Patent GB-694021-A (GB)
Title: Apparatus employing bodies of semiconducting material
IPC: H03F3/04
Filed: 1948-09-10
Abstract: 694,021. Semi-conductor amplifiers. WESTERN ELECTRIC CO., Inc. Sept. 10, 1948 [Feb. 26, 1948; June 17, 1948]. Nos. 23808/48 and 23809/48. Class 40 (iv). [Also in Groups II, XXXVI and XL (c)] An electric circuit element comprises a body of semi-conductor material having a first electrode making a rectifier connection, a low resistance second electrode and a third electrode, and is such that if a forward bias is applied between the first and second electrodes, and an opposite bias between the third and second electrodes, an input signal applied to the first electrode results in an amplified signal in the output circuit including the third electrode. The device may be used for amplifying or generating electric oscillations. In Figs. 1 and 1A, a block of N-type germanium is provided with a low resistance base electrode contact 2, and two rectifier contacts 5 and 6, a rectifier contact being regarded as a high resistance contact, the effective resistance of which varies according to the direction of the applied voltage. The two rectifier contacts are placed close together, the " emitter " electrode 5 being biased a fraction of a volt in the direction of easy current flow, and the " collector electrode 6 being biased between - 5 and - 50 volts in the reverse direction. If - a signal is applied between the emitter 5 and the base electrode 2, an amplified signal output appears in the load circuit connected between the collector 6 and the base electrode 2. A table of operating characteristics is given. The functional similarity between the emitter, base and collector electrodes and the cathode, control grid and anode respectively of a thermionic valve is discussed and reference made to circuits corresponding to " grounded grid," "cathode follower," and to oscillator circuits. Alternative constructions are described in which the collector electrode may comprise two or more elements symmetrically disposed about the emitter, or be in the form of a ring or two semi-circular metal spots, or may consist of the plated surface of a cone. Alternatively, the emitter and collector electrodes may consist respectively of the two plated sides of a wedgeshaped piece of insulating material providing contacts of linear form. The semi-conducting block 1 of N-type material is regarded as having a thin P-type layer 3 on the surface of the block, the two layers being separated by a high resistance barrier 4. The thin layer may be a " chemical " layer produced by variations in the impurity content, or a " physical " layer in a block having a uniform concentration of donor impurities, due to the conditions at the surface. Reference is made to the use of silicon, germanium, cuprous oxide or silicon carbide as the basic semi-conducting material, and the use of impurities such as phosphorus, antimony, arsenic, boron or aluminium to provide desired N or P characteristics. A block of silicon material suitable for the device and having a thin layer of opposite conductivity type may be prepared as described in Specifications 592,260, 592,303 or 669,399, [all in Group XXXVI], or a block of N-type germanium may be prepared as described in Specification 632,942, [Group XXXVI]. After being plated to provide the low resistance contact, the block of N-type germanium may be subjected to anodic oxidization treatment in which the surface to be treated is covered with polymerized glycol borate, and then subjected to electric currents. When assembled with the contact arrangement as shown in Fig. 1, an electrical forming process may be carried out in which a potential in excess of the peak back voltage is applied between one or both of the electrodes 5 and 6 and the base electrode 2. The effective resistance of the barrier 4 may be increased by restricting the area of the block subjected to the anodic oxidization treatment to provide a bowl-shaped barrier (Fig. 11), or by machining the block (Fig. 10). The points of contacts 5 and 6 may be prepared as described in Specification 595,601, [Group XXXVI], or the contacts may consist of evaporated gold metal spots. Preferably the area of these point contacts is large compared with the depth of the P layer. The block may consist of P-type material with a thin N-type layer, or may comprise different basic materials for the N and P-type layers respectively. The theoretical aspect is discussed in the Specification. A substantial proportion of the current from emitter 5 consists of positive holes which tend to flow along the P layer 3 before finally penetrating the barrier 4 to reach the base electrode 2. Current in the N portion of the block may consist of electrons flowing towards the barrier 4 to neutralize the positive holes. The current spreading from emitter 5 tends to be collected by the electrode 6 due to the strong field across the layer and barrier in the region of this electrode. The current from emitter 5 is materially affected by changes in the potential of this electrode, but the proportion of emitter current collected by the electrode 6 (i.e. positive holes) is not greatly affected by changes in the potential of the collector electrode. Consequently signals applied to the low impedance emitter circuit appear in amplified form in the high impedance collector circuit. The increase in the number of positive holes at the collector tends to increase the other component of the collector current consisting of electrons flowing from the base electrode 2. Diagrams of electron energy throughout the P layer are given, and factors relating to the optimum thickness of the P layer are discussed. Specification 694,023 and U.S.A. Specification 2,402,839 also are referred to.