Patent GB-706514-A (GB)
Title: Improvements in or relating to electric rectifying devices
Inventor: REEVES ALEC HARLEY
IPC: H03K23/00
Filed: 1951-10-12
Abstract: 706,514. Multistable-state circuits using semiconductors. STANDARD TELEPHONES & CABLES, Ltd. Oct. 3, 1952 [Oct. 12, 1951], No. 23860/51. Class 40 (6). [Also in Group XXXVI] A negative resistance device comprises a N- type semi-conductor having a base electrode and a rectifying electrode composed of a material including donor type impurity, which has been subjected to an electro-forming current sufficient to produce a negative resistance portion in the forward voltage-current characteristic. The device enables low time-constant circuits to be provided for high frequency applications. In Fig. 1, an N-type germanium or silicon crystal has a base electrode 2, and a whisker electrode 4 of silver or copper containing a small quantity of donor material such as arsenic or phosphorus in contact with a surface which has been etched. A switch 15 enables a condenser 12 to discharge through a relay 8, the contacts 7 of which momentarily connect a current source 5 across electrodes 2 and 4. Source 5 may be about 40 volts A.C. or D.C. and is applied through variable resistance 9 to provide a current pulse of about 1 or 2 amps. This current may result in the whisker being welded to the germanium. Condenser 11 is also connected across electrodes 2 and 4 during the electroforming to avoid transient voltages. According to the Provisional Specification a drop of oil may be put on the electrode point to stop sparking, and a massive metal support may be used to cool the crystal. A metal film electrode may be used instead of the whisker electrode, as described in Specification 686,915, [Group XXXVI]. A cathode-ray oscillograph represented by deflecting plates 16-19 in conjunction with an alternating current test source 21 mav be provided to obtain the voltage current characteristic of the crystal rectifier. Curve 28 in Fig. 2 shows the forward characteristic of the rectifier initially, and curve 23, 25, 27, that obtained after the electroforming treatment. If the value of the load resistor 20 is less than that of the negative resistance portion 25, the corresponding load line 29 will cut the characteristic at three points and since any point on portion 25 is unstable the arrangement provides a trigger circuit in which the operating point jumps from bend 24 to point 32, and from bend 26 to point 33. In some cases the characteristic may have a more pronounced Z-bend so that the portion 25 becomes a positive resistance, and a load resistance of any value may then be arranged to cut the curve at three points to provide a trigger circuit. This effect may be due to the whisker electrode comprising, in effect, two point contacts in parallel, one having a normal characteristic and one the negative resistance characteristic of Fig. 2. Fig. 4 shows an application of the rectifiers described, in which rectifier 41 derives a train of short pulses from a sine wave, and rectifier 56 is arranged as a binary counter. Rectifier 41 is biased in the forward direction by battery 47 to a point just below bend 24 in the characteristic. A high frequency sine wave is applied to the rectifier via transformer 50, which alternately switches rectifier 41 between high and low current conditions. These substantially rectangular waveforms B (Fig. 5) are differentiated by capacitor 65 and resistance 43 to produce a train of short pulses C which are applied to the base electrode of rectifier 56 which is also biased to near trigger conditions. Inductor 58, arranged to be resonant at a higher frequency than the input, is shock excited by the short pulses C, so that at point 67 on curve D rectifier 56 is switched to its high current condition, and at point 69, to its low current condition. The short pulses 55 corresponding to the trailing edges 53 of the rectangular pulses are smaller, due to the backlash in the trigger circuit, and due also to the time constants involved, these have no effect on rectifier 56. If pulse 71 is differentiated, a single positive pulse can be obtained in response to two pulses 54 in graph C. Specification 650,007 also is referred to.