Patent GB-728244-A (GB)
Title: Improvements in and relating to germanium photocells
IPC: H01L31/06
Filed: 1951-10-17
Abstract: 728,244. Photo-electric devices. GENERAL ELECTRIC CO. Oct. 17, 1952 [Oct. 19, 1951], No. 26095/52. Class 37. [Also in Group XL (b)] A photo-electric device comprises a thin germanium wafer having a P-N junction with ohmic contacts to the P and N portions, and means for directing light through the wafer to impinge perpendicularly on the junction. In Fig. 1, an insulating casing 11 supporting a lens 12, encloses a germanium wafer 13 which has a P-N junction parallel to its main surface. Electrode 17, which may consist of a number of metal foils, is connected to the upper surface of wafer 13. The germanium is mounted on a plate 21 which is supported on a fernico plug 14 connected to electrode 15. Light is directed by lens 12 on to the surface of the germanium to produce photo-voltaic and photo-conductive effects. The P-N junction is formed by applying a thin film 20 (Fig. 2) of significant impurity (acceptor or donor according to whether the upper portion is to be P or N type respectively), and heating to diffuse the material into the germanium preferably in an atmosphere of argon. Reference is made to Specifications 727,900 and 728,129 and the depth and extent of diffusion depends on the temperature and duration of the treatment, which is selected so that the P-N junction occurs at the desired depth. What remains of the film 20 after treatment may be. removed by an etching process. The plate 21 may also comprise donor or acceptor material so that the conductivity of the lower portion of the germanium wafer may be modified by heat treatment to effect diffusion of impurity. Acceptor materials may consist of aluminium, gallium or indium, and donors of antimony, phosphorus or arsenic. Plate 21 may be constituted by a layer of solder comprising, for example, 85 per cent lead and 15 per cent antimony or indium. The position of the P-N junction may be located by cutting the wafer at a sharp angle and moving a hot metal probe along the surface resulting in a thermo-voltaic effect; the resulting voltage changes polarity as the junction is passed. In place of film 20, the impurity may be applied to the surface in the form of a grid, any material remaining after the heat treatment being removed by an etching process which results in grooves appearing around the grid formation which prevent the short-circuiting of any portions of the P-N junction which extend to the surface. Fig. 4 shows an alternative photo-electric arrangement in which a drop 40 of solder material comprising significant impurity is placed on the surface of a germanium disc 35, and opposite impurity material 41 extends around the lower periphery of the disc. Heat treatment effects diffusion to provide a P-N junction 42. The germanium is shaped as a convex lens to direct light on to the junction. Electrodes 38 and 37 contact the P and N portions of the germanium. Etching treatment may be applied to provide a groove around impurity drop 40. The thickness of the germanium through which the light penetrates to reach the junction may be modified to control the spectrum to which the device responds, since germanium absorbs ultra-violet more than infra-red. The film 20 in the Fig. 1 example may also act to reduce reflection from the surface. The device will also operate as a rectifier.