Piezoelectricity is the linear coupling between strain and electric polarisation in non-centrosymmetric crystals, and it is currently being re-engineered as a thin-film functional layer for millimetre-wave RF filters, programmable photonics, cryogenic quantum hardware and cell-scale acoustic tools.
Summary
A piezoelectric material converts mechanical strain into electric polarisation and, in reverse, converts an applied field into strain. The strength of the coupling is set by the piezoelectric tensor (commonly quoted as the coefficient d33 in pC/N, or e33 in the stress form), the elastic stiffness C33 and the clamped permittivity. These are not independent: one 2026 analysis resolves a long-standing gap between first-principles permittivity for scandium aluminium nitride (about 11.7) and measured values near 15 by showing the difference is the inverse piezoelectric effect inflating the lattice, with an effective permittivity of epsilon_33^S + e_33^2/C_33. A parallel theory paper finds that in strongly polar wurtzites (AlN, AlScN, AlBN, GaN, ZnO) electrostriction and elastic moduli are themselves polarisation-dependent, so the piezoelectric coefficient is strongly nonlinear in polarisation, including the counterintuitive case where reducing polarisation increases the strain response.
As a functional layer, the interesting question is not whether the effect exists but which material can be deposited on a wafer, at what coupling, with what loss, and whether it survives the surrounding process. Three materials families dominate the 2026 record here. Aluminium scandium nitride, patterned into periodically polarised (P3F) stacks, has produced 19 GHz bulk acoustic wave ladder-lattice filters with 1.3 dB minimum insertion loss, 6.26% fractional 3 dB bandwidth, 30 dB average out-of-band rejection and a 348 x 476 micrometre footprint, converting single-ended to balanced signals without a balun. Thin-film barium titanate has been shown as surface acoustic wave resonators with effective electromechanical coupling k2eff of 0.14 at 5.2 GHz, operation to 7.8 GHz, an extracted d33eff of 53 pC/N comparable to bulk BTO, and 100 ns ferroelectric switching for reconfigurable front-ends, characterised from room temperature down to millikelvin. Potassium sodium niobate, long celebrated in bulk ceramics but blocked from microelectronics by thermodynamic stability and planar process incompatibility, has been reworked on 8-inch wafers, where Mn-doped films need a Na-rich composition above 70 at.% (not the bulk equimolar optimum) to suppress pyrochlore and phase segregation and give fully (001)-oriented columnar growth.
The application pull is broad rather than concentrated. Piezoelectric actuators sit under silicon nitride waveguides in a fully monolithic, all-CMOS piezo-optomechanical photonic platform, with 200 mm photonic wafers built by back-end-of-line processing directly on completed CMOS driver wafers, motivated by architectures needing thousands to millions of reprogrammable photonic devices per chip. In quantum hardware, a piezoelectric actuator provides the feedback path that cools two translational modes of a magnetically levitated milligram sensor to below 2 picometre amplitude and below 10 mK, at Q factors of 3.8 x 10^6 and 5.5 x 10^6, and a piezoelectric-2DEG heterostructure is analysed as a quantum-limited acoustoelectric phonon amplifier. At the low-glamour end, a single piezoelectric transducer plus a model-free control algorithm delivers two-dimensional manipulation of individual cells across three cell lines, and a piezo-driven micropipette loads particles from 170 nm to 6 micrometres into in-vacuum optical traps with trapping efficiency as high as 93%.
Evidence base
- 2 June 2026: periodically polarised AlScN BAW ladder-lattice filters at 19 GHz achieved 1.3 dB minimum insertion loss, 6.26% fractional 3 dB bandwidth and 30 dB average out-of-band rejection in a 348 x 476 micrometre footprint, with no balun required.
- 18 June 2026: thin-film barium titanate SAW resonators showed k2eff of 0.14 at 5.2 GHz, operation to 7.8 GHz, extracted d33eff of 53 pC/N comparable to bulk, and 100 ns low-voltage switching, characterised from room temperature to millikelvin.
- 3 July 2026: a fully monolithic all-CMOS platform placed piezoelectric actuators under silicon nitride waveguides, with 200 mm photonic wafers built by back-end-of-line processing on completed commercial CMOS driver wafers.
- 5 June 2026: Mn-doped K1-xNaxNbO3 on 8-inch wafers required a Na-rich composition above 70 at.%, diverging from the bulk equimolar optimum, to suppress pyrochlore and phase segregation and obtain complete (001) polar orientation.
- 6 May 2026: the discrepancy between calculated ScAlN permittivity of about 11.7 and measured values near 15 was attributed to the inverse piezoelectric effect, with epsilon_eff = epsilon_33^S + e_33^2/C_33.
- 14 May 2026: series parasitic components were shown to produce apparent ferroelectric D-E hysteresis in non-ferroelectric capacitors even under the double-wave method, with suspicious loops identified in published work and strain-field loops proposed as the more reliable test.
Open questions
- Does the Na-rich Mn-doped KNN process reproduce across wafers and lots at 8 inches, and what piezoelectric coefficient and leakage does it deliver in a released device rather than a blanket film?
- Do the 19 GHz P3F AlScN filters retain 1.3 dB insertion loss and 30 dB rejection at power, temperature and over lifetime, and can the periodic polarisation be manufactured at volume yield?
- How many piezoelectric actuators per die does the 200 mm CMOS back-end-of-line photonic process yield, and what is the per-actuator power and crosstalk at that density?
- Can the predicted nonlinear, polarisation-dependent piezoelectric response in wurtzites be measured directly, including the anomalous regime where lower polarisation gives larger strain?
- Does the chirality-derived enhancement in selenium nanowires survive translation from piezoresponse force microscopy on single wires to a working harvester output?
Assessment drafted 2026-08-31 from up to 18 KB sources using the technology-scorecard framework; scores are a draft read pending review.