Resonant body transistors

Abstract

Timing components are the heartbeat of consumer electronics as almost all electronic systems need a highly stable reference source for synchronization between its sub-systems. Over the past few decades, quartz crystals have provided highly accurate frequency references and demonstrated a continuing and sustainable presence with improved performance. On the other hand, Micro-Electro-Mechanical (MEM) resonators are micro-meter scale mechanical devices fabricated on silicon wafers with CMOS compatible processes and materials. The research on MEM resonators started in the 60's when a vibrating metal beam was proposed as the gate of a MOS transistor [1]. Pioneering work on the use of MEM resonators for frequency reference applications has been initiated in the early 90's at University of California at Berkeley and later blossomed at University of Michigan [2]. Subsequently, growing interest in wireless applications has generated tremendous technological progress in the field of radio frequency micro-electro-mechanical systems (RF MEMS) and transformed the MEM resonator technology based on IC-compatible micromachining processes and materials such as semiconductors, polysilicon or metals in a strong competitor position to the quartz crystal. Today, majority of the MEM resonators exploit the principles of capacitive excitation and detection via deep sub-micron air-gaps. However, MEM resonators with capacitively transduced signals are passive devices that show limited scaling potential in terms of impedance and signal-to-noise ratio. Inspired by the resonant gate transistor [1], vibrating or resonant body transistors (VBT or RBT) have been proposed for the first time in 2007–2008 [3–4], by embedding a field effect transistor in the body of vibrating beams, Fig. 1 with lateral gates coupled via narrow air-gaps. The resonant body transistor is an active resonator with intrinsic gain mechanisms, Fig. 2: the output of RBT is the drain current of the transistor and not the capacitive current. They have the unique advantage of enabling combined modulation of charge and piezoresistance (or mobility), which are effective at very small scale and controllable by the device design. The device small signal equivalent circuit is a hybrid between a resonator (RLC resonant circuit) and a transistor (current sources), Fig. 2a. The gain mechanisms are mirrored in the current sources depending on the transistor transconductance, which is voltage-tuneable (Fig. 3) and reaches its maximum at the resonance frequency (Fig. 2b). Absolute gain in resonant transistors is demonstrated in Fig. 4. In Fig. 5a bulk mode, piezoresistive gain resonant transistors based on multiple coupled beams shows the highest quality factor in RBTs reported to date (Q∼105) and a Q x f > 2 1012, comparable with quartz. Recently, a high frequency (>10GHz) version of the RBT, with internal dielectric transduction, has been reported in [5] showing a record Q x f higher than 1013. A 70MHz square bulk-mode resonator with four gates demonstrating significant signal gain and lower motional resistance than the same design in capacitive operation is depicted in Fig. 6.