High-$Q$ Gallium Nitride Thickness-Shear Baw Resonators with Reduced Temperature Sensitivty

This paper reports on a high quality factor ($Q$) gallium nitride (GaN) thickness-shear (TS) bulk acoustic wave resonator with a reduced temperature coefficient of frequency (TCF). $4.2\mu \mathrm{m}$-thick AlGaN/GaN-on-Si substrate is used to create high-Q resonators operating in TS mode, as well as high-order width-extensional modes (WEn). The temperature characteristic of TS and WE resonators are measured and compared highlighting the considerable reduction in TCF of the shear mode. GaN resonator prototypes are presented operating in TS mode at 573MHz, with a $Q$ of 2700, and in WE3, WE5, and WE7 modes at 239MHz, 407MHz, and 564MHz, with $Q\mathrm{s}$ of 8700, 7200 and 2300, respectively. A TCF of-17.9 ppm/°C is measured for the TS mode, which is significantly lower compared to WE3,5,7 modes with measured TCF of $-24 \text{ppm}/^{\circ}\mathrm{C}\pm 0.3\ \text{ppm}/^{\circ}\mathrm{C}$. The large (> 6 ppm/°C) improvement in temperature sensitivity, along with the promise of monolithic integration with HEMT electronics highlights the potential of TS GaN resonators for realization of frequency stable and radiation hard integrated oscillators.