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This paper reports on the design, implementation and characterization of a low phase-noise 27 MHz MEMS oscillator with sub-ppm temperature instability based on a high-Q composite bulk acoustic wave (BAW) resonator. An array of silicon dioxide (SiO2) pillars has been uniformly embedded in the body of a piezoelectrically transduced silicon resonator to compensate its negative temperature coefficient of frequency (TCF). Using this technique, an overall frequency drift of 83 ppm is achieved for the resonator over the temperature range of -20°C to 100°C while resonator Q remains greater than 7,500 in atmospheric pressure. An electronically compensated oscillator using this resonator exhibits sub-ppm temperature instability with a consistent phase noise (PN) behavior over the entire temperature range and a value of -101 dBc/Hz at 1 kHz offset-frequency. Long-term stability measurement has been carried out for both temperature-compensated resonator and oscillator in an environmental chamber to study their stability over time.