US2741686A - Electric resistor - Google Patents

Description

April 10, 1956 H. Q. NORTH ETAL ELECTRIC RESISTOR Original Filed April 24, 1947 Fig.5.

C h m 1 mwz y m e F Vee A E F VULTS I80 I60 I40 I20 I170 60 60 ll 20 Their Attornqs.

ELECTRIC RESISTOR Harper 6:}. North, Los Angeles, Calif., andlvlargaret E.

Kershaw, Believue, Wash., assignors to' General Electric Company, a corporation of New York Original application April 24, 1947, Serial No, 743,492. Divided and this application January 20, 1951, Serial No. 206,964

7 Claims. (Cl. 201-63) The invention described and claimed herein, is a division of application Serial Number 743,492, originally filed on April 24, 1947, as a joint application of Harper Q. North and Margaret E. Kershaw, now Pat. No. 2,704,818. This present application has been divided from the above-mentioned previous application, in order to set forth the true joint invention of'Harper' Q. North and Margaret E. Kershaw since it has been found that certain portions of the subject matter described in the previous joint application, actually comprises the sole invention of Harper Q. North.

The invention described herein relates to asymmetrically conductive devices of thetypeembodying a semiconductor .rember in small area contact with'a conducting electrode, and more particularly to methods of making such devices of the type wherein the semiconductor member is welded or alloyedwith theconductor comprising the contacting electrode.

The use in radiocommunication and other fields of microwave energy, as for instance energy at a frequency of thousands of megacycles, 'has'made it difficult to design and build suitable vacuum tubes, with inter electrode capacitances small enough and electron transit time short enough to permit satisfactory operation of equipment employing such frequencies. Crystal diodes, which have been developed to replace vacuum tubes at ultra and super high frequency applications have proved useful, and it is accordingly an object of the present invention to provide a method of making improved crystal "diodes particularly adapted for use at such high-radio frequencies.

Another object is to provide a method of making a crystal diode for use at high voltages which will possess a high back resistance and low forward resistance.

A further object is to provide a method of making welded crystal diodes particularly adapted to the generation of harmonics at frequenciesof the order of'thousands of megacycles.

A still further object is to provide a method of welding together the electrodes ofan asymmetricallyconductive device, which method enables the production of a device having the improved characteristics set forth in the fore going objects.

The novel features which we believe to be characteristic of the invention are set forthwith particularity in the appended claims. The invention itself, however, both as to its organization and methodof operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig. l is an enlarged view of a crystal diode in cutaway side view; Fig. 2 is a diagram of a circuit useful in manufacturing a crystal diode according to the inventionfFig. 3 and Pig. 4 are diagrammatic representations of the electrical characteristics of crystal diodes of several types; and Fig. 5 is a greatly enlarged view of the tip of a'whis'ker from a diode according to the invention which has been torn away from the surface of the semiconductor-pellet.

Referring now to Fig; l of the drawings," the complete ite States Patent iCC diode comprises a shell 1 in the form of a tube, which may be of steel and about /4 inch in diameter and /3 inch in length, which houses a cylindrical silver plug 2 having a copper conductor 3 extending axially therethrough and a pellet 4 of the semiconductor germanium soldered at the end or" the copper conductor flush with the lower face of the silver plug. The germanium semiconductor pellet may be about .06 inch in diameter and 0.02 inch thick and preferably consists of germanium with a minimum of impurities, or germanium to which'has been added about .2 percent by weight of tin or about .2 or .5 atomic percent of antimony which may act'as donors as discussed hereinafter. The soldered connection is preferably made by plating the germanium pellet with rhodium, which solder wets much more readily than it does germanium. The pellet is then soldered to the ,pretinned face or". plug 2 across the end of conductor'3, tin being a suitable solder.

Also within the shell 1 is a steel rod 5 about .06 inch in diameter and .2 inch long having a copper conductor '6 connected at one end, such as by insertion and soldering into a round hole drilled in the end, the rod 5 carrying a pointed electrode or cat-whisker 7 of platinum-l0 percent ruthenium which is spot-welded to a small projection 8 upstanding from the upper end of the rod. The rod is firmly held in place withinthe shell by an insulating glass head 9 which sealsto the steel'shell 1 and to the steel rod 5, the bead being of a glass of which the thermal coefiicient of'expansion approximatelymatches that'of the rod and shell. A mica washer 10 fits firmly against the bead and between rods and shell 1.

in constructing a diode of the type shown in Fig. l, the cat-whisker 7 is first welded to rod projection 8, the glass ead is slipped over the rod, and then the rod and bead are slipped into the shell and externally clamped such as by a suitable jig in desired relative positions. The, parts are now heated for 7 minutes in an 800 C. air furnace to melt the head, the parts are removed from the furnace and the bead pressed flat into the configuration shown with the mica washer in place using a cold steel plunger or die against the mica washer It; and a suitable dieagainst the opposite face of the head. The assembly may now be annealed at 400 C. in air and then permitted to cool slovuy. The platinum-l0 percent ruthenium cat-whisker 7 is preferably of .0015 inch diameter and of overall length before bending of about .1 inch. It issharpened at the contact tip to provide an area of contact of the 'cloth and finally with fine magnesium oxide on cloth.

Such polishing yields a uniform clean surface relatively free from strain which apparently results in uniformly low loss diodes for wave-lengths as short as l centimeter.

The silver plug is now inserted in'the shell land is slowly pressed down until contact is just established between the tip of the cat-whisker 7 and the face of the germanium pellet. is-established may be indicated by deflection of a meter included in an electrical circuit comprising essentiallya source of potential and a current meter. Connections to the circuit are conveniently arranged throughconiductors 3 and 6. A suitable circuitis shown'in. Fig. 72

comprising potential source 11 meten12, current limiting The exact position at which contact variable resistance element 13, and switch 14 all in series across the diode.

After contact is just established between cat-whisker 7 and the pellet, in accordance with our invention, a pressure of about 150 milligrams between the tip and the pellet is obtained by pressing the silver plug .0005 inch further into the shell. This distance of .0005 inch represents the deflection of the cat-whisker bent as shown when the tip is subjected to about 150 milligrams pressure for platinum-l percent ruthenium wire of .0015 inch diameter.

Preferably, the silver plug is now soldered or glued about the periphery of its upper face to the inside of the shell, though a pressed fit of the plug in the shell will usually be sufficient to hold the parts without solder. Glue or solder may be desirable as an hermetic seal to prevent moisture from later damaging the pellet or catwhisker.

The next step in the construction is to pass a direct current through the diode, in the forward direction, that is using the cat-whisker as an anode and the pellet as a cathode, to weld or fuse the tip of the cat-whisker to we face of the pellet.-

One method by which such direct current welding may be accomplished, is by passing extremely high currents through the diode for a very short period of time, usually less than one second, by such means as rapidly discharging a capacitor through the diode. In accordance with our improved method of direct current welding, however, a direct current of predetermined magnitude derived from a substantially constant voltage source is passed through the cat-whisker to the semiconductor member for a more extended period of time. This latter method may be practiced by using the circuit of Fig. 2. Resistance 13 is made high when the switch 14 is closed and then slowly lowered to increase the current to the desired maximum. Asymmetrically conductive devices welded in accordance with this latter method usually exhibit improved high frequency and high voltage characteristics hereinafter described.

For a catwhisker sharpened to a point of about .0005 inch radius, and pressed against the pellet face by about 150 milligrams so as to give an area of contact of approximately 10- square inches, a satisfactory weld is obtained by passing for a few seconds, such as from 3 to 5 seconds, a current from whisker to pellet of between 100 and 400 milliamperes. A weld made by a current of 250 milliamperes has been found to have a tensile strength of 35,000 pounds per square inch, being able to support a tensile force of 500 milligrams. Currents less than 250 milliamperes are preferably employed when the area of contact is less than square inches, and when the diode is to be used for wave-lengths shorter than 10 centimeters.

In the manufacture of diodes for microwave applications which are hereinafter referred to as high frequencies to include those frequencies at which the interelectrode capacitance and noise voltages are of importance, it has been found desirable to use a small area of contact between whisker and pellet, of the order of 10- square inches or less, a small pressure between whisker and pellet of not more than 150 milligrams, low welding currents of not more than approximately 250 milliamperes, and a germanium pellet doped preferably with .2 atomic percent of antimony, in which the antimony serves as a donor to decrease the resistivity of the pellet. Units so constructed display very low forward resistance; a substantially logarithmic current-voltage characteristic over three decades of current, and except for the spreading resistance which is nonlinear with current, over five decades of current; a high back resistance; and a capacitance across the rectifying boundary layer, of approximately .2 micromicrofarad, the total interelectrode capacitance of the unit constructed as described being approximately 1 micromicrofarad.

Units comprising an antimony doped pellet and with a contact area of less than 10' square inches from the use of a pressure of 50 milligrams between the whisker and the surface of the semiconductor, and a welding current of 50 milliamperes, have proven useful as wavelengths less than 1 cm, having a capacitance of even less than .2 mmfd. across the barrier layer. Such units have been used successfully as harmonic generators of 4 millimeter wave energy.

Referring to Fig. 3, a family of curves of forward current, on a logarithmic vertical scale, against forward voltage, on a horizontal linear scale, is shown for a welded microwave diode with an antimony-doped germanium pellet, curve A; for a welded diode with a tindoped germanium pellet, curve B; and for comparison, for a non-welded silicon pellet diode of a type heretofore known, curve C. There is also shown by a dashed line D extending from the linear portion of curve A a plot of the current theoretically obtainable for an antimonydoped germanium welded diode if the spreading resistance could be compensated for. It will be noted that the practical unit, curve A, provides substantially linear (logrithmic) response from a current of .001 milliampere up to more than 1 milliampere, the characteristics of other types of crystal diodes being linear for much shorter ranges, curves B and C. The very low forward resistance, indicated by the steepness of the curve A, and the wide range of current linearity make the welded antimony-doped germanium diode particularly suited to microwave applications.

In the construction of diodes for use at lower frequencies and higher voltages, it has been found desirable to use germanium pellets without added donors, or pellets doped with .2 atomic percent tin, the latter being also suitable for some high frequency applications.

The cat-whisker may be sharpened to a point with a radius of between .00005 and .0001 inch, a pressure of 150 milligrams may be applied between whisker and pellet, and welding currents of about 250 to 400 milliamperes are suitable.

Diodes made in accordance with the above for high voltages may have a forward resistance of ohms at +1 volt and up to l megohm back resistance at 1 volt. The back resistance is maintained at a high level to as much as -100 volts or more in some units.

The characteristics of high voltage welded diodes are indicated in Fig. 4 by a curve E, representing a welded contact diode comprising a pellet of germanium without added donors, and by curve F for a diode comprising a tin-doped germanium pellet. While the tin-doped unit provides a lower forward resistance, passing a little more than 16 milliamperes at 1 volt, the back voltage characteristics of the unit with the un-doped pellet are more desirable, passing one milliampere at a voltage of about volts (curve B) whereas the tin doped unit passes this current at about (-)58 volts. T he back voltage characteristics of antimony-doped germanium diodes have been found to be such as to make such units generally unsuitable for high voltage operation. It will be apparent that the steep logarithmic current-voltage characteristic curve of the welded germanium diodes makes them useful as voltage regulators, as Well as modulators and harmonic generators.

The nature of the weld may be better understood from Fig. 5 which is reproduced from a photomicrograph of the tip of a whisker which had been broken away from the surface of the germanium pellet after welding. The small bead 15 on the top is separated from the body of the whisker 7 by a neck-like portion 16. Parts of the head 15 apparently comprises the weld consisting of the alloyed metals. The throat 16 is approximately .0001 inch in cross sectional diameter. Since a force of 35,000 pounds per square inch is necessary to break away the tip of thewhisker from the pellet, as shown, the weld made in accordance with the invention is strong mechanically.

In general it may be said that in producing diodes for higher frequencies, less welding current should be used since the area of contact should be smaller than for lower frequency uses. Welding the contact decreases the noise generated by the diode at high radio frequencies and currents of about 250 milliamperes have been found to give optimum performance characters for units intended for use at 3 centimeter wavelength, providing an interelectrode capacitance across the barrier layer of about 0.2 mmfd. Currents of 100 milliamperes have been found to be more in order for welding the contact of diodes for use at l centimeter wavelength. There is somewhat more inherent noise in the latter case but the capacitance across the rectifying contact will be somewhat smaller than 0.2 mmfd.

Although we have described above one method for accomplishing the objects of our invention many modifications may be made, and we intend, by the appended claims, to cover all such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a high frequency germanium diode, the method of making stable the characteristics of the contact between a sharply pointed electrode and the surface of a germanium pellet which comprises pressing the point of said electrode against said surface with a force of between 50 and 150 milligrams and passing for an appreciable time between 50 and 250 milliamperes of direct current in the forward direction from said electrode to said pellet through said point.

2. The method of making a germanium crystal diode for use at wave lengths less than centimeters which comprises placing the point of a pointed platinumruthenium wire whisker in contact with an area of the surface of a germanium member less than 10- square inches and passing a direct current of not more than 250 milliamperes through the contact between said point and said surface to weld said wire to said member.

3. The method of making a germanium crystal diode suitable for use at radio frequencies which comprises pressing the point of a pointed platinum alloy whisker against the surface of a member comprising germanium with a force to provide a contact area therebetween of approximately 10* to 10* square inches, and passing approximately 50 to 250 milliamperes of direct current in the forward direction between said whisker and said member for a few seconds.

4. The method of making a high frequency germanium rectifier which comprises the steps of polishing to optical smoothness the surface of a germanium pellet, placing in contact with said surface the point of a pointed metallic electrode, forcing said point against said surface with a force to provide an area of contact of approximately .10- to 10- square inches, and passing a direct current of from 500,000 to 5,000,000 arnperes per square inch of Contact in the forward direction between said electrode and said pellet through said point whereby said electrode is welded to said pellet.

5. The method of making a germanium crystal diode suitable for use at Wave lengths less than 1 centimeter, which method comprises pressing the point of a sharply pointed platinum alloy wire against the surface of a member comprising germanium with antimony as a donor with a force to provide a contact area therebetween of not more than 10 square inches, and passing a direct current of the order of milliamperes in the forward direction between said wire and said member for a few seconds.

6. The method of making a germanium crystal diode suitable for use at radio frequencies, which method comprises placing the point of a pointed metallic electrode in contact with the surface of a germanium pellet containing approximately 0.2 to 0.5 atomic percent antimony, forcing said point against said surface to provide an area of contact less than 10 square inches, and passinga direct current of not more than 250 milliamperes in the forward direction between said whisker and said member for a few seconds.

7. An ultra-high frequency germanium diode suitable for use at wave lengths less than 1 centimeter, comprising a germanium member, and a pointed platinum alloy wire welded at the point to said germanium member, said pointed wire having a minimum cross-sectional welded area of not more than 10- square inches.

OTHER REFERENCES North, 1.: Applied Physics, vol. 17, November 1946, pages 912-923.

Claims (1)

1. IN A HIGH FREQUENCY GERMANIUM DIODE, THE METHOD OF MAKING STABLE THE CHARACTERISTICS OF THE CONTACT BETWEEN A SHARPLY POINTED ELECTRODE AND THE SURFACE OF A GERMANIUM PELLET WHICH COMPRISES PRESSING THE POINT OF SAID ELECTRODE AGAINST SAID SURFACE WITH A FORCE OF BETWEEN 50 TO 150 MILLIGRAMS AND PASSING FOR AN APPRECIABLE TIME BETWEEN 50 AND 250 MILLIAMPERES OF DIRECT CURRENT IN THE FORWARD DIRECTION FROM SAID ELECTRODE TO SAID PELLET THROUGH SAID POINT.

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