Patent GB-715834-A (GB)
Title: Semiconductive negative-resistance devices and methods of making them
IPC: H01C7/10
Filed: 1951-12-07
Abstract: 715,834. Semi-conductor circuits. WESTERN ELECTRIC CO., Inc. Dec. 7, 1951 [Dec. 8, 1950], No. 28724/51. Class 40 (6). [Also in Group XXXVI] A negative resistance device comprises an extrinsic semi-conductor body with donor and acceptor impurities in near balance, means for heating one portion to provide intrinsic conductivity, and means for injecting minority carriers from the intrinsic portion into the extrinsic body. Fig. 1 shows the device, comprising a semiconductor body 13 having an ohmic contact to rod 14 and engaged by a gold-plated tungsten wire contact 27. This point contact 27 could be replaced by a larger area contact. Fig. 2 shows the current voltage characteristic of the device including the negative resistance portion. The semi-conductor material has extrinsic conductivity provided by a relatively high impurity content of both donors and acceptors which are almost balanced, resulting in high resistivity, short injected charge carrier lifetime and substantially no rectification properties. In the example described, acceptors slightly outnumber donors so that the material is P-type. The material is such that intrinsic conductivity (i.e, the state when the number of holes and electrons produced thermally exceeds the number provided by the impurities) occurs at a relatively low temperature (e.g. 200‹ C.). A bias current is passed through the device and an intrinsic conductivity region is produced near the point contact, since the heating effect in this area is greater. If the point contact is negative, electrons are repelled from the intrinsic region into the extrinsic P-type body, resulting in the production of an intermediate region containing both electrons and holes; the number of holes in this region is increased due to the presence of the electrons. The resistance of this intermediate region is thus greatly reduced and this provides the negative resistance effect. The material is also such that the lifetime of injected carriers is short which enables the negative resistance effect to be maintained at higher operating frequencies. If the point contact is positive a negative resistance portion still obtains due to the production of the intrinsic conductivity region, but in this case the effect ceases at a much lower operating frequency as no intermediate region having injected charges of opposite sign exists. The semi-conductor material may be prepared by the steps (1) fusing a mixture of germanium with about .0003 per cent by weight of antimony in helium at 1200‹ C. (2) Progressively cooling as described in Specification 632,942. (3) Heating in vacuo at a temperature between 600‹ C. and 650‹ C. in contact with aluminium oxide, and cooling at a relatively fast rate. Alternatively, in place of fusing the mixture of germanium and antimony, the heat treatment may be applied to wafers of germanium which have been dipped into a suspension of antimony in distilled water. The semiconductor body is copper plated for soldering to rod 14, and the contact surface etched. After assembly, a current of .5 amps. may be passed through the device for electro forming. Silicon and tellurium may also be used in place of germanium. Fig. 5 shows a high speed switching circuit. The device 30 is biased to the position left of P in Fig. 2, the point contact being negative, so that normally the load 32 takes very little current. The application of an aiding pulse between terminals 33 and 34 carries the device into the region where its resistance is much reduced so that the load 32 is effectively switched into circuit. A pulse of opposite polarity is used to switch the circuit back into its original condition.