Piezoresistive versus Piezoelectric Transduction of GaN Micromechanical Resonators

Gallium nitride (GaN), in contrast to other commonly used piezoceramics, is a semiconductor that exhibits piezoresistive in addition to piezoelectric effects. The large piezoresponse – combined piezoelectric and piezoresistive effects – of GaN points out possible applications of GaN-based material systems in resonant devices. While the static piezoresistive response of GaN is small [1], the time-dependent piezoresponse of GaN electromechanical devices is much larger than that of its other semiconductor rivals, namely Si and SiC; 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. In this paper, utilizing the piezoresponse of GaN, we present the design and measurement results of various types of GaN micromechanical resonators and compare the advantages and drawbacks of each method. We use GaN grown on Si (111) to have the ability to remove the substrate easily and selectively using isotropic or anisotropic etching methods. A general schematic shown in Fig. 1 demonstrates versatile resonant devices that we implemented using GaN-on-Si substrates. The first class of devices are bulk-mode resonators, consisting of acoustic filters with interdigitated (IDT) electrodes, and bulk acoustic wave (BAW) resonators operated in thickness mode (Fig. 2). These types of devices exploit only the piezoelectric response of the GaN layer that is sandwiched between two electrodes. The bottom electrode can be either a metal layer that is sputtered from the backside or a silicon layer that is the device layer of a starting SOI substrate (Fig. 2 (ad)). The latter choice offers higher Q and better power handling capability, while the earlier choice (GaN with thin metal electrodes) offers the highest electromechanical coupling efficiency, especially when the d33 piezoelectric coefficient of GaN is used. Such BAW resonators can be used for timing [3] or resonant sensing applications [4] and can be monolithically integrated with HEMTs with a few modifications to the HEMT baseline fabrication process. It should be noted that the GaN BAW filters can only be used for narrowband applications as the piezoelectric coupling coefficient (kt 2) of GaN is relatively small. The second type of devices are resonant HEMTs, wherein two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is used as the bottom electrode of the piezoelectric actuation, as well as the sense channel of the pickup HEMT. In …