  Achieving monolithic integration of passive acoustic wave devices, in
particular RF filters, with active devices such as RF amplifiers and switches,
is the optimal solution to meet the challenging communication requirements of
mobile devices, especially as we move towards the 6G era. This requires a
significant ($\approx$100x) reduction in the size of the RF passives, from
mm$^2$ footprints in current devices to tens of ${\mu}m^2$ in future systems.
Applying ideas from integrated photonics, we demonstrate that high frequency
(>3 GHz) sound can be efficiently guided in ${\mu}$m-scale gallium nitride(GaN)
waveguides by exploiting the strong velocity contrast available in the GaN on
silicon carbide (SiC) platform. Given the established use of GaN devices in RF
amplifiers, our work opens up the possibility of building monolithically
integrated RF front-ends in GaN-on-SiC.
