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In this article, a 2.4-GHz micro-electromechanical systems (MEMS)-based oscillator is designed and demonstrated based on $g_{\text {m}}$ -boosting Colpitts structure. A dynamic self-body-biasing scheme is proposed to shorten startup time without introducing any extra units or additional parasitics to the resonant tank. Darlington cell is also employed to enhance negative transconductance and improve the oscillation stability, especially under a low supply voltage operating condition. The oscillator is integrated with a 2.4-GHz MEMS resonator with high $Q$ characteristic. The tail current source is designed at the boundary of the current- and voltage-limited regime for a superior phase noise performance. The measurement result shows that the phase noise at 10- and 100-kHz offsets is −112.92 and −138.23 dBc/Hz, respectively. A figure-of-merit of 226 dBc/Hz is achieved. The measured startup time is $\sim 5 \mu \text{s}$ . The core oscillator consumes a dc power of 0.98 mW from a 0.7-V supply voltage.

Flexural mode MEMS resonators are ideal candidates for resonant microsensors. However, their high motional resistance in air restricts the performance of corresponding oscillators and consequently the sensor performance. In this work, we report a piezoelectric MEMS oscillator based on a flexural mode membrane resonator array for in-air resonant sensors. Array design and piezoelectric transduction of the membrane resonators facilitate a low motional resistance and a high power handling capability. At the resonator level, the electrode pattern is optimized to further reduce the motional resistance, and the nonlinearity of the resonator is analyzed to fully exploit its high power handling for oscillator design. At the oscillator level, transimpedance and Pierce circuits are designed, analyzed and characterized. Theoretical calculations well fit measured results, both for the white and 1/f2 phase noise of the transimpedance oscillator and for the Allan deviation below an integration time of 0.1 s of the Pierce oscillator. The Pierce oscillator achieves a phase noise of −119 dBc/Hz at a 1 kHz offset and a −151 dBc/Hz noise floor. The frequency resolution of the Pierce oscillator reaches 0.024 Hz. To the best of our knowledge, the measured phase noise and frequency resolution are the best among reported low-frequency piezoelectric MEMS oscillators for in-air resonant sensors. The proposed solution could be applied for a variety of sensing scenarios, such as mass, pressure, acceleration and strain sensing. A theoretical resolution as low as 15 $\text{p}\varepsilon $ is expected if it is utilized as a strain sensor. [2023-0128]

It is a longstanding issue that high-order width-extensional (WE) piezoelectric microelectromechanical system (MEMS) resonators suffer low quality factors (Q-factors). In this study, it is observed that the mode distortion occurs in each unit when it couples with each other to constitute the high-order WE-mode resonator, leading to a decreased Q-factor. Based on this finding, we propose a new Q-factor boost strategy by improving mode matching degree between adjacent units from the view of dividing the high-order resonator into units. Both simulation and experimental results show a significant improvement in mode matching degree between adjacent units with varied unit length. Resonator’s Q-factor measured in air improved by 53% compared with the conventional resonator with constant unit length. It is believed that the proposed strategy could apply to other WE-mode MEMS resonators with different orders or dimensions; and Q-factor can be further improved by combining the proposed method with traditional energy reflection methods. [2023-0201]

It is a long-lasting issue that process variations, such as residual stress and poor dimension definition, deteriorate resonant frequency control of a piezoelectric micromachined ultrasonic transducer (PMUT). This letter presents a tapered cantilever cluster PMUT with tight frequency control. The PMUT could release residual stress via cantilever deformation and its front cavity could precisely define cantilever boundary. Theoretical analysis and test results show that the impact of residual stress on resonant frequency has been eliminated. Frequency precision and accuracy are evaluated across die, wafer and lot. Across wafer frequency uniformity and relative deviation of the proposed design are 0.8% and 1%, respectively, which are approximately an order of magnitude better than those of conventional design. The proposed PMUT demonstrates superior frequency control among reported PMUTs till date, meeting the 1% frequency control requirement for the first time. The measured transmitting and receiving sensitivities are 1.15 Pa/V/mm2 at a distance of 5 mm and 0.15 mV/Pa, respectively. The design is simple and cost effective to implement without stringent fabrication process requirements such as trimming or etch optimization. It is competitive in air-coupled applications where tight frequency control for frequency matching is a must, such as ultrasonic gas flowmeter and beam-forming array. [2023-0139]