50 $\Omega$
input. The higher phase velocity, combined with lower electrode damping,
enables high quality factors with moderate electrode pitch, and provides a
viable route towards high-frequency piezoelectric devices. The GaN platform,
with its ability to guide and localize high-frequency sound on the surface of a
chip with access to high-performance active devices, will serve as a key
building block for monolithically integrated RF front-ends.
