micromachines Article Switchable Transducers in GaN MEMS Resonators: Performance Comparison and Analysis Imtiaz Ahmed * and Dana Weinstein   Citation: Ahmed, I.; Weinstein, D. Switchable Transducers in GaN MEMS Resonators: Performance Comparison and Analysis. Micromachines 2021, 12, 461. https://doi.org/10.3390/mi12040461 Academic Editors: Marc Faucher and Mina Rais-Zadeh Received: 15 March 2021 Accepted: 15 April 2021 Published: 19 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA; danaw@purdue.edu * Correspondence: ahmed111@purdue.edu; Tel.: +1-843-283-9624 Abstract: This work presents a comprehensive comparison of switchable electromechanical trans- ducers in an AlN/GaN heterostructure toward the goal of reconfigurable RF building blocks in next-generation ad hoc radios. The transducers’ inherent switching was achieved by depleting a 2D electron gas (2DEG) channel, allowing an RF signal launched by interdigital transducers (IDTs) to effectively excite the symmetric (S o ) Lamb mode of vibration in the piezoelectric membrane. Different configurations for applying DC bias to the channel for electromechanical actuation in the piezoelectric are discussed. Complete suppression of the mechanical mode was achieved with the transducers in the OFF state. Equivalent circuit models were developed to extract parameters from measurements by fitting in both ON and OFF states. This is the first time that an extensive comparative study of the performance of different switchable transducers in their ON/OFF state is presented along with frequency scaling of the resonant mode. The switchable transducer with Ohmic IDTs and a Schottky control gate showed superior performance among the designs under consideration. Keywords: MEMS resonator; GaN; heterostructure; 2DEG; switchable; piezoelectric; transducers; symmetric lamb mode; phonon trap cavity 1. Introduction Rapid evolution in wireless technology and increasing demand for high-bandwidth communication for 5G/6G and the Internet of Things (IoT) has necessitated a growing number of components in radio front-end modules in an increasingly overcrowded radio frequency (RF) spectrum. Low-cost ad hoc radios have drawn consumer interest, enabling new devices such as MEMS resonators for on-chip clocking (e.g., for massive MIMO), frequency-selective notch and passband filters, and spectral sensing due to their smaller footprint and low power consumption [1]. Additional losses associated with in-line solid- state or electromechanical RF switches in reconfigurable systems have spurred MEMS resonators integrated with out-of-line switches to improve the system’s overall noise figure and reduce system-level size and weight. Switchable RF filters based on electrostatic MEMS resonators provide inherent switching using the DC bias necessary for their linear actuation. However, these resonators and corresponding filters typically exhibit high insertion loss due to their low electromechanical coupling [2]. Intrinsically switchable piezoelectric MEMS resonators with high quality factor (Q) and coupling coefficient (k 2 ) could provide a much-needed solution for compact, low-loss, tunable RF filters and oscillators in the GHz regime for wideband communication. Gallium nitride (GaN) has been explored extensively as an electromechanical mate- rial due to its high coupling coefficient (k 2 ~2%), acoustic velocity (~8000 ms −1 ), and low viscoelastic losses (f·Q~2.5 × 10 13 ) that enable high-Q MEMS resonators with a scaling capability up to millimeter-wave frequencies [3–8]. GaN is also a wide bandgap semicon- ductor with high electron mobility and breakdown field, making it ideal for high-power and high-frequency applications in radio base stations and hand-held devices [9,10]. GaN high-speed electronics can also be integrated monolithically with MEMS resonators to Micromachines 2021, 12, 461. https://doi.org/10.3390/mi12040461 https://www.mdpi.com/journal/micromachines