Improvements in transistors and the manufacture thereof

摘要

842,103. Transistors. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 20, 1956 [Nov. 22, 1955], No. 35502/56. Class 37. A transistor specifically designed for switching circuits is made by converting the outer portion of a wafer of first conductivity type to the opposite type by gaseous diffusion, heating the wafer in a vacuum, obtaining from the wafer a wafer with surfaces of both conductivity types exposed and etching away the portion of opposite type conductivity to make it of predetermined thickness, cutting out a small dice and soldering to the surface, of opposite type conductivity a circular ohmic connection in the centre of which an impurity dot is fused into the dice, a second impurity dot being fused into the surface of the first conductivity type. A slice is cut from a monocrystalline piece of P-type germanium with a resistivity of 2 ohms/centimetre, the major planes of the slice being parallel to low index planes of the crystal. The slice is placed in an atmosphere of 106 arsenic atoms/cc in hydrogen held at 800‹C for 19 hours to produce an N-type region for a depth of 0À001 inches below each surface of the slice. After diffusion the resistivity of the N-type region increases exponentially with depth from zero at the surface to a maximum at the P-N junction and decreases from the junction until the normal resistivity of the germanium is reached. Should an alloy emitter be formed as a spot on the surface of the N-type region the barrier junction will extend over the resistivity gradient and in consequences its injection efficienty will vary over the barrier. To obtain an injection efficiency constant over the barrier junction, the crystal is heated in a vacuum for an hour, after the diffusion of the arsenic impurity, so that the arsenic diffuses outwards from the surface and forms a layer of about 0 À0002 inches depth with a constant resistivity of 0À1 ohms/ centimeter. An alloy junction formed in this depth will have an injection efficiency which is constant over the junction. To from a transistor the crystal slice is cut to form P-N wafers and to obtain a precise thickness of the N-type region (base), so that it may have a specific breakdown voltage and frequency-response, the edge of a wafer is marked by an etch line to indicate the approximate position of the junction, the exact position being found by a barium titanate deposition method. The wafer is then ground down to a base thickness close to that desired and is etched away until the desired breakdown voltage as measured by probes, is obtained The wafer is then diced and a tab 5, Fig. 1, with a hollow frustro-conical projection is soldered to make an ohmic connection with the base 2 of the dice, the surface surrounded by the tab being used to form an alloy emitter 8 by pressing or firing a sphere of indium on to the surface. A larger sphere of indium forms an ohmic contact with the collector 3. Terminal connections are passed through a seal 13 so that the transistor may be sealed in a can 14 containing mesitylene.