Patent GB-694034-A (GB)
Title: Electrical devices utilizing semiconductor materials for the translation of electriccurrents
IPC: H01L29/00
Filed: 1950-03-24
Abstract: 694,034. Semi-conductor amplifiers; exchange systems. WESTERN ELECTRIC CO., Inc. March 24, 1950 [March 31, 1949], No. 7420/50. Class 40 (iv). [Also in Groups XL (a), XL (b) and XL (c)] An electrical device comprises a body of semi-conductor material with an electrode applied to the surface thereof and having a film of insulating material on the surface of the body in the vicinity of the electrode, and means for projecting electric charges on the insulated film to produce a surface charge which affects the contact resistance of the electrode. Fig. 1 shows an evacuated envelope containing an electron gun assembly comprising cathode 2, control grid 3, beam focusing electrodes 4 and beam deflecting electrodes 5, to produce an electron beam which impinges on a target consisting of a body of semi-conductor 7 connected to a metal base 8. A point contact collector electrode 10 which is biased by battery 12 to form a high resistance contact engages the surface of the semi-conductor 7. The surface of the semi-conductor 7 is provided with a thin film of insulation which may be produced by oxidizing the surface of a block of material such as germanium or silicon. The surface film should be a good insulator and should emit secondary electrons in a ratio greater than unity. The secondary emission ratio may be improved by evaporating droplets of suitable material such as nickel or magnesium on to the surface of the insulating material. An auxiliary electrode 18 is located in front of the block 7 to control the flow of secondary electrons from its surface. If electrode 18 is biased negatively so that no secondary electrons are drawn from the block and a short positive pulse is applied to grid 3 so that a beam pulse impinges on the block 7 in the locality of collector electrode 10, the collector current is considerably increased and remains at the higher level after the cessation of the beam pulse. The period of increased collectorcurrent may be made to extend for several seconds or for hours according to the character and thickness of the insulating film. The effect is apparently due to the negative surface charge existing on the insulating film. If, initially, electrode 18 is biased positively when a beam pulse is applied, so that secondary, electrons are drawn away from the block, there is no detectable change in the collector current. If a beam pulse is applied with electrode 18 biased positively during the period of increased collector current due to the prior application of a beam pulse with electrode 18 biased negatively, the collector current immediately falls to its original small value. The effect of the value of the beam current and of the location of the beam relative to the collector electrode is discussed. The invention is distinguished from the known induced conductivity effect which would occur if the semi-conductor block had no insulating film. Fig. 11 shows the invention applied to a supervisory system for telephone calls whereby a sequence of pulses in one conductor, as in a time division multiplex system, are converted into enduring signals in a plurality of output conductors. Supervisory signals consisting of a recurring train of positive pulses are received over a line, each pulse train consisting of a marker pulse and twelve available " time-slots " any of which may contain a pulse. The pulse train is repeated at regular intervals and occupies only a small fraction of the time cycle, so that the line may also carry other signals. Each " time slot " corresponds to one of twelve incoming toll lines and is used to operate the respective calling equipment, The supervisory pulse train is selected by selector 57, and each pulse is applied to grid 33 of the electron gun assembly so that a beam impulse is produced. The marker pulses in conjunction with square-wave generator 52 and sawtooth generator 54 cause the beam path to sweep over twelve collector electrodes 43 spaced along two germanium strips 39 (Fig. 12). An erasing electrode 45 (corresponding to electrode 18 of Fig. 1) is provided for each collector electrode 43. Electrode 45 is normally at a negative potential relative to the strip 39 due to battery 41, so that when an input pulse is received, the appropriate collector 43 passes increased current to operate relay 44 and light the calling lamp 59 associated with the toll line corresponding to the time position of the pulse. When the operator answers and plugs into the appropriate jack 61 to complete the circuit to a subscriber's line, a positive voltage from battery 49 is applied to the appropriate electrode 45, so that when the next associated incoming pulse is received, tht collector current drops to its normal low value releasing relay 44 and extinguishing lamp 59. A modification is described, Fig. 18 (not shown), in which the electron beam is continuously operative and the erasing electrode, which now serves all the collector electrodes, is normally positively biased. The incoming pulses drive the erasing electrode negative thereby increasing the current of the appropriate collector. This arrangement causes the calling lamp to be extinguished when the series of recurring pulses constituting a particular calling signal ceases. As an alternative to the use of the erasing electrode, the decay time of the increased collector current may be made such that the operated relay restores after a predetermined period. Figs. 13 to 16 and 19 (not shown) describe alternative constructions for the target which may consist of a layer of semiconductor with a surface film mounted on a metal sheet which is fixed to an insulating block. The collector electrodes in insulated tubes pass through holes in the material layer before making point contact with the semi-conductor surface. The target may be circular or spiral-shaped. The auxiliary erasing electrode may consist of a wire grid located in front of the semi-conductor sheet. Specification 694,023 is referred to.