Patent GB-1021237-A (GB)
Title: Ultrasonic wave transmission device
IPC: H03H9/13
Filed: 1962-11-15
Abstract: 1,021,237. Electromechanical transmission devices. WESTERN ELECTRIC CO. Inc. Nov. 15, 1962 [Nov. 17, 1961], No. 43188/62. Heading H3U. An electromechanical wave transmission device comprises a wave path including a body of low resistivity piezoelectric semi-conductive material and means for producing a depletion layer in the body. The velocity of propagation of an acoustic wave within a piezoelectric medium depends upon the resistivity of the medium. Hence the velocity of the wave in the region of the depletion layer is greater than it is in the rest of the semi-conductor body. By electrically varying the depth of the layer the transmission characteristics of the device may be varied. The body may comprise a single crystal and consists of a semi-conductor which can be rendered sufficiently non-conductive to exhibit a piezo-electric effect, e.g. GaAs, GaP, GaSb, InAs, InSb, BP, AlP, AlAs, AlSb, CdS, ZnS, ZnO, CdSn, ZnSn, MgTe. One half 10, Fig. 1, of the body is conditioned to have N-type conductivity and the other half, 11, P-type conductivity, the junction 12 between the halves being back-biased by a potential applied between electrodes 19, 20 in such a manner that a depletion layer 24 is formed. This layer extends at right-angles to the path of acoustic wave energy propagated between piezo-electric transducers 13, 14. The delay of the device varies with the magnitude of the back-bias which determines the thickness of the depletion layer and hence the proportion of high-velocity material in the path. The device of Fig. 1 may be elaborated by increasing the number of depletion layers, as shown in Fig. 5. This is done by epitaxially depositing alternate layers of N and P-type material until the whole body is built up. Subsequently, all the N-type layers are joined together by diffusing an N-type impurity into one longitudinal face of the body and the P-type layers are joined by diffusing a P-type impurity into the other. Upon application of a potential between electrodes 57, 58, a depletion layer 59 of serpentine form develops in the body. By adjusting the potential, the distance between successive depletion zones may be varied, and where this distance approximates to a half-wavelength or a quarter-wavelength of the propagated wave, the device behaves as a band-stop or band-pass filter. The steepness of the band depends upon the impedance discontinuity at each interface, the latter increasing with the piezo-electric constant of the material. If the discontinuities are not sharp, the device behaves as a dispersive delay line. In a modification, a single longitudinal depletion layer 49, Fig. 3, is formed in the body. The wave velocity is the weighted average of the slow normal semi-conductive velocity and the high depletion-layer velocity. Maximum delay and dispersion are obtained when the thickness of the layer is equal to or less than the wavelength of the propagated energy. Boron phosphide is a preferred material since the velocity of acoustic energy in this substance is high. The device may make use of depletion layers formed by back-biased non-ohmic contacts, in which case one or both electrodes on the longitudinal faces of the body may provide the contact. Suitable materials are gold on GaAs or platinum on CaS. In the cases of CdS and ZnO a depletion layer may be formed at the junction between high-resistivity and N-type zones of the same material.