Dispersion relations for acoustic surface waves in piezoelectrics with periodic metal strips applied to the surface are developed. Particular emphasis is given to the modification in effective piezoelectric coupling due to strip geometry, and the details of the dispersion relations around the stop bands. The general results are applied to discuss devices such as multistrip directional couplers and strip-coupled amplifiers.

Acoustic surface waves in piezoelectric materials with periodic metal strips on the surface

The multistrip coupler performs the function of a directional coupler for freely propagating surface acoustic waves on a piezoelectrically active substrate. Its operation is analyzed in terms of a transmission line based equivalent circuit. Expressions are obtained for the directionality (transmission and reflection) in terms of the number of coupler strips and the acoustic frequency. Theory and experiment are shown to be in very good agreement. 50-percent metallization is found to give optimum performance; progressive increase in the proportion of the coupler area covered with metal is shown to involve a progressive change from an inline field model to a crossed-field model. Outside a stopband region it is found that a simplified expression for the directionality is valid; this greatly facilitates component design. Design criteria for multistrip components are discussed in terms of substrate and bandwidth requirements. An analysis is included of resistive and capacitative effects on coupling, and problems associated with coupling between dissimilar materials are discussed.

Theory and Design of the Surface Acoustic Wave Multistrip Coupler

This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q.

https://web.eecs.umich.edu/~minar/pdf/srep05617.pdf

Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators

Since the mid-1960s, there has been a strong upsurge of interest in the propagation of surface acoustic waves on solids. The stimulus for this interest came from the realisation that simple, compact, and potentially cheap surface-acoustic-wave devices could be made which performed valuable signal-processing functions principally at intermediate frequencies. In a very short space of time these devices have met demanding specifications and have been accepted for military systems.This article reviews the recent work on surface acoustic waves (s.a.w.s) with the main emphasis on devices and applications. The introductory section provides basic background information on the nature, propagation and transduction of s.a.w.s. In some s.a.w. devices, propagation of the surface wave is influenced by surface structures which reflect, couple, guide or amplify the wave, and these are reviewed prior to the main section which is devoted to the more important s.a.w. devices, i.e. delay lines, tapped delay lines, frequency filters, oscillators, fixed and programmable analogue matched filters, convolvers and correlators, and finally, devices related to displays.Surface-wave devices are beginning to have an impact in both radar and communications systems. Both these areas of application are reviewed with particular reference to code generation and matched filtering (pulse compression) for high-performance radars, correlation systems for simple radars, synthetic-clutter generation for clutter-reference radars, compressive receivers, processors for fusing and filters for acquisition of synchronisation in communication systems.At appropriate points in the review, basic limitations of s.a.w. devices are indicated and the competition offered by other devices is assessed.

Surface-acoustic-wave components, devices and applications

Experimental observations are reported of high-frequency currents associated with thin photoconducting cadmium sulphide platelets having flat and parallel faces. They appear under conditions when `round-trip gain' due to acousto-electric amplification is achieved. They are correlated with departures from Ohm's law. The results are confined to conditions under which a linear theory is believed applicable. The linear theory is outlined and comparison is made with a variety of experimental observations. Semiquantitative agreement is obtained.

https://iopscience.iop.org/article/10.1088/0022-3719/2/1/324/pdf

High-frequency acoustoelectric currents and departures from Ohm's law in thin platelets of CdS I. Linear behaviour

The distribution of acoustic intensity within a surface-wave multistrip coupler on lithium niobate has been observed using a laser probe. The number of metal strips required for complete transfer of energy between coupled tracks has been determined as a function of acoustic frequency. Two stopbands were observed in the frequency range explored. A simple analytic expression is presented which is in good agreement with experiment away from the vicinity of the stopbands. It may be regarded as useful for coupler design; it is used to demonstrate that the 3 dB fractional bandwidth of the coupler may exceed 120%.

E.G.S. Paige

Papers

Frequency-dependent behaviour of an acoustic-surface-wave multistrip coupler

Gigahertz acousto-electric oscillations in zinc oxide