Surface acoustic waves on GaAs/AlxGa1-xAs heterostructures

Gate-Tunable Quantum Acoustoelectric Transport in Graphene

Transport probes the motion of quasi-particles in response to external excitations. Apart from the well-known electric and thermoelectric transport, acoustoelectric transport induced by traveling acoustic waves has rarely been explored. Here, by adopting hybrid nanodevices integrated with piezoelectric substrates, we establish a simple design of acoustoelectric transport with gate tunability. We fabricate dual-gated acoustoelectric devices based on hBN-encapsulated graphene on LiNbO3. Longitudinal and transverse acoustoelectric voltages are generated by launching a pulsed surface acoustic wave. The gate dependence of zero-field longitudinal acoustoelectric signal presents strikingly similar profiles to that of Hall resistivity, providing a valid approach for extracting carrier density without magnetic field. In magnetic fields, acoustoelectric quantum oscillations appear due to Landau quantization, which are more robust and pronounced than Shubnikov-de Haas oscillations. Our work demonstrates a feasible acoustoelectric setup with gate tunability, which can be extended to the broad scope of various van der Waals materials.

Quantum Oscillations in Graphene Using Surface Acoustic Wave Resonators

Surface acoustic waves (SAWs) provide a contactless method for measuring wave-vector-dependent conductivity. This technique has been used to discover emergent length scales in the fractional quantum Hall regime of traditional, semiconductor-based heterostructures. SAWs would appear to be an ideal match for van der Waals heterostructures, but the right combination of substrate and experimental geometry to allow access to the quantum transport regime has not yet been found. We demonstrate that SAW resonant cavities fabricated on LiNbO_{3} substrates can be used to access the quantum Hall regime of high-mobility, hexagonal boron nitride encapsulated, graphene heterostructures. Our work establishes SAW resonant cavities as a viable platform for performing contactless conductivity measurements in the quantum transport regime of van der Waals materials.

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles

Phase change metasurfaces based on VO₂, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO₂ and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO₂ film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO₃ substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO₃ substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO₂/TiO₂/LiNbO₃ composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.

Mechanical Detection of the De Haas-van Alphen Effect in Graphene

In our work, we study the dynamics of a graphene Corbino disk supported by a gold mechanical resonator in the presence of a magnetic field. We demonstrate here that our graphene/gold mechanical structure exhibits a nontrivial resonance frequency dependence on the applied magnetic field, showing how this feature is indicative of the de Haas-van Alphen effect in the graphene Corbino disk. Relying on the mechanical resonances of the Au structure, our detection scheme is essentially independent of the material considered and can be applied for dHvA measurements on any conducting 2D material. In particular, the scheme is expected to be an important tool in studies of centrosymmetric transition metal dichalcogenide (TMD) crystals, shedding new light on hidden magnetization and interaction effects.

Acoustically Induced Giant Synthetic Hall Voltages in Graphene

Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in nonlocal experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wave induces a longitudinal and a giant synthetic Hall voltage in the absence of external magnetic fields. The superposition of a synthetic Hall potential and a conventional Hall voltage can annihilate the sample's transverse potential at large external magnetic fields. Surface acoustic waves thus provide a promising and facile avenue for the exploitation of gauge fields in large planar graphene systems.

Systematical Study of the Basic Properties of Surface Acoustic Wave Devices Based on ZnO and GaN Multilayers

Recently, surface acoustic wave (SAW) devices based on layered structures are a popular area of research. Multilayered structures, including ZnO and GaN, have shown great performance and can be applied in diverse fields. Meanwhile, thin films, such as AlGaN and n-ZnO, can be added to these structures to form a 2-D electron gas (2DEG) which makes the devices tunable. This work systematically studies the basic properties of SAW devices based on ZnO and GaN multilayers via COMSOL Multiphysics. The sorts of structures with different crystal orientations are simulated, and various acoustic modes are considered. Results show that a range of phase velocity from about 2700 m/s to 6500 m/s can be achieved, and devices based on ZnO and GaN multilayers can meet the requirements of the electromechanical coupling coefficient from about 0 to 7%. Every structure’s unique properties are valuable for diverse applications. For example, c-ZnO/c-GaN/c-sapphire structure can be used for high-frequency and large-bandwidth SAW devices, while SAW devices based on a-ZnO/a-GaN/r-sapphire and 2DEG are suitable for programmable SAW sensors. This work has great reference value for future research into SAW devices.

Proposal for Plasmon Spectroscopy of Fluctuations in Low-Dimensional Superconductors

We propose to employ an optical spectroscopy technique to monitor the superconductivity and properties of superconductors in the fluctuating regime. This technique is operational close to the plasmon resonance frequency of the material, and it intimately connects with the superconducting fluctuations slightly above the critical temperature T_{c}. We find the Aslamazov-Larkin corrections to ac linear and dc nonlinear electric currents in a generic two-dimensional superconductor exposed to an external longitudinal electromagnetic field. First, we study the plasmon resonance of normal electrons near T_{c}, taking into account their interaction with superconducting fluctuations, and show that fluctuating Cooper pairs reveal a redshift of the plasmon dispersion and an additional mechanism of plasmon scattering, which surpasses both the electron-impurity and the Landau dampings. Second, we demonstrate the emergence of a drag effect of superconducting fluctuations by the external field resulting in considerable, experimentally measurable corrections to the electric current in the vicinity of the plasmon resonance.

Surface acoustic wave-based ultraviolet photodetectors: a review

Over the past decade, ultraviolet (UV) detection has been a subject of major interest for both research scientists and engineers because of its important applications in both the civil and military fields. The rapid development of interdisciplinary research has enabled the realization of UV detectors based on a variety of principles. Among these devices, UV detectors based on surface acoustic wave (SAW) technology offer unique advantages of remote wireless operation capability and zero power consumption. This article provides a comprehensive review of the working principles, important parameters, and the acoustic wave and materials types used in SAW-based UV detectors. The research and development status of these detectors are discussed and the most commonly used methods to optimize device performance are also summarized. Novel types of acoustic UV detectors based on thin film bulk acoustic resonators (FBARs) and Lamb wave resonators (LMRs) are briefly introduced. Finally, future development challenges are proposed and suggestions for future directions are provided to aid the development of this important research field.

Acoustogalvanic Effect in Dirac and Weyl Semimetals

The acoustogalvanic effect is proposed as a nonlinear mechanism to generate a direct electric current by passing acoustic waves in Dirac and Weyl semimetals. Unlike the standard acoustoelectric effect, which relies on the sound-induced deformation potential and the corresponding electric field, the acoustogalvanic one originates from the pseudoelectromagnetic fields, which are not subject to screening. The longitudinal acoustogalvanic current scales at least quadratically with the relaxation time, which is in contrast to the photogalvanic current where the scaling is linear. Because of the interplay of pseudoelectric and pseudomagnetic fields, the current could show a nontrivial dependence on the direction of sound wave propagation. Being within the experimental reach, the effect can be utilized to probe dynamical deformations and corresponding pseudoelectromagnetic fields, which are yet to be experimentally observed in Weyl and Dirac semimetals.

Acoustoelectric Study of Microwave-Induced Current Domains

Surface acoustic waves (SAW) have been utilized to investigate the properties of a two-dimensional electron system subjected to a perpendicular magnetic field and monochromatic microwave radiation in the regime where the so-called microwave-induced zero-resistance states form. Contrary to conventional magnetotransport in Hall bar and van der Pauw geometries, the collimated SAW beam probes only the bulk of the electronic system exposed to this wave. Clear signatures appear in the SAW propagation velocity, corroborating that neither contacts nor sample edges are a root source for their emergence. By virtue of the directional nature of this probing method and with the assistance of theoretical modeling, we were able to demonstrate that the SAW response depends on the angle between its propagation vector and the orientation of domains that spontaneously form when zero-resistance is observed in transport. This confirms in unprecedented manner the formation of an inhomogeneous phase under these nonequilibrium conditions.

Quantum simulation of particle creation in curved space-time

Conversion of vacuum fluctuations into real particles was first predicted by L. Parker considering an expanding universe, followed in S. Hawking's work on black hole radiation. Since their experimental observation is challenging, analogue systems have gained attention in the verification of this concept. Here we propose an experimental set-up consisting of two adjacent piezoelectric semiconducting layers, one of them carrying dynamic quantum dots (DQDs), and the other being p-doped with an attached gate on top, which introduces a space-dependent layer conductivity. The propagation of surface acoustic waves (SAWs) on the latter layer is governed by a wave equation with an effective metric. In the frame of the DQDs, this space- and time-dependent metric possesses a sonic horizon for SAWs and resembles that of a two dimensional non-rotating and uncharged black hole to some extent. The non-thermal steady state of the DQD spin indicates particle creation in form of piezophonons.

Real-Time Electron and Hole Transport Dynamics in Halide Perovskite Nanowires

For optoelectronic devices, high transport mobilities of electrons and holes are desirable, which, moreover, should be close to identical. Acousto-optoelectric spectroscopy is employed to probe the spatiotemporal dynamics of both electrons and holes inside CsPbI3 nanowires. These dynamics are induced without the need for electrical contacts simply by the piezoelectric field of a surface acoustic wave. Its radio frequency of fSAW = 324 MHz natively avoids spurious contributions from ion migration typically occurring in these materials. The observed dynamic modulation of the photoluminescence is faithfully reproduced by solving the drift and diffusion currents of electrons and holes induced by the surface acoustic wave. These calculations confirm that the mobilities of electrons and holes are equal and quantify them to be μe = μh = 3 ± 1 cm2 V-1 s-1. Additionally, carrier loss due to surface recombination is shown to be largely suppressed in CsPbI3 nanowires. Both findings mark significant advantages over traditional compound semiconductors, in particular, GaAs, for applications in future optoelectronic and photovoltaic devices. The demonstrated sublifetime modulation of the optical emission may find direct application in switchable perovskite light-emitting devices employing mature surface acoustic wave technology.

Generation and enhancement of surface acoustic waves on a highly doped p-type GaAs substrate

Surface acoustic waves (SAWs) have been widely studied due to their unique advantage to couple the mechanical, electrical, and optical characteristics of semiconductor materials and have successfully been used in many industrial applications. In this work, we report a design that uses piezoelectric material Zinc Oxide (ZnO) to enhance the generation and propagation of SAWs on the surface of a highly doped p-type Gallium Arsenide (GaAs) substrate, which is more extensively used in optoelectronic devices than intrinsic GaAs structures. To maximize the piezoelectricity and successfully generate SAWs, high quality c-axis orientation of the ZnO film is needed; thus we experiment and develop optimized recipes of a radio frequency (RF) magnetron sputtering system to deposit ZnO on the GaAs substrate. To further optimize the SAW performance, an intermediate Silicon Oxide (SiO2) layer is added between the ZnO film and GaAs substrate. Additionally, we test samples with varied thickness of ZnO films and dimensions of interdigital transducer (IDT) fingers to figure out their individual effect on SAW properties. The results and techniques demonstrated in this paper will provide guidance for further studies on enhancing SAWs propagating along many other doped semiconductor materials. This combination of acoustics and optoelectronics in doped semiconductors is a promising start to building enhanced and hybrid devices in various fields.

Valley Acoustoelectric Effect

We report on the novel valley acoustoelectric effect, which can arise in a 2D material, like a transition metal dichalcogenide monolayer, residing on a piezoelectric substrate. The essence of this effect lies in the emergence of a drag electric current (and a spin current) due to a propagating surface acoustic wave. This current consists of three contributions, one independent of the valley index and proportional to the acoustic wave vector, the other arising due to the trigonal warping of the electron dispersion, and the third one is due to the Berry phase, which Bloch electrons acquire traveling along the crystal. As a result, there appear components of the current orthogonal to the acoustic wave vector. Further, we build an angular pattern, encompassing nontrivial topological properties of the acoustoelectric current, and suggest a way to run and measure the conventional diffusive, warping, and acoustoelectric valley Hall currents independently. We develop a theory, which opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acoustoelectronic devices.

Employing graphene acoustoelectric switch by dual surface acoustic wave transducers

We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. We operate two pairs of inter-digital transducers (IDTs) to launch surface acoustic waves (SAWs) on a LiNbO3 substrate and utilize graphene as a channel material to sustain acoustoelectric current Iae induced by SAWs. By cooperatively tuning the input power on the IDTs, we can manipulate the propagation direction of Iae such that the measured Iae can be deliberately controlled to be positive, negative, or even zero. We define the zero-crossing Iae as [Formula: see text], and then demonstrate that Iae can be switched with a ratio [Formula: see text] at a rate up to few tens kHz. Our device with an accessible operation scheme provides a means to convert incoming acoustic waves modulated by digitized data sequence onto electric signals with frequency band suitable for digital audio modulation. Consequently, it could potentially open a route for developing graphene-based logic devices in large-scale integration electronics.

Current Flow in the Bubble and Stripe Phases

The spontaneous ordering of spins and charges in geometric patterns is currently under scrutiny in a number of different material systems. A topic of particular interest is the interaction of such ordered phases with itinerant electrons driven by an externally imposed current. It not only provides important information on the charge ordering itself but potentially also allows manipulating the shape and symmetry of the underlying pattern if current flow is strong enough. Unfortunately, conventional transport methods probing the macroscopic resistance suffer from the fact that the voltage drop along the sample edges provides only indirect information on the bulk properties because a complex current distribution is elicited by the inhomogeneous ground state. Here, we promote the use of surface acoustic waves to study these broken-symmetry phases and specifically address the bubble and stripe phases emerging in high-quality two-dimensional electron systems in GaAs/AlGaAs heterostructures as prototypical examples. When driving a unidirectional current, we find a surprising discrepancy between the sound propagation probing the bulk of the sample and the voltage drop along the sample edges. Our results prove that the current-induced modifications observed in resistive transport measurements are in fact a local phenomenon only, leaving the majority of the sample unaltered. More generally, our findings shed new light on the extent to which these ordered electron phases are impacted by an external current and underline the intrinsic advantages of acoustic measurements for the study of such inhomogeneous phases.

Graphene Surface Acoustic Wave Sensor for Simultaneous Detection of Charge and Mass

We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO₃ substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (I_A) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (V_Eg) in solution. We found that I_A is controlled by V_Eg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the I_A-V_Eg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus, we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing.

Acoustoelectric Current in Graphene Nanoribbons

Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.

Scalable fabrication of a hybrid field-effect and acousto-electric device by direct growth of monolayer MoS2/LiNbO3

Lithium niobate is the archetypical ferroelectric material and the substrate of choice for numerous applications including surface acoustic wave radio frequencies devices and integrated optics. It offers a unique combination of substantial piezoelectric and birefringent properties, yet its lack of optical activity and semiconducting transport hamper application in optoelectronics. Here we fabricate and characterize a hybrid MoS₂/LiNbO₃ acousto-electric device via a scalable route that uses millimetre-scale direct chemical vapour deposition of MoS₂ followed by lithographic definition of a field-effect transistor structure on top. The prototypical device exhibits electrical characteristics competitive with MoS₂ devices on silicon. Surface acoustic waves excited on the substrate can manipulate and probe the electrical transport in the monolayer device in a contact-free manner. We realize both a sound-driven battery and an acoustic photodetector. Our findings open directions to non-invasive investigation of electrical properties of monolayer films.

Lithium niobate has piezoelectric and birefringent properties useful for optics, but it is not readily integrated with traditional optoelectronics. Here, the authors create a device that senses photoconductance in semiconducting molybdenum disulfide using surface acoustic waves excited in lithium niobate.

Collective lipid bilayer dynamics excited by surface acoustic waves

We use standing surface acoustic waves to induce coherent phonons in model lipid multilayers deposited on a piezoelectric surface. Probing the structure by phase-controlled stroboscopic x-ray pulses we find that the internal lipid bilayer electron density profile oscillates in response to the externally driven motion of the lipid film. The structural response to the well-controlled motion is a strong indication that bilayer structure and membrane fluctuations are intrinsically coupled, even though these structural changes are averaged out in equilibrium and time integrating measurements. Here the effects are revealed by a timing scheme with temporal resolution on the picosecond scale in combination with the sub-nm spatial resolution, enabled by high brilliance synchrotron x-ray reflectivity.

Propagation characteristics of surface acoustic waves in single-electron transport devices and the electrical measurement

We focused on investigating the propagation characteristics of surface acoustic waves (SAWs) in SAW-based single-electron-transport (SAW/SET) devices. SAW/SET devices operating in the gigahertz frequency range were fabricated on a GaAs/Al(x)Ga(1-x)As heterostructure. The transmittance and reflection performances of SAW/SET devices were obtained via the electrical measurement. Modeling based on coupling-of-modes theory was introduced to analyze SAW/ SET devices. Based on this model, many second-order effects such as propagation loss and the mechanical and electrical loadings caused by electrode perturbation of the surface were taken into account. Multiple-transit reflections of SAWs were also contained in the calculation. The calculated transmittance and reflection performances were in good agreement with the measured data. Furthermore, we proposed four interference types which could satisfactorily explain the periodic oscillations presented in the acoustoelectric current versus frequency characteristics.

Study of the acoustoelectric effect for SAW sensors

Research has recently begun on the use of ultrathin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., because the basic material parameters may change because of film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, and which also correspond to the mid-point between the open and short-circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on a YZ LiNbO3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, exposure to hydrogen gas, or both. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

A ZnO nanorod-based SAW oscillator system for ultraviolet detection

A high-precision ultraviolet (UV) detector combining ZnO nanostructure and a dual delay line surface acoustic wave (SAW) oscillator system is presented. The UV detector is made of ZnO nanorods on a 128 degrees YX-LiNbO(3)-based two-port SAW oscillator. The ZnO nanorod synthesized by chemical solution method is used as a UV sensing material. The center frequency of the SAW device is at 145 MHz. A dual delay line SAW oscillator system was constructed to eliminate external environmental fluctuations. Under illumination of a UV source consisting of an Xe lamp and a monochromator, frequency shifts of the UV detector were measured. A maximum frequency shift of over 40 kHz was observed under 365 nm illumination for several on-off cycles, indicating the ZnO nanorod-based detector was sensitive to UV light and with good repeatability. Moreover, frequency shifts reached a value of 19 kHz after 365 nm was turned on for 10 s, which implies a real-time high-sensitivity UV sensor was successfully fabricated. Results show a ZnO nanostructure-based SAW oscillator system is a promising candidate for a real-time, fast-response, high-precision UV detector.

Shock waves in nanomechanical resonators

We study the formation of shock waves in a nanomechanical resonator with an embedded two-dimensional electron gas using surface acoustic waves. The mechanical displacement of the nanoresonator is read out via the induced acoustoelectric current. Applying acoustical standing waves, we are able to determine the so-called anomalous acoustocurrent. This current is found only in the regime of shock wave formation.

Self-induced acoustic transparency in semiconductor quantum films

We develop a quantum theory of the nonlinear interaction between intense surface acoustic waves and electrons in a quantum well in the regime of moving quantum wires and dots. In the quantum nonlinear regime, the sound attenuation exhibits quantum oscillations and dramatically decreases with increasing quantization. In the case of dynamically created electron dots formed by two acoustic waves, the waves can propagate without any dissipation in the limit of high sound intensity and, hence, the electron quantum film acts as an acoustically quasitransparent material.

Novel concepts for GaAs/LiNbO(3) layered systems and their device applications

Thin semiconductor quantum well structures fused onto LiNbO(3 ) substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.

Voltage controlled SAW velocity in GaAs/LiNbO(3)-hybrids

The combination of the electronic properties of semiconductor heterojunctions and the acoustic properties of piezoelectric materials yields very promising surface acoustic wave (SAW) hybrid systems. Quasi-monolithical integration of thin GaAs/InGaAs/AlGaAs-quantum well structures on LiNbO(3) SAW devices is achieved using the epitaxial lift-off (ELO) technique. The conductivity of the two-dimensional electron system in the quantum well, which can be controlled via field effect, modifies the velocity of the SAW. Due to the high electromechanical coupling coefficient of LiNbO(3) a large phase shift can be obtained. As an example for this new class of voltage-tunable single chip SAW devices, a voltage-controlled oscillator (VCO) is presented in which the output frequency can be tuned by an applied gate voltage.