Patent GB-925039-A (GB)
Title: Semiconductor potentiometric device
IPC: H01L27/06
Filed: 1961-08-31
Abstract: 925,039. Electric analogue calculating; semi-conductor devices. WESTINGHOUSE ELECTRIC CORPORATION. Aug. 31, 1961 [Oct. 28, 1960], No. 31372/61. Class 37. In an apparatus for obtaining an output which is a function of at least two input signals there are included at least two rectifying junctions operating in the exponential range of their forward characteristics and biased in the forward direction by the input signals so that a voltage proportional to the logarithm of the input is developed in each input circuit, and an output circuit connected across the two junctions and containing a third rectifying junction operating in the exponential part of its characteristic to give a current therethrough proportional to the antilogarithm of the combined voltage across the first and second rectifying junctions. A multiplier circuit for performing the function of a linear potentiometer may comprise two input supply voltages x and y (Fig. 1) connected across the diodes V 1  and V 2  in series with the high resistances 22 and 24, respectively. The current flowing through each rectifier is then effectively proportional to the input voltages x and y, and each rectifier is operated in the part of its characteristic where the voltage across it is proportional to the logarithm of the current flowing through it. The sum of the voltages across the two diodes is imposed across a third diode which is operated in the part of its forward characteristic where the current through it, I c , measured on ammeter 27, is an antilogarithmic function of the voltage across it. Hence I c  is proportional to the product xy of the input voltages. Further diode circuits (Figs. 2, 3, 4, 5, not shown) are described showing how, using the same principle, a logarithmic potentiometer and a divider may be devised, how temperature compensation may be introduced, and how the meter may be isolated from the output rectifying circuit using a transistor. Fig. 6 shows a further circuit which performs a multiplication function, and also lends itself to a monolithic semi-conductor construction. Between diodes V 1  and V 2 , which act in the same way as those in Fig. 1, an oppositely poled diode V 3  supplied from a constant potential K through resistor 66 is included. This has the effect of dividing the output by a constant k, so that the multiplying function for the voltages x and y is preserved. The emitter base junction of the input transistor 73 acts as the output diode 26, and the meter 74 is included in the collector circuit of this transistor. The whole of the circuit indicated in solid lines may be fabricated as a single monolithic device (see Fig. 10). A P-type silicon wafer, which may be prepared from a section of a dendritic crystal, is treated by diffusing on to its surface a doping material such as antimony, arsenic or phosphorus in a furnace with a temperature range of 600-1250‹ C., to form an N-type layer. The wafer is thereafter treated to form the hole 96, the troughs 97, 99 and 100 and the recessed area 106, either by sand blasting, ultrasonic drilling or etching, which in any case is necessary in the final stage to smooth out roughened surfaces. Masking material such as wax is used to protect those areas which do not need treatment. N-type material for the emitter portions 110 and collector portions 111 and N-type ohmic contacts may be secured to the appropriate surfaces of the wafer by disposing thereon foils comprised of an alloy of, say, gold and antimony (0.5-1% of the alloy) and alloying or fusing these foils into the surface by heating in vacuum at a temperature of 400- 700 ‹ C. P-type ohmic contacts may be formed from suitable material, e.g. boron, aluminium, gallium or indium or a gold foil alloyed with about 0.1% boron. Gold may be used for neutral ohmic contacts. In addition to silicon . as the wafer material, germanium or semiconducting compounds comprised of stoichiometric proportions of elements from Group III, e.g. gallium, aluminium or indium, and V, e.g. arsenic, phosphorus and antimony, of the Periodic Table may be used. Specification 913,674 is referred to.