AlGaN/GaN Micromechanical Devices and Resonant HEMTs

Piezoelectrically actuated micromechanical resonators have been a subject of extensive research for the past decade with the main goal of replacing quartz resonators in timing applications. Aluminum nitride (AlN) has been the main contender as a piezoelectric ceramic replacement for quartz since its low-temperature sputtering process has been developed. In recent years, gallium nitride (GaN) has received some attention as a material of choice for micromechanical resonators mainly due to its piezoelectric properties [1]. GaN, in contrast to other commonly used piezoceramics, is a semiconductor that exhibits piezoresistive in addition to piezoelectric effects. Although neither the static piezoresistive nor the piezoelectric response of GaN is particularly large, the combined piezoelectric and piezoresistive effects – the piezoresponse – of GaN is significant. This property of GaN can be utilized to implement micromechanical resonators with unique structures having combinatory transduction mechanisms. It has been shown that the time-dependent piezoresponse of GaN electromechanical devices is much larger than that of its other semiconductor rivals as a result of significant piezoelectric contribution to the overall response. Hence, GaN with a large gauge factor in a heterostructure [2] has an advantage over other piezoresistive materials for time-dependent applications. Micromechanical resonators are classic examples of such time-dependent systems. Utilizing the large piezoresponse of GaN, our group has shown the highest-quality factor (Q) of >13,000, highest-frequency of > 8 GHz, as well as the highest-performance GaN micromechanical resonators with the highest measured frequency × Q values of ~ 10 13. In addition, making use of the stress-sensitivity of the two dimensional electron gas (2DEG) present at an AlGaN/GaN interface, we have developed resonant high electron mobility transistors (HEMTs) with unprecedented acoustic properties. For all such devices, we use GaN grown on Si (111) to have the ability to remove the substrate selectively using isotropic or anisotropic etching methods. Fig. 1 shows a general schematic of versatile resonant devices presented in this paper. The first class of devices are passive piezoelectric resonators operated in their bulk acoustic resonant mode, consisting of a piezoelectric layer and a set of interdigitated transducer (IDT) electrodes. These types of devices exploit only the piezoelectric response of the GaN layer. Depending on whether a bottom electrode is used or not, the device makes use of the thickness mode piezoelectric coefficient (d33) or is laterally driven using the d31 coefficient of GaN. In the former case, the bottom electrode could be either a metal …