Patent GB-861021-A (GB)
Title: Improvements in or relating to photosensitive semiconductor devices
IPC: C04B35/46
Filed: 1957-05-31
Abstract: 861,021. Photo-cells. RAULAND CORPORATION. May 31, 1957 [May 31, 1956], No. 17324/57. Class 37. A method of manufacturing a photo-conductive device comprises the following steps: coating a substrate with a material consisting primarily of at least one of the compounds cadmium sulphide and cadmium selenide and including minor amounts of a donor element from Groups III A  and VII A  of the Periodic Table and of an inhibitor element which is copper, silver or manganese, and firing the coated substrate in a surround containing at least one of said compounds at a temperature exceeding 650‹ C. for a time sufficient to sinter the material to the substrate and to develop photo-conductive properties therein. The substrate is then cooled and spaced conductive coatings applied to form electrodes. In the case of a CdS coating the electrodes can be formed by painting with silver paste and can be separated by a gap of about 1 mm. The inhibitor element can be applied by evaporation, to the extent of 10-<SP>3</SP>/mgrs/cm.<SP>2</SP>, to a substrate previously coated with the remaining components of the material. Alternatively the material can be coated on the substrate as a water suspended slurry, the slurry being formed by mixing together the compound or compounds in powdered form, a chloride, a copper salt and water in such proportions that the concentrations of chlorine and copper are of the order of 10-3 parts by weight of the compound or compounds. The compound can be cadmium sulphide on a glass substrate and the firing is at 650-750 C., preferably at approximately 710‹ C. for one half to two hours, in a surround of powdered cadmium sulphide. Alternatively the compound can be cadmium selenide on a quartz substrate and the firing is at 800- 900‹ C., preferably at 870‹ C. for one half to two hours, in a surround of powdered cadmium selenide. The method can include the additional sensitizing step or steps of heating the sintered coating in an oxygen-containing atmosphere such as air at temperature of 250-325‹ C., the heating being continued until the coating exhibits a maximum dark conductivity and the coating then being rapidly cooled, e.g. by quenching in a neutral oil or other liquid. Before the heating in the oxygen-containing atmosphere, the sintered coating can have applied to it, e.g. by spraying a dilute solution of a copper salt, e.g. cupric chloride having a concentration of the order of 10 parts per million. Fig. 3 shows a crucible arrangement for use in the sintering step, the arrangement comprising a glass vial 30, an aluminium oxide or steatite ceramic disc 31, a glass disc 32 with the coating 33 facing upwardly and a ceramic wafer 34, followed by another coated glass disc 35 and further ceramic discs and coated glass discs. The last disc is a ceramic wafer 36. A cover 37 of glass or quartz is provided. The vial 30 is positioned within a quartz crucible 38 filled with commercial grade CdS powder 39 and covered by a quartz lid 40. Fig. 4 shows one form of completed photo-cell 41 without a housing, the cell comprising a glass disc 42 with a sintered CdS coating 43, two electrodes 44, 45 separated by a gap 46 approximately 1 mm. wide, and two leads 48 and 49. Illumination of that portion of the coating 43 exposed through the gap 46 causes the electrical resistance between the electrodes 44 and 45 to vary to permit control of circuit characteristics. Fig. 5 shows a housing for the photo-cell 41-46, 48, 49, comprising a cup-shaped casing 50 with a highly transparent base portion 51, the whole preferably being made by machining from a rod of polymerized methyl methacrylate, polystyrene or other transparent plastic. The leads 48 and 49 are mounted in longitudinal grooves in the outer surface of a retaining cylinder 53 and are pressed into contact with the electrodes 44 and 45, silver paste or other conductive cement 54 and 55 being used at the junctions. Cylinder 53 is preferably filled with wax 56 which extends beyond the cylinder to a wax plug 57. The sidewalls of the housing are provided with a non-conductive opaque coating 58. In the construction shown in Fig. 6 the sensitivity of the photo-cell is increased by providing a constant-width curved gap 66 between the electrodes 64 and 65 which are applied to the CdS coating 63 by silk screening using a graphite suspension in alcohol such as Neolube. Operating characteristics of the device are discussed in detail with reference to graphs.