The Resonant Junction Transistor
Vibrating RF MEMS resonators have emerged as a potential solution for implementing monolithically integrated filters and frequency references for applications that require extreme scaling of size and weight. Electrostatically transduced resonators in particular have received attention as on-chip stable high frequency references due to their compatibility with existing CMOS processes and high quality factor (Q) often exceeding 10,000. Many of these electrostatic resonators, however, use extremely small air-gaps, which pose significant reliability problems, or use very thick device layer (> 10 [mu]m) silicon-on-insulator (SOI) substrates, which require significant changes to existing SOI CMOS processes and significantly increases cost. This dissertation presents a novel transduction mechanism using the depletion forces in pn-diodes to achieve efficient transduction at frequencies exceeding 1 GHz without sacrificing Q, fabricated in a process that can be completely integrated into a typical SOI RF CMOS front end process flow without any significant changes. This dissertation presents the theory of actuation and sensing, fabrication process, and experimental results for these pn-diode internally transduced RF MEMS resonators fabricated in the Cornell Nanoscale Science and Technology Facility (CNF). Measurements were performed using a pseudo-differential setup to demonstrate the feasibility of using such devices in conjunction with simple interfacing electronics. Experimental results indicate Q = 18,000 at a resonant frequency of 3.72 GHz, yielding the highest reported electrically measured room temperature f-Q product in silicon to date of 6.69x1013 Hz. The high Q of these devices - approaching the material limit in silicon - can be attributed in part to the simple transducer using only a homogeneous doped singlecrystal silicon structure. Such devices might therefore be useful for investigating the intrinsic acoustic loss mechanisms not only in silicon, but also in any other semiconducting material whose electrical properties can be controlled by doping. In this dissertation, we use these devices to present the first experimental verification of Landau-Rumer phonon-phonon scattering in single-crystal silicon at gigahertz frequencies via temperature measurements. In addition, data presented in this dissertation at low temperatures below 50 K indicates that electron-phonon scattering may play a greater role in limiting the Q of gigahertz silicon mechanical resonators than previously believed. While pseudo-differential measurements were able to yield a distinct second-order transmission response, the need for a differential environment may be a limitation for certain systems where a single-ended architecture is necessary, especially for ultra low-power or portable applications. To solve this problem, we make use of the piezoresistive property of single-crystal silicon (i.e., change in resistivity due to elastic strain) to sense the output motional current. This sensing mechanism eliminates the direct feedthrough path from input to output formed by the static capacitance of the transducers and allows for electrical detection of the mechanical resonance using a simple two-port RF measurement setup. In this dissertation, we employ a variation of the presented pn-diode transduced RF MEMS resonator which uses piezoresistive sensing through the lightly doped channel in an embedded junction field effect transistor (JFET) - the resonant junction transistor. Using this device, we demonstrate a micromechanical resonator with Q = 25,900 at a resonant frequency of 1.61 GHz. The frequency selective acoustic transconductance induced via the piezoresistive effect is 171 [mu]S for a drain current of 143 [mu]A, yielding an acoustic transconductance to bias current ratio (ga/ID) of 1.2 V-1. When integrated with SOI RF CMOS transistors that can provide power gain at the resonant frequency, this value puts this device in the realm of practicality for monolithic, direct synthesis of high-frequency local oscillator (LO) signals for low-power transceivers.
rf mems; acoustic resonator; jfet; quality factor
Bhave, Sunil A.
Lal, Amit; Kan, Edwin Chihchuan; Molnar, Alyosha Christopher
Ph. D., Electrical Engineering
Doctor of Philosophy
dissertation or thesis