Patent GB-807959-A (GB)
Title: Fused junction semiconductor devices
IPC: H01L21/228
Filed: 1956-01-30
Abstract: 807,959. Semi-conductor devices. HUGHES AIRCRAFT CO. Jan. 30,1956 [Feb. 25, 1955], No. 2973/56. Class 37. A method of making a PN junction comprises depositing on a crystalline semi-conductor body (which is a chemical element, e.g. Si or Ge) of one conductivity type a molten layer of a solvent for the semi-conductor which is also an activator in purity for the opposite conductivity type or contains such an activator, the body having been previously heated to at least the eutectic temperature of the solvent and semiconductor, and then cooling the body to form a PN junction by recrystallization of the dissolved semi-conductor. Suitable solvents are Au, Pt, Ag, Sn and Al, which is also an acceptor impurity ; suitable donors are P, As, Sb and suitable acceptors Al, Ga and In. In one embodiment the surface of a wafer 30 (Fig. 1) of an N-type Si single crystal so cut that a relatively densely populated crystallographic plane, e.g. 111, 110, 100, 112 lies in the wafer surface, is lapped, etched by immersion for 30 seconds in a mixture of equal proportions of nitric, hydrochloric and acetic acids, rinsed first in distilled water, then in methyl alcohol and placed on an electrically heated graphite element 16 housed in vacuum chamber 20. A1 wire is then wound round the sawtooth filament 24 of, e.g., Wo, which is wetted by Al, the chamber evacuated to 10<SP>-4</SP> mm. of Hg and the platform heated to 800  C. Current is passed through the filament until the Al melts and is then increased to evaporate the Al on to the Si. The Si body is then cooled to 200 C. at a rate of 0.2 C./second and removed from the chamber. For optimum results the Si surface should be heated to between 700 and 900 C., the evaporated layer 0.2 to 10 mils. thick and the rate of deposition greater than <SP>1</SP>/ 100  mil./sec. A layer of eutectic SiAl alloy which recrystallizes out on top of the recrystallized Si serves as an ohmic contact electrode. A PN junction diode is manufactured by applying a predetermined quantity of Au containing ¢-1 per cent Sb to the other face of the wafer, heated to 500  C., before the above junction-forming process, using the apparatus shown in Fig. 1, to provide a second ohmic contact. After both treatments the body may be divided into a number of elements which are each provided with terminal wires attached to opposite faces of the wafer by solder or a thermosetting paste including Au. A PNP transistor (Fig. 4) may be produced by application of Al first to one side and then to the other of an N-type Si wafer by the above described method. One of the junctions so formed is used as the collector and the other as an emitter electrode. The area of the emitter junction 42 may be made smaller than that of the collector 44 by masking one face of the wafer to restrict the deposit to its centre. An annular Au Sb alloy layer 50 in the form of a ring is then fused as described in connection with the diode to the previouslymasked part of the wafer surface to form an ohmic base contact. Terminal wires 46, 47, 51 are attached by solder or a gold-containing thermosetting paste to the emitter 48, collector 49, and base 50 contacts respectively.