Techniques are disclosed for forming group III material-nitride (III-N) microelectromechanical systems (MEMS) structures on a group IV substrate, such as a silicon, silicon germanium, or germanium substrate. In some cases, the techniques include forming a III-N layer on the substrate and optionally on shallow trench isolation (STI) material, and then releasing the III-N layer by etching to form a free portion of the III-N layer suspended over the substrate. The techniques may include, for example, using a wet etch process that selectively etches the substrate and/or STI material, but does not etch the III-N material (or etches the III-N material at a substantially slower rate). Piezoresistive elements can be formed on the III-N layer to, for example, detect vibrations or deflection in the free/suspended portion of the III-N layer. Accordingly, MEMS sensors can be formed using the techniques, such as accelerometers, gyroscopes, and pressure sensors, for example.