High-quality epitaxial AlN films have been deposited on GaN/sapphire using helicon sputtering at a temperature of 300 C. The surface acoustic wave (SAW) characteristics, in terms of insertion loss, stopband rejection, and electromechanical coupling coefficient, of SAW devices fabricated on AlN/GaN/sapphire are much superior than those fabricated on GaN/ sapphire. The investigation of environmental effects, including temperature and relative humidity, shows that the ambient stability can be improved with the deposition of an AlN film on GaN/sapphire. An oscillator fabricated using an AlN/GaN/ sapphire-based SAW device was developed for application in sensors. The composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and utilize their semiconducting, optoelectronic, and piezoelectric properties.