US2863106A - Crystal rectifier and manufacture thereof - Google Patents

Description

De@ 2, 1958 P. E. LIGHTY4 ET AL 2,863,106

CRYSTAL' RECTIFIER AND MANUFACTURE THEREOF t I \I,.'

Hx I Wl United States Patent C CRYSTAL RECTIFIER AND MAN UFACTURE THEREOF Paul E. Lighty, Lafayette, John J. Albanes, Bayonne, and John H. Gesell, Clifton, N. J., assiguors to International Telephone and Telegraph Corporation, a corporation of Maryland Application July 9, 1953, Serial No. 367,058

,13 Claims. (Cl. 317-236) This invention relates to semiconductor devices and more particularly to improved point-contact crystal rectifiers and method of making same. i

Among the problems encountered in the manufacture of semiconductor devices, particularly crystal rectiers of the germanium and silicon type, `is the obtaining of crystal diodes that have stable and reproducible electrical characteristics. This calls for a diode structure that is impervious to moisture and that is mechanically rigid, particularly one having a stable contact between the terminal wires and the semiconductor crystal. Until now, these reproducible electrical properties have been diicult to obtain without resorting to elaborate techniques unsuitable for modern mass-production methods.

It is an object of the present invention, therefore, to provide a new and improved electrical crystal contact device which avoids the above-mentioned disadvantages of such prior devices. Itis a further object to provide a method lfor the rapid and economical assembly of such point-contact devices.

Further objects of this invention and features thereof will become apparent from the following drawings, in which:

Fig. l is a sectional view of the crystal diode;

Fig. 2 is a sectional View of the press and Whisker plug assembly prior to 'activation of the press;

Fig. 3 is a sectional view of the same press and Whisker plug assembly of Fig. 2 following activation of the press;

Fig. 4 is a plan view, partially in section, taken along the lines 4-4 of Fig. `2, of a part lof the die used in the press shown in Figs. 2 and 3;

Fig. 5 is a partial sectional view ofthe press cavity and Whisker plug assembly prior to activation lof the press, showing a preferred method of this invention; and

Fig. 6 is a partial sectional view of the diode tube and Whisker plug assembly following activation of the press shown in Fig. 5 showing a preferred embodiment of this invention. i

It is a feature of this invention that the diode may be `readily manufactured by combining three individual subassemblies. It is an important feature of the manufacture of two of these subassemblies that any soft metal, preferably lead or various alloys thereof, in the form of either metallic powder, shot, tubing, beads, pellets, Wire, granules, or other convenient unit is used to concurrently join together under pressure various elements making up the lsubassemblies which are then combined in the crystal diode.

Referring to Fig. l, the diode tube 1 may consist of any rigid insulating material. We prefer to use au unglazed, nonporous ceramic tube for this diode tube although other materials such as glass, alumina, titania, and various plastics such as polymonochlorotriuoroethylene and polytetrauoroethylene may equally well be used. The subassembly unit we designate as the Whisker plug assembly comprises a support pin 2, preferably made of nickel 'although any similar metallic conralfice ductor of the proper degree of rigidity may equally well be used, joined together to an S-shaped point-contact wire 3, preferably made of platinum or a platinumruthenium alloy; other materials such as tungsten, Phosphor bronze, silver7 and silver alloyed with tin, palladium, gold or copper may also be used, although we have found a wire composed of platinum and 10% ruthenium preferable for our purpose. This Whisker electrode and metallic pin are held together in rigid relationship to one another by a body of metal 4. One of the important features of this invention and one that makes this diode eminently suitable for mass-production techniques in an eicient and economical manner lies in the ease with which this Whisker electrode and support pin are joined together by this body of metal by use of a single, concurrent, compression-molding operation. Various metals and alloys may be used in carrying out the features of this invention; but We have found that the use of metallic lead or various lead alloys the most satisfactory for the practice of this invention. An alloy of lead and antimony, preferably composed of 90% lead and 10% antimony by weight, has the desired physical properties for joining together the Whisker electrode and support pin. These are joined together by a compression-molding technique, in a manner subsequently to be described.

For the subassembly designated as the crystal plug assembly, a support pin 5 preferably of nickel is used although other alternative conductors may be used, as mentioned for the support pin 2 which is part of the Whisker plug assembly. This support pin 5 is comolded with a metal 6, which is readily deformable under pressure. This metal is o'f the same composition as described for metal 4 of the Whisker plug assembly. To the end of support pin 5 the semiconductor die or slab 7 is attached. This semiconductor may be composed of germanium, silicon, aluminum-antimony alloy or other similar semiconductive material found to give rectifying action in diode devices. Where germanium is used, the semiconductor is prepared by the reduction in a hydrogen, nitrogen or helium atmosphere of previously purified germanium dioxide to metallic germanium. During or subsequent to this reduction process, various additives, such as antimony or arsenic as donor atoms, or gallium, indium or aluminum as acceptor atoms, may be added to the germanium to produce desired electrical properties. We have found that for the obtaining of stable, uniform semiconductor material having reproducible electrical properties, the use of single-crystal material is desirable. The semiconductor, either before or after being diced to proper dimensions, may be etched or similarly treated to produce desired surface characteristics. Where single crystalline material is used for the semiconductor, it is apparent that a certain latitude will exist with respect to the specic location of the Whisker point on the semiconductor surface without affecting the electrical properties of the assembled crystal diode. This semiconductor 7 may be attached to the support pin 5 in any of several manners such as welding, soldering, or by the use of a conductive element 8. We have found that the use of a polyethoxyline-type cement containing silver or a silver-antimony alloy .to render it conductive is highly suitable for the purposes o'f this invention. The metal plugs 4 and 6 are shaped in a tapered manner so that a portion of their diameters are smaller than the inside diameter of the ceramic tube. They are then readily force fitted into the ceramic tube under pressure in such a manner that shearing off of some of the soft metal ensues, thereby insuring a good fit to the diode tube 1. Under usual assembly procedures, the diode tube is held firmly in a horizontal plane with the respective ,plug assemblies indexed on 3 either side and connected electrically so that upon application of a slow steady pressure simultaneously on both plugs these assemblies enter the tube uniformly and indicate when the Whisker makes electrical contact with the semiconductor die. The Whisker is then advanced automatically an additional .G02 to .003 inch after electrical contact has been established in order to secure the requisite pressure on the semiconductor surface. This assembly is then rotated into position for end-sealing of the tube. Measured amounts of a polyethoxyline-type cement 9 color-coded black is used for the crystal plug assembly. This cement is forced into position in the appropriate end of the diode tube, and the diode is passed through a curing oven to set the cement. Following this, a polyethoxyline-type cement 10, color-coded White, and used for the Whisker plug assembly, is forced into the remaining untreated end of the diode tube and similarly cured. Terminal or lead Wires 11 and 12 are then butt-Welded to the end of the support pins. We prefer the use of tinned copper-clad steel Wire for these terminal Wires although alternative materials such as a tin-nickel-iron alloy may also be used. The principal criterion for the choice of these terminal Wires is the practicability of butt-Welding them to the support pins.

In Fig. 2 is shown a schematic representation of the press and Whisker plug assembly prior to operation. The Whisker Wire 3 from the supply reel is inserted through the chase 13 of the mold 14 and, in certain embodiments of this invention, particularly those using metallic powder, granules or pellets, given a crimp such that on insertion of the nickel pin 2 contact will be made between the end of the Wire and the pin. The metallic powder is then introduced into the mold cavity 15, followed by insertion of the nickel pin 2. Pressure is then applied by forcing the piston or plunger to the fully lowered position, as shown in Fig. 3. During the contacting of the lead with the crimped Whisker Wire, this Wire is flattened against the support pin, as shown in Fig. 3. .The plunger is then Withdrawn, the Wire crimped in the desired resilient S-shape, and then cut. This entire operation of inserting the Whisker Wire, the moldable metal, and the support pin, applying the pressure, and removing the finished product may be readily accomplished by a semi-skilled operator in approximately tive seconds. The technique is therefore eminently suitable for mass-production methods. Also, this method may be readily adapted to automatic equipment and techniques.

In Figs. 2, 3 and 4 We have shown one embodiment of several possible variations in the design of the die assembly. Fig. 4 shows a plan view, partially in section of this particular embodiment. In this, the lower portion of the die assembly consists of a series of concentric annular grooves. lt has been found that When lead shot is used not only does this annular arrangement serve to center the material of the shot, but that a firmer bond is obtained between the lead shot and the Whisker electrode. Obviously, many similar modifications in the design of the plunger and cavity assembly may be made without departing from the spirit of this invention.

We have found that Where powder is used, certain difliculties are encountered in maintaining the surface of the lead powder free of oxide unless careful precautions are exercised. We therefore prefer, ordinarily, to use the moldable metal in the form of a solid body such, for example, as lead shot. We have `found that the antimony-lead alloy of approximately 90% lead and antimony is readily shaped by the pressures used and yet has suiiicient rigidity to maintain its shape under normal circumstances. Therefore, We especially prefer the use of this lead-antimo-ny alloy although other metals that may be shaped under pressure may also be used. In Figure 5 is shown a partial sectional of the press cavity prior to activation of the press, showing a preferred embodiment of our process, namely, the use of lead shot 16. It is immaterial in this embodiment Whether the Whisker Wire 3 is straight or slightly crimped.

Where lead in the form of shot is used, and under the pressures that are applied, the Whisker 3 and pin 2 of the Whisker plug assembly, as shown in Fig. 6 are firmly embedded in the lead, although in this particular embodiment they usually do not touch one another. The conductivity of the lead is such, however, as to provide a substantially invarient electrical path from the Whisker 3 to the pin 2. The circular grooves in the face of the die, Fig. 4, provides for iiow of lead completely about the stem of the Whisker 3 as indicated at 17, Fig. 6.

In the case of the crystal plug assembly, the support pin is inserted in the mold and indexed so that upon completion of the molding cycle the end of this pin is flush With or slightly lower than the finished surface of the lead. The manner of molding the lead powder or shot is essentially similar to that used for the Whisker plug assembly.

The foregoing technique of comolding using a readily deformable metal allows the use in germanium crystal diodes of a much lsmaller size of germanium die than that already employed. Furthermore, the tolerances of assembly in making the subassemblies is such that ready and adequate contact is made by the point of the Whisker and the germanium. This renders the method eminently suitable for mass-production techniques.

While We have described above the principles of our invention in connection with specific apparatus and method steps, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

l. A crystal rectifier comprising a rigid housing of electrically insulating material, the inner surface of which defines a right circular cylinder, a crystal plug assembly and a Whisker plug assembly in which said Whisker plug assembly comprises a Whisker electrode and a support pin, a body of metal embedding one end of said Whisker electrode and one end of said support pin, said body of metal being shaped so that a part of it is spaced from and tapers with respect to said inner surface.

2. In a crystal rectifier, according to claim 1, in which said body of metal consists essentially of lead and antimony.

3. In a crystal rectifier according to claim 1, in Which to 95% by Weight of lead and 5 to 10% by Weight i of antimony are present in said body of metal.

4. A crystal rectifier comprising a rigid tube of electrically insulating material, a crystal plug assembly and a Whisker plug assembly, in which said Whisker plug assembly comprises a Whisker electrode and a support pin, a body of metal embedding one end of said Whisker electrode and one end of said support pin, said body of metal being shaped so that a part of it is of smaller diameter than the inside diameter of said tube.

5. A crystal rectifier, according to claim 4, in which said body of metal includes lead.

6. A crystal rectifier, according to claim 4, in which said crystal comprises germanium.

7. A crystal rectifier comprising a rigid tube of electrically insulating material and a Whisker plug assembly, in which said Whisker plug assembly comprises a Whisker electrode and a support pin, a body of metal embedding one end of said Whisker electrode and one end of said support pin, said body of metal being shaped so that a part of it is of smaller diameter than the inside diameter of said tube.

8. A method of manufacturing a crystal rectifier comprising placing an end of a Whisker electrode, a quantity of metal in solid form and an end of a support pin in a mold, and compressing said metal into a compact body having said Whisker end and said pin end embedded therein.

9. A method according to claim 8, in which said metal prior to said compressing step is in the form of a metallic powder consisting principally of lead.

10. A method according to claim 8, in which said metal prior to said compressing step is in the form of shot consisting principally of lead.

11. A method according to claim 8, in Which said Whisker electrode end is crimped prior to said compressing step.

12. A method according to claim 8, in which the placing of said Whisker electrode end in said mold comprises inserting it through an opening in one side of said mold, said side having an inner surface which comprises a series of grooves.

13. A method according to claim 12, in which said grooves are concentric annular grooves with said opening at their center.

References Cited in the file of this patent UNTED STATES PATENTS 2,269,861 Rennie Ian. 13, 1942 2,436,028 Pfann et al. Nov. 4, 1947 t 2,472,938 Brittain et al n June 14, 1949 2,572,801 Casellini Oct. 23, 1951 2,633,489 Kiuman Mar. 31, 1953

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