Patent GB-906030-A (GB)
Title: Improvements in and relating to solid state electrical devices
Inventor: WRIGHT GORDON THOMAS
IPC: H01L29/72
Filed: 1957-10-16
Abstract: 906,030. Semi-conductor devices. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 10, 1958 [Oct. 16, 1957], No. 32380/57. Class 37. In a device comprising a body of crystalline insulating material (resistivity greater than 10<SP>6</SP> ohm cm.) with first and second electrodes on its opposite faces, the surface layer of the body beneath the first electrode is treated to render it semi-conductive to form a contact through which for certain bias conditions charge carriers enter the body to produce a space charge limited current which may be modulated by voltages applied to a third electrode on or in the body. A typical device (Fig. 1) comprises a crystal 13 of cadmium sulphide, provided with a silver paste (or colloidal graphite) cathode on one face and donor doped at the opposite face by heating an evaporated layer 14 of indium in contact with it in an inert or reducing gas or vacuo. The indium electrode 14 is then soldered to a silver ribbon 15 and the silver electrode soldered to a nickel-alloy base 16. As an alternative the indium layer may be removed and replaced'by a silver electrode to facilitate use at high temperatures. A further arrangement comprises a thin sintered zinc sulphide layer, one surface of which has been rendered semi-conducting by electron bombardment. In both devices the third electrode is applied to the insulator at any suitable point. Fig. 4 illustrates a device in which the control electrode, consisting of parallel bars of nickel or gold, is embedded in a cadmium sulphide crystal. The device is formed by evaporating the grid on the surface of a cadmium sulphide crystal and subsequently growing an extension of the crystal over the grid from the vapour phase. Silver electrodes are electro-plated on opposite faces of the crystal after one face has been donor doped by exposure to cadmium vapour. If a high permittivity insulating material is used the space charge field due to occupied trapping centres is reduced with a consequential increase of current for given biases. Such a device may be made by doping one surface of a barium titanate crystal with samarium, attaching electrodes of colloidal graphite or cold-setting silver paste to both surfaces, and encapsulating. Alternatively, the crystal surface is doped by diffusion of strontium to form a solid solution of strontium titanate in barium titanate with interstitial strontium atoms. In modifications of the three last-mentioned devices the control electrode and anode are in the form of interdigitated bars of aluminium or silver vacuum evaporated on the surface opposite the cathode. The current in the devices is responsive to radiation and to transverse magnetic fields. Devices are also envisaged in which both faces of the insulator are rendered semi-conductive, these having similar forward and reverse characteristics. In the Provisional Specification rectifying diodes comprising a ferro-electric or highpermittivity insulating or semi-conductor material such as barium titanate or lead zirconate between metal or semi-conductor electrodes are disclosed. In an alternative arrangement an insulating layer of aluminium oxide 10<SP>-7</SP> cm. thick is interposed between the ferroelectric and one of the electrodes which is conveniently of aluminium. An additional electrode may be provided to vary the remanent polarization and permittivity of the ferro-electric and thus control the current in the device. The current in all these devices using ferro-electric material is a function of applied pressure.