A high-Q AlGaN/GaN phonon trap with integrated hemt read-out

This paper presents novel phonon traps, the main building blocks of self-sustained all-GaN oscillators, implemented on an AlGaN/GaN electro-acoustic platform. The geometry of acoustic cavities has been engineered to efficiently trap the energy in the central region of devices, where the interdigitated excitation/sense electrodes and HEMT read-outs are located. Such an energy trapping method obviates the need for narrow tethers. This facilitates routing of multiple lines connected to the source, gate, and drain of the read-out HEMTs as well as allowing co-integration of different interdigitated transducer (IDT) sets for efficient piezoelectric lateral field excitation. Furthermore, elimination of narrow tethers improves the power handling capability of the GaN resonators, making them suitable for high-power applications. Two types of AlGaN/GaN-based devices are discussed in this work, both operating in the ninth-order width-extensional mode that is excited using Schottky IDTs. In the first device, another set of Schottky IDTs it used for sensing, while the second device utilizes a HEMT to read-out the signal. An unloaded quality factor (Q) of ~13,000 has been measured at room temperature and atmospheric pressure for a device operating at ~740 MHz, resulting in a frequency × Q value of 0.96×10¹³, which is the highest value reported to date for GaN-based resonators and is very close to the intrinsic limits for AlN and GaN, set by phonon-phonon interactions.