Integrated CMOS-MEMS free–free beam resonators using pull-in mechanism to enable deep-submicrometer electrode-to-resonator gap spacing without interference in their mechanical boundary conditions (BCs) have been demonstrated simultaneously with low motional impedance and high . The key to attaining high relies on a decoupling design between pull-in frames for gap reduction and mechanical BCs of resonators. In addition, the use of metal– composite structures has been proved to greatly benefit the thermal stability of CMOS-MEMS resonators. Furthermore, tuning electrodes underneath pull-in frames were designed to offer “quasi-linear” frequency tuning capability where linear relationship between tuning voltage and frequency was achieved. In this paper, CMOS-MEMS free–free beam resonators with gap spacings of 110, 210, and 275 nm, respectively, were tested under direct one-port measurement in vacuum, demonstrating a resonator greater than 2000 and a motional impedance as low as 112 and, at the same time, allowing quasi-linear frequency tuning to achieve a total tuning range of 5000 ppm and a sensitivity of 83.3 ppm/V at 11.5 MHz with zero dc power consumption. Such a resonator monolithically integrated with a CMOS amplifier, totally occupying a die area of only , was also tested with enhanced performance, benefiting future timing reference and RF synthesizing applications. [2011-0223]