Techniques are disclosed for monolithic co-integration of thin-film bulk acoustic resonator (TFBAR, also called FBAR) devices and III-N semiconductor transistor devices. In accordance with some embodiments, one or more TFBAR devices including a polycrystalline layer of a piezoelectric III-N semiconductor material may be formed alongside one or more III-N semiconductor transistor devices including a monocrystalline layer of III-N semiconductor material, over a commonly shared semiconductor substrate. In some embodiments, either (or both) the monocrystalline and the polycrystalline layers may include gallium nitride (GaN), for example. In accordance with some embodiments, the monocrystalline and polycrystalline layers may be formed simultaneously over the shared substrate, for instance, via an epitaxial or other suitable process. This simultaneous formation may simplify the overall fabrication process, realizing cost and time savings, at least in some instances.