Patent GB-816799-A (GB)
Title: Improvements in or relating to semi-conductor devices and to methods of making them
IPC: F25B9/00
Filed: 1956-06-15
Abstract: 816,799. Semi-conductor devices. WESTERN ELECTRIC CO. Inc. June 15, 1956 [June 28, 1955], No. 18592/56. Class 37. [Also in Group XXIII] A method of making a semi-conductor device comprises heating a glass-forming composition comprising a significant impurity in contact with the surface of a semi-conductor body to at least partially fuse the composition and to diffuse the impurity into the surface to alter its conductivity or conductivity type. In one embodiment borosilicate and phosphate glazes containing finely-divided silver are fused respectively to opposite faces of a circular wafer of N-type silicon to form a PN junction body. After etching the edges of the wafer the two faces are copper-plated and tinned and electrodes soldered thereto to complete a rectifier. A rectifier may also be made by fusing a phosphate glaze containing platinium flake to one face and the edge of such a wafer, and a borate glaze containing rhodium to the opposite face. After the firing the borate glaze and the thin underlying P-type silicon layer are removed from all but the small central region of the latter face by an etching process. Electrodes are then applied as before. A photo-voltaic cell (Fig. 8) is made by fusing a clear borosilicate glaze 72 to the upper face, the edge, and the periphery of the lower face of an N-type Si wafer 71, the glaze applied to the edge and lower face containing rhodium, and fusing a phosphate glaze 73 containing silver to the centre of the lower face. The body apart from the annular uncoated region between the phosphate and borate glazes is then protected by wax and the annular part etched to clean the PN junction formed under coating 73. Electrodes 75 are then applied as described before. A photo-cell (Fig. 2) may alternatively be made by the following steps: (1) etching a wafer of N-type Si in a mixture of nitric and hydrofluoric acids; (2) painting a clear glaze 21 comprising boron aluminium, barium and silicon oxides suspended in a bindersolvent mixture known as Acryloid A-10 on one face of the wafer and a similar glaze 11, containing platinum powder on the edges and on the periphery of the other face; (3) firing in air for 30 minutes at 1050 C.; (4) etching the unglazed part of the other face to expose the PN junction and then sand-blasting; and (5) making electrical connections 14 to the glazed and unglazed parts of the lower face. In another embodiment a slice of N-type Si of 10-15 ohm cm. resistivity is heated to 1150 C. in a sealed tube for 16 hours with, but out of contact with, a bead of boron trioxide in a rarefied helium atmosphere. In the process the trioxide is vapour deposited on the silicon and diffuses into it to form a P-type surface layer beneath a glaze which may be removed by rinsing in hot water. Embodiments are also described in which boron trioxide, a borate glaze, or gallium oxide is applied in the form of a suspension with an organic binder in a suitable organic solvent, for instance with polymerized ethyl acetate in toluene, to N-type Si bodies which are then dried by a preliminary firing, and then fired to produce a glazed surface. Details of other glaze compositions, which are preferably chosen to match the expansion coefficient of the semi-conductor, of suitable organic binders and solvents therefor and of firing processes are given in the Specfication.