Micromachined Heterostructured Lamb Mode Waveguides for Acoustoelectric Signal Processing

This work explores micromachined heterostructured waveguides that leverage strong interactions between acoustic phonons and electrons to enable radio frequency (RF) signal amplification or attenuation. A thin-film piezoelectric-on-semiconductor stack is tailored to generate high electromechanical coupling Lamb waves that are impacted by high mobility electrons within a microacoustic waveguide. Lamb waves are generated by RF signal via interdigital electrodes on the piezoelectric layer; by applying a voltage to the semiconductor layer so that the electrons lead mechanical waves, the RF signal is amplified through the acoustoelectric (AE) effect. Conversely, the signals propagating faster or in the opposite direction of the electron flow undergo attenuation, rendering the waveguide nonreciprocal. Research on the AE effect dates to the mid-20th century and until now has been mostly focused on surface acoustic waves (SAWs). In this work, enabled by high-quality bonded thin films of lithium niobate (LN) and silicon (Si), it is shown that fundamental symmetric (
$S0$
) Lamb mode waveguides at 100 s of MHz can achieve more than 40 dB of AE gain and strong nonreciprocal transmission with less than 10 mW of bias power consumption. This could enable implementation of switches, delay lines, isolators, and circulators, which are critical for interference cancellation and full-duplex radio. The AE effect is observed to be stronger in some higher
$S0$
harmonics, allowing for scaling to higher frequencies with optimized electrodes that have more relaxed critical dimensions. In addition, up to 5.5-dB sustained terminal gain measured in this work implies the potential for transistor-less amplifiers that could be implemented in concert with more traditional acoustic devices in a single-chip manner.