Patent GB-1005219-A (GB)
Title: Improvements in or relating to methods and devices for modifying acoustic wave signals
IPC: H03H9/54
Filed: 1962-04-18
Abstract: 1,005,219. Semi-conductor devices. WESTERN ELECTRIC CO. Inc. April 18, 1962 [April 29, 1961], No. 15003/62. Heading H1K. [Also in Division H3] An acoustic signal is amplified, attenuated, delayed or otherwise modified by passing it through a body of material which is at the same time piezoelectric and semi-conductive. Suitable materials, in either single crystal or polycrystalline form, are e.g. ZnO, CdS, AIN, InAs, CdSe, CdTe, GaAs and ZnS. Since these materials are piezoelectric, an alternating electric field travels in the material at the same velocity as the acoustic wave if the direction of propagation is correctly related to the crystal - lographic axes of the material. This field tends to generate electric currents which cause bunching of the charge carriers in the material. Movement of the bunched carriers under the influence of a D.C. biasing field causes further alternating electric fields which, through the piezoelectric effect may either amplify or attenuate the original acoustic wave. It is stated in the Specification that for a given material and fixed electric field a frequency # for maximum gain or loss is given by where p is the resistivity of the material, # is the dielectric constant multiplied by 8À85 x 10<SP>-14</SP> farads/cm., V D  is the average drift velocity of the carriers in the electric field and V s  is the velocity of sound. The drift velocity in question is the component along the direction of propagation of the acoustic wave, complete coincidence of the two directions being unnecessary. Fig. 1 shows a typical curve of gain against the ratio V D /V S  for a fixed ratio of # to 1/p; other curves for different values of this parameter show a similar symmetry about the ordinate through + 1. It can be seen that only when the component V D  is the same direction as the acoustic propagation and is greater than V s  can amplification occur. For other values of V D  attenuation occurs. Since V D α E (the electric field) and the rest of the terms in the equation are dependent upon the properties of the material chosen, E is the only practicable variable. An acoustic wave amplifier may take the form shown in Fig. 3 in which 11, 12 are depletion-layer transducers of the type described in Specification 995,850 formed at either end of a body 10 of GaAs cut from a single crystal. For a ratio of 1/p to # of 0.5 (curve of Fig. 1), # must be made equal to 320 Me/s., and for a value V D /V S  equal to 1À5 (maximum gain) a field E equal to 210 v/cm. must be applied between the transducers. It is apparent that the device is non-reciprocal, since the working point for reverse propagation (V D /V S  negative) is g where attenutation occurs. Two other possible working points for non- reciprocal attenuation are c and d. Other values of 1/## are possible for which the gain in one direction may be made about equal to the loss in the other (see Fig. 2, not shown). Since some semi-conductors are photosensitive, the intensity of the illumination may be used to vary the conductivity. The lower the resistivity, the higher the frequency of operation of the device, a practical range of resistivities being 1 to 10<SP>6</SP>ohm/cms. A delay line using multiple reflection of an acoustic wave between input and output transducers 52, 53 is shown in Fig. 5. A D.C. field is applied to the semiconductive transmission body 50 by means of electrodes 56, 57. The device operates at the points f, g of Fig. 1 which provide maximum gain in one direction and negligible attenuation in the other. In another modification, oscillations are produced by providing a feedback path between input and output in a device having gain (Fig. 4A, not shown). At higher frequencies, a re-entrant electromagnetic resonant cavity 30, Fig. 4B, replaces both input and output transducers. It is stated that if the transmission body 31 is situated as shown and subjected to a D.C. field, continuous oscillations are produced. In the modification shown in Fig. 6, use is made of the fact that the acoustic wave is refracted in a non-uniform field such as that provided within the transmission body 60 by electrodes 65, 66. The effect is non-reciprocal so that an acoustic wave from left to right couples transducers 62, 63 and a wave from right to left couples transducers 61, 62. The device thus provides a three-port circulator. The refraction phenomenon may be used to provide switching, the presence or absence of a field being used to determine the direction of propagation of the wave towards one or another of two output transducers (Fig. 7, not shown). Specifications 942,488, 958,690, 959,293 and 964,589 also are referred to.