Acoustic Charge Transport Research Articles

Contact modelling of heterojunction acoustic transport devices

A method is presented for contact modelling of heterojunction acoustic charge transport devices (HACT). The method is used to optimise the ohmic contact dimensions, especially contact depths. It uses a solution of the two-dimensional Poisson equation, the continuity equations for electrons and holes, the current densities, the displacement charges resulting from the acoustic wave as well surface state densities. This solution gives the potential and charge distribution in the charge injection region of HACT devices for different depths of ohmic contacts. The channel current densities are computed as a function of gate applied voltages. This technique is developed to calculate the I–V characteristics; then the most suitable choice of the ohmic contact depth taking into account these I–V characteristics is given. This method is applied to calculate the optimal depth of ohmic contacts in the case of a conventional HACT (n-HACT). This result will be very useful for HACT structure design.

Single-electron acoustic charge transport on shallow-etched channels in a perpendicular magnetic field

The effect of a perpendicular magnetic field on the quantized current induced by a surface acoustic wave in a quasi-one-dimensional channel is studied. The channel was defined in a GaAs heterostructure by a shallow-etching technique. The quantized current in this constriction displayed superior precision (\ifmmode\pm\else\textpm\fi{}60 ppm) and robustness to dc bias relative to that observed previously in Schottky-gate-defined channels. In a perpendicular magnetic field commensurability oscillations were observed in the interval of current between quantized plateaus. The phase of the oscillations changes when a plateau is approached, which leads to an oscillatory change in the slope of the plateaus.

Modelling of heterojunction acoustic charge transport devices

A method of modelling heterojunction acoustic charge transport (HACT) in GaAs has been developed. This model allows for nonuniform impurity doping profile, variable epitaxial layer configuration, and arbitrary structural design of the input electrode. The acoustic wave potential is incorporated as a time- and space-varying doping density that adds directly to the impurity doping density. The wave-induced doping density is obtained from the piezoelectric displacement charge that accompanies the acoustic wave. The method uses a solution of the two-dimensional Poisson equation to obtain the potential and charge distribution in the charge injection region of an HACT device.

Imaging of acoustic charge transport in semiconductor heterostructures by surface acoustic waves

We demonstrate room-temperature acoustic charge transport of electrons and holes in an InGaAs/GaAs heterostructure. The carriers are optically generated by interband absorption and then separated, stored, and transported in the piezoelectric potential superlattice of a surface acoustic wave. The charge distribution is detected with a spatial resolution of a few acoustic wavelengths by a second orthogonal probe beam, genererated by so-called tapered transducers. The image information is given as a phase shift signal in frequency space and allows for the direct comparison of the number of generated and transported carriers. Regions of mere carrier drag and full carrier capture and transport are observed simultaneously.

Single-electron acoustic charge transport by two counterpropagating surface acoustic wave beams

Single-electron acoustic charge transport by two counterpropagating surface acoustic wave beams

Reflection characteristics of obliquely incident surface acoustic waves from groove and aluminum gratings on {100}-cut gallium arsenide

In this article we present an experimental study of the surface acoustic wave behavior of a slanted reflector array on {100}-cut, 〈110〉-propagating GaAs. The thorough examination of both Al stripes and grooves on GaAs unveiled previously unknown information about the behavior of slanted gratings on {100}-cut GaAs. We discovered that etched grooves provide a strong reflection coefficient (C=0.29) while Al stripes do not (C=0.014). These data were obtained from measurements of the reflection characteristics for slanted gratings on GaAs. Potential device applications include an ultralow power acoustic charge transport device which does not require an interdigital transducer.

Single-electron transport in a one-dimensional channel by high-frequency surface acoustic waves

We report a detailed experimental study of the quantized acoustoelectric current induced by a surface acoustic wave in a one-dimensional channel defined in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure by a split gate. The current measured as a function of the gate voltage demonstrates quantized plateaus in units of $I=ef$ where $e$ is the electron charge and $f$ is the surface acoustic wave frequency, the effect first observed by Shilton et al. The quantization is due to trapping of electrons in the moving potential wells induced by the surface acoustic wave, with the number of electrons in each well controlled by electron-electron repulsion. The experimental results demonstrate that acoustic charge transport in a one-dimensional channel may be a viable means of producing a standard of electrical current.

Amplitude-dependent effects of the acoustoelectric voltage and acoustic charge transport in [formula omitted] heterostructure in the quantum Hall regime

The acoustoelectric effect induced by the coupling between a surface acoustic wave and a two-dimensional electron system is investigated on GaAs AlGaAs heterojunctions at 4.2 K in the quantum Hall effect regime. This work was performed to study two-dimensional acoustoelectric non-linearities. We report on amplitude-dependent effects observed in the acoustoelectric voltage in the open geometry. These effects are qualitatively similar to those found on a CdS thick film. We present data suggesting that an acoustic charge-transport behaviour could be present in our experiment. This charge-transport signature provides further evidence for the presence of non-linear effects since the estimated electric field threshold value for non-linear effects reproduced similar non-linear behaviour observed recently in the three-dimensional (3D) piezoelectric semiconductor InSb.

A review of progress in acoustic charge transport technology.

Acoustic charge transport (ACT) devices are essentially charge coupled devices for which the potential propagating with a surface acoustic wave (SAW) in a piezoelectric semiconductor traps and transports charge. In this paper a physical description of device operation for which both acoustic and electronic attributes play a key role is given. Next, the various acoustic and electronic modeling tools that have been developed to aid in the design of ACT devices will be discussed. There have been several architectures which have been reported to date and the virtues of these approaches will be compared. There will be a discussion of some of the problems that have kept the technology from gaining widespread acceptance and some predictions of the areas of research which will be the most promising.

Waveguide for an acoustic charge transport device

A waveguide for surface acoustic waves comprises a substrate for supporting propagation of surface acoustic waves along a direction at a substrate velocity and a dielectric structure disposed above the substrate along the propagation direction of the surface acoustic waves. The structure includes a central portion disposed along the propagation direction, first and second intermediate portions respectively disposed along each opposed side of the central portion along the propagation direction, and first and second outer portions respectively disposed along each outer side of the first and second intermediate portions along the propagation direction. The central, intermediate and outer portions each has first, second and third intrinsic surface acoustic wave propagation velocities along the propagation direction of the surface acoustic waves, respectively and the first velocity is intermediate of the second and third velocities.

Acoustoelectric charge injection in semiconductors

A considerable change of trapped and free electric charge is observed in piezoelectric semiconductors in the presence of a traveling acoustic wave. The electric field, induced by the ultrasound, alters the electric equilibrium of the semiconductor sample, resulting in an accumulation of majority carriers at the surface with a consequent decrease in surface resistance. In specific cases, charge injection occurs at the semiconductor-metal contact area due to the large electric field induced by the acoustic wave. This effect, here referred to as the Acoustoelectric Charge Injection, was also investigated for the case in which a Surface Acoustic Wave (SAW) is propagating along the metalized surface of a semiconductor. The injected charge is experimentally measured having a exponential time decay typical of a deep trap level, thus suggesting that Acoustic Charge Injection can modify the transient behavior of high-speed analog signal processing devices based on SAW, acoustooptic, and acoustic charge transport (ACT) phenomena. Experimental results and theoretical calculations are presented for CdS samples and for a metalized GaAs-epilayer grown on semi-insulating GaAs substrate. >

Yield modeling of acoustic charge transport transversal filters

This paper presents a yield model for acoustic charge transport transversal filters. This model differs from previous IC yield models in that it does not assume that individual failures of the nondestructive sensing taps necessarily cause a device failure. A redundancy in the number of taps included in the design is explained. Poisson statistics are used to describe the tap failures, weighted over a uniform defect density distribution. A representative design example is presented. The minimum number of taps needed to realize the filter is calculated, and tap weights for various numbers of redundant taps are calculated. The critical area for device failure is calculated for each level of redundancy. Yield is predicted for a range of defect densities and redundancies. To verify the model, a Monte Carlo simulation is performed on an equivalent circuit model of the device. The results of the yield model are then compared to the Monte Carlo simulation. Better than 95% agreement was obtained for the Poisson model with redundant taps ranging from 30% to 150% over the minimum. >

Theoretical calculations of charge confinement in a pn/sup ~/np heterojunction acoustic charge transport device

An alternative structure for heterojunction acoustic charge transport (HACT) devices has been devised and analyzed. The GaAs/AlGaAs structure uses a pn/sup ~/np doping profile near the surface of the device to create a charge transport layer and provide top vertical confinement. This is contrary to previous n-type HACT structures which rely on residual surface states and a heterojunction discontinuity for the same functions. The use of the pn/sup ~/np doping as the channel depletion mechanism makes the device insensitive to the residual surface state density, thus providing a more robust design. In addition, the use of the back np junction enables widening of the transport layer thereby increasing the amount of charge that can be transported by the acoustic wave. As a result of the increased charge capacity it is expected that the pn/sup -/np ACT device will exhibit a greater dynamic range and current than previous HACT designs. The analysis of the device structure is accomplished herein using a two dimensional hydrodynamic simulation code, Semiconductor Total Energy Balance Simulator in two Dimensions (STEBS-2D), which has been modified to account for the potential created by the surface acoustic wave. The calculated results indicate that an order of magnitude enhancement in charge capacity is possible using the new structure. Transfer efficiency calculations for several different lifetimes in the transport layer show high efficiency values with a Shockley Read Hall lifetime of 10 nsec.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Surface acoustic wave properties of ZnO films on {001}-cut 〈110〉-propagating GaAs substrates

GaAs has been employed as a material for acoustic charge transport (ACT) devices principally because it is a piezoelectric semiconductor. However, because GaAs is a weakly piezoelectric material, the surface acoustic wave (SAW) interdigital transducer (IDT) drive power required to achieve charge transport is typically about 27 dBm. An enhancement of the piezoelectric coupling could potentially improve device lifetime, reliability, dynamic range, and decrease device power consumption. To this end the use of a ZnO film on top of an ACT-like substrate to enhance the piezoelectric coupling has been investigated. Moreover, the ZnO film structure would also make it possible to monolithically integrate SAW devices and GaAs electronics. In order to provide a basis for the design of such devices, SAW properties are reported, including velocity, effective piezoelectric coupling constant, and attenuation, measured on ZnO films sputtered on {001}-cut 〈110〉-propagating GaAs substrates. The measurements have been performed for the different film thicknesses over the range of 1.6–4 μm and with films of different grain sizes. IDTs operating between 180 and 360 MHz were fabricated, and a knife-edge laser probe was used to measure the SAW propagation. The measured data for the velocity shows good agreement with theoretical values. The quality of the dc triode-sputtered films was superior to that of the rf magnetron-sputtered ones due to higher K2 and less attenuation. The value of K2 for the 1.6-μm-thick dc triode-sputtered one was measured to be 0.75% which is comparable with 1.07% for bulk crystalline ZnO.

Text pattern matching based on a high-speed signalprocessor

The prototype implementation of a high-speed text search processor is presented. The system uses a programmable analogue signal processing device based on acoustic charge transport technology. Several text search operations can be performed, including operations with ‘don&apos;t care’ terms, at throughput rates approaching 90 million characters per second.

A base-band NDS electrode configuration for HACT devices

Heterostructure Acoustic Charge Transport (HACT) devices have been fabricated with a new nondestructive sense (NDS) electrode structure that provides for the recovery of base-band signals without the use of an integrating capacitor. This electrode structure provides an output signal comprising an RF carrier at the SAW frequency, amplitude modulated by the sampled input signal which has been delayed by a period proportional to the output electrodes distance from the input diode. The output of the NDS electrode structure is subsequently demodulated to provide the base-band signal. >

Applications of acoustic charge transport

The emerging technology of acoustic charge transport (ACT), which has enabled the implementation of powerful signal processors capable of handling high-speed RF and microwave signals in their natural analog domain, is reviewed. The development of ACT technology has led to the implementation of fully programmable analog signal processors capable of performing over 45 billion multiply-and-accumulate operations per second. These processors are finding application as adaptive filters and equalizers in a variety of commercial and military systems. The operation and performance capabilities of the ACT device are described, and seven applications now under development are discussed.

A physically based small-signal circuit model for heterostructure acoustic charge transport devices

A physically based small-signal circuit model for GaAs-AlGaAs Schottky gate heterostructure acoustic charge transport (HACT) devices is presented. Analytical expressions for the instantaneous and average channel current as a function of gate voltage are obtained from physical device parameters. The charge injection model is based on subthreshold current models for GaAs MESFETs. It is shown that the shape of the sampling aperture of the charge injection operation is approximately Gaussian. Good agreement is obtained between the measured DC channel current versus gate voltage and that predicted by the model. Equivalent circuits for the transfer and output sensing operations and expressions for noise sources due to the physical processes that occur within the device are developed. Thermal, shot, and transfer noise are treated. The form of the analytic expressions for frequency response and noise figure allows easy implementation on commercially available CAE software. Simulations of both gain and noise figure performed on Libra show good agreement with measured data. >

New analog memory devices demonstrated using heterojunction acoustic charge transport device technology

A new analog memory (AM) has been demonstrated using heterojunction acoustic charge transport (HACT) technology. The initial HACT AMs had storage time capabilities of 100 μs at 25 °C. The Schottky electrode storage array had 36 memory cells, each cell holding 2 charge packets, giving a maximum signal frequency of half the Nyquist frequency. Hold voltages in the 6–8 V range were found to be adequate, and effective storage was obtained at the same dc transport current level that resulted in the best charge transport efficiency. Storage times in the millisecond range are possible with cooling.

The effects of grating size on acoustic charge transport properties on GaAs

Gratings in the channels of acoustic charge transport devices on GaAs create perturbing electric fields that can upset the charge transport process, principally by reducing the charge carrying capacity. An experimental device is designed to test the effect of 1.0, 1.3, and 1.6 μm gratings on the acoustic charge transport properties. The measured charge capacity losses are found to be less than the measurement tolerances, for transport parameters typical of delay line devices. In addition, for parameters typical of charge storage devices, the measured charge capacity losses are higher, but are still low. Furthermore, an approximate theory to predict the charge capacity loss is presented; its predictions are in reasonable agreement with experimental results.