1 GHZ GAN RESONANT BODY TRANSISTORS WITH ENHANCED OFF-RESONANCE REJECTION

We present 1 GHz Gallium Nitride (GaN) Resonant Body Transistors (RBTs) with order-of-magnitude improvement in offresonance rejection relative to previously reported devices. This is realized by strong mechanical coupling of a 1-port resonator with a second resonator in which a High Electron Mobility Transistor is used for sensing. As the first demonstration of GaN RBTs in goldfree technology, this work enables Qs up to 3077 at 1 GHz, with f·Q product of 3.1×10. We also demonstrate an acoustic transconductance (gem) of 72 μS, >2× higher than previously demonstrated GaN RBTs. With such high Q and gem, as well as reduced feed-through, these GaN RBTs can enable high performance, low phase-noise oscillators monolithically integrated with high power electronics for clocking. INTRODUCTION As third and fourth generation (3G, 4G) wireless communications have emerged over the past few years the demand for higher bandwidth wireless data transfer has increased. This has placed great importance on the performance of radio base stations, requiring low phase-noise frequency sources that exhibit immunity to nearby interferers and excellent power handling [1]. With quality factors in the thousands at GHz frequencies, small footprints, and the ability to achieve multiple frequencies on the same chip, MEMS resonators can provide building blocks for low phase-noise oscillators operating over a wide frequency range. Single-stage frequency sources are typically preferred for reduced cost, size, weight and power. As a wide bandgap semiconductor, GaN provides high electron velocities, charge densities (1×10 cm in AlGaN/GaN), and critical electric fields, ideal for high power (>10W/mm), high frequency (>300 GHz) integrated circuits. As a result, GaN High Electron Mobility Transistors (HEMTs) have been increasingly used for microwave monolithically integrated circuits (MMICs) and high power electronics. The ability to amplify and deliver higher power densities than gallium arsenide makes GaN a great candidate for use in single-stage oscillators without the need for additional amplifiers. GaN also exhibits excellent electromechanical properties, such as high piezoelectric coefficients (electromechanical coupling kT up to 2% in FBARs [2]), high acoustic velocities and low acoustic losses. Hence, this technology offers a platform for seamless integration of high performance RF MEMS resonators with high power, high frequency electronics. Monolithic integration of MEMS resonators with ICs would lead to reduced matching constraints and parasitics, which ultimately translate into lower phasenoise and reduced jitter clocks, especially at high frequency. For successful implementation in low phase-noise oscillators, these resonators must satisfy two requirements [1]: (1) High net signal-to-noise ratio and (2) small half-power bandwidth, i.e., high Q. For broadband operation, resonators must be scaled to multiGHz frequencies. At such high frequencies, passive resonators suffer from capacitive feed-through between drive and sense transducers which overwhelms the mechanical peak and lowers the signal-to-noise ratio (SNR). Active Field Effect Transistor (FET) sensing has been shown to overcome this challenge by providing internal electromechanical amplification of the resonant signal. In Silicon, FET sensing has enabled resonators over 37 GHz [3]. In GaN, multi-GHz HEMT-sensed resonators have been demonstrated in contour mode [4] and thickness mode resonators [5]. These previously demonstrated RBTs implement a single resonant cavity containing both drive transducer and sense transistor, resulting in direct electrical coupling between the RF input and transistor terminals (Fig. 1(a)), which increases the broadband floor of the device. Instead, we propose an RBT in which drive transducer and sense transistor are embedded in two electrically isolated, mechanically coupled resonators (Fig. 1(b)). While mechanical coupling of two or more passive MEMS resonators has been previously used to achieve tunable, low insertion-loss filters [6], this is the first implementation that takes advantage of active transistor sensing. PRINCIPLE OF OPERATION Fig. 2 shows the SEM of a mechanically coupled GaN RBT. Acoustic waves are launched piezoelectrically with an interdigitated transducer (IDT) in resonator 1. This first resonator is mechanically coupled to resonator 2, via an array of beams of length equal to half wavelength of the beam vibration at the desired resonant frequency. This guarantees that the two bars will resonate in phase Figure 1: Schematic of GaN Resonant Body Transistor. (a) Placement of drive transducer and sense transistor in the same acoustic cavity leads to broadband feed-through. (b) Electrical isolation between drive transducer and sense transistor is done by mechanically coupling two resonators. HEMT Drive GaN, ε = 8.9 (a) Previous designs Broadband feedthrough HEMT Drive