Here we present the first demonstration and in-depth study of unreleased acoustic resonators in 14nm FinFET technology in the IEEE X band, which offer a zero-barrier-to-entry solution for high Q, small footprint, resonant tanks integrated seamlessly in advanced CMOS nodes. These devices leverage phononic waveguides for acoustic confinement, and exploit MOS capacitors and transistors inherent to the technology to electromechanically drive and sense acoustic vibrations. Sixteen device variations are analyzed across thirty bias points to discern the impact of phononic confinement, gate length, and termination scheme on resonator properties. The limiting factor in FinFET resonator performance among design variations tested is shown to be Back End of Line (BEOL) confinement, with devices with acoustic waveguides incorporating Mx and Cx metal layers exhibiting 2.2x higher average quality factor (Q) and peak amplitude, with maximum Q increasing from 115 to 181 and maximum amplitude scaling from 0.8 to 4.5 uS. A detailed analysis of biasing in the highest performing device shows good fit with a derived model, which addresses the velocity saturated piezoresistive effect for the first time in active resonant transistors. Peak differential transconductance that is dominated by changes in the silicon band-structure, as expected from an analysis that includes contributions from the piezoresistive effect, electrostatic modulation, and silicon bandgap modulation.
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