Patent GB-897992-A (GB)
Title: Improvements in or relating to electrical-circuit elements
IPC: H01B1/10
Filed: 1958-07-15
Abstract: 897,992. Semi-conductor devices. PHILIPS ELECTRICAL INDUSTRIES Ltd. July 15, 1958 [July 15, 1957], No. 22638/58. Class 37. A semi-conductor device which may be changed from a state of symmetrical conductivity to one of assymmetrical conductivity for applied voltages above a certain frequency by application of a polarizing voltage comprises two semi-conductor zones separated by electret material. The device may comprise semiconductor particles dispersed in a mass of electret material binding them together. An electret material is defined as a dielectric material capable of retaining polarization for longer than the relaxation time. The semiconductor material should be one in which the effective mobilities of holes and electrons differ markedly and in which the storage capacity for the current carriers of higher effective mobility is greater than for those of lower effective mobility. In one embodiment 10<10> ohm. cm. N- type cadmium sulphide, made by heating cadmium sulphide with 10<-4> gram mols. of gallium nitrate and copper nitrate in hydrogen sulphide for 2 hours at 950 C., is mixed in a volume ratio of 4: 1 with a lithium rich glass enamel. A thin layer of the mixture is deposited on a glass plate from a suspension in butyl acetate, heated for 3 minutes at 600 C. to consolidate it and then provided with linear electrodes by evaporation. The device is normally non-conducting in the unpolarized state but conducts symmetrically when illuminated. If a direct voltage of 300 is applied for 5 seconds across the device while it is illuminated and heated to 110 C. the device becomes polarized. Subsequently the device remains normally non-conducting but when illuminated it rectifies currents of commercial mains frequency while remaining symmetrically conducting for direct and very low-frequency alternating currents. These properties are retained for a period of weeks, whereas if the lithium rich glass is replaced by a sodium glass the properties are lost more rapidly. The direction of rectification may be reversed by heating again and repolarizing the electret by application of a direct voltage of 300 in the reverse direction. The heating may be produced by an external source or by use of a polarizing voltage of sufficient magnitude to generate the heat internally. If the high dark resistance cadmium sulphide is replaced by 10<5> ohm cm. N-type cadmium sulphide or selenide the device becomes normally symmetrically conductive in the absence of light and rectifying when polarized. A device comprising P- and N-type zones spaced by an electret zone is also suggested which would be rectifying even in the non-polarized state. A memory circuit utilizing a device comprising an electret zone 20 between two photoconductive insulator zones is shown in Fig. 4. The circuit comprises a reversible direct-voltage source 28 for determining the polarization of the device and a source of high-frequency alternating voltage 30 in the input, and a load resistor 22 and low-pass filter 23, 24, 25 in the output circuit. The device acts as a half-wave rectifier when polarized and as a resistor when unpolarized so that when an alternating read-out signal is applied by closing switch 29 the output at terminals 31, 32 is zero if the device is unpolarized, or positive or negative according to the sense of polarization of the device. A device similar to that shown in Fig. 4 but provided with a raster consisting of an opaque plate with a series of equidistant apertures each provided with a contact for polarizing the electret at that point is also described. In this case with the alternating read-out signal applied to all the contacts the stored information may be read out from the layers underlying any aperture or combination of apertures by illuminating the relevant apertures. The theory on which the devices are based is discussed.