Modal analysis of out-of-plane vibrations in switchable piezoelectric Gallium Nitride micromechanical resonators

This paper analyzes higher order out-of-plane vibrations in a piezoelectric Gallium Nitride (GaN) MEMS resonator that utilizes the two-dimensional electron gas (2DEG) as a switchable and embedded electrode. More specifically, we show that the out-of-plane vibration mode that provides the strongest electromechanical resonance (~12MHz) has an associated stress field that allows charges generated from the orthogonal stresses (σ_x and σ_y) via the piezoelectric effect to add up constructively. Besides, we show that the 2DEG bottom electrode can be depleted to reduce the resonant peak till it is indistinguishable from the noise floor. The on-off ratio between the resonant peaks at zero gate bias vs. 2DEG depletion is 41dB and resonator has a quality factor of 3,900.