Patent GB-988326-A (GB)
Title: Semiconductor potentiometer
IPC: H01L29/00
Filed: 1961-08-25
Abstract: 988,326. Semi-conductor devices. WESTINGHOUSE ELECTRIC CORPORATION. Aug. 25, 1961 [Oct. 20, 1960], No. 30725/61. Heading H1K. An apparatus functioning as a non-linear potentiometer includes a semi-conductor device having first, second, and third successive layers of constant thickness lying adjacent to each other and forming first and second PN junctions. The width or the resistivity of at least one of the layers varies continuously and non-linearly from one end of the layer to the other. A potential gradient is established along the length of the resistive path formed by the second layer (which lies between the two junctions), and by varying the bias potential which determines the gradient, conduction of the different portions of the first PN junction is controlled in a manner such that the resistance appearing between the two outer layers varies continuously and non-linearly with respect to the applied bias. The device shown in Fig. 6 is obtained from an N-type silicon wafer doped to 10-100 ohm./cm. with arsenic, antimony, or phosphorus. The high resistivity collector layer 11 is obtained by exposing the wafer to an atmosphere of indium, gallium, aluminium, or boron. Oxide masking may be used to limit diffusion to the desired area, or overall diffusion may be allowed and the unwanted parts of the P-type zone removed by etching and/or abrasion. All except the lower surface is then masked and a low resistivity emitter layer 15 is formed by a similar diffusion process. Ohmic contacts 20, 21, 22 are alloyed to the P-type layers using boron-doped gold foil and ohmic contacts 23, 24 are formed on the base layer 12 using antimony-doped gold foil. Oxide masking may be used to prevent the contacts from overlapping adjacent layers. The collector layer is shaped by suitably masking the layer and etching or sandblasting. A wax mask is suggested for use with an etchant consisting of nitric, acetic, and hydrofluoric acids. In operation the emitter layer 15 is effectively at a uniform potential because of its low resistivity. Because of its high resistivity the base layer may have a voltage gradient along its length. The emitter/base contacts 21, 24 are subjected to a standing bias B upon which is impressed the signal Vi. The emitter/base contacts 20, 23 at the opposite end from the collector contact 25 have a bias of B+y, y being variable. The bias B is insufficient for the transistor to conduct, but as the additional bias y is increased the transistor conducts over increasing areas from the end remote from the collector connection 25. The collector current thus depends on both voltage Vi and voltage y so that the device behaves as a potentiometer in which the signal Vi corresponds to the full winding voltage and the voltage y corresponds to the tapping point. The " low " of the potentiometer is determined by the shape of the collector layer or, in alternative embodiments, by the resistivity profile of one of the layers. The operation of the device of the invention is initially illustrated with reference to a device having emitter, base and collector layers of uniform width and resistivity. Specification 913,674 is referred to.