US2608611A

US2608611A - Selenium rectifier including tellurium and method of making it

Info

Publication number: US2608611A
Application number: US110684A
Authority: US
Inventors: John N Shive
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1949-08-17
Filing date: 1949-08-17
Publication date: 1952-08-26
Anticipated expiration: 1969-08-26
Also published as: BE495728A; FR1018535A; NL80468C; DE919360C; NL154007B; GB668059A

Description

J. N. SHIVE Aug. 26, 1952 SELENIUM RECTIFIER, INCLUDING TELLURIUM AND METHOD OF MAKING IT Filed Aug. 17, 1949 2 Sl-IEETS--SHEET 1 IN 5 N TOR By J. M SH VE A TTORNE V Aug. 26, 1952 J. N. SHIVE 2,608,611

SELENIUM RECTIFIER, INCLUDING TELLURIUM AND METHOD OF MAKING IT Filed Aug. 17, 1949 2 SHEETS-SHEET 2 A T TOR/V5 V Patented Aug. 26, 1952 SELENIUM RECTIFIER INCLUDING TEL- V V .LURIUM AND METHOD OF MAKING IT John N. Shive, Plainfield, N. J., assignor to Bell- .Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 17, 1949, Serial No. 110,684

This invention relates to selenium rectifiers and to methods of making them. a

Devices of the typeto which this invention pertains comprise, in general, a metallic backin member or electrode, for example of nickel or aluminum,a layer of selenium upon one face ofithebacking member, and a front electrode, for example of a metal. such as tin, or an alloy, such as oftin, bismuthand cadmium, upon the outer face of the selenium layer. The electrical characteristics of such devices, in the forward or conducting direction, are dependent upon three. components namely the .main rectifying barrier betweenthe selenium and the front electrode, the selenium layer and the barrier between the seleniumjand the backing member or electroda Desirably, of' course, the resistance in the forward direction shouldbe low and the operating characteristics should be stable. 7 The barrier between the. selenium and the backing member, commonly refe'rred to as the secondary barrier, has a direction of rectification opposite to that of the main barrier, that is the one between the selenium and the front electrode. Thus, it adds tothe forward resistance. Further, it has been found that in presently known devices whereas the resistance of the selenium layer-remains substantially constant and the resistance of;the main barrier increases but slightly with time, the resistance of the secondary barrier increases substantially. This results in degradation of the performance of such devices, particularly in a substantial increase in the forward resistance.

Prior attempts to reduce the secondary barrier resistance have involved the addition of a halogen, notably iodine, to the selenium or, particularly in the case of aluminum back electrodes, the provision of a layer of bismuth between the selenium and the aluminum. Although these effect some improvement in decreasing the secondary barrier resistance, this resistance still is subject to i'ncreasewith age. Further, other undesirable properties maybe introduced thereby. For example, the halogen addition increases the sensitivity of the rectifien to atmospheric moisiure and renders. the rectifier morerdifficult to orm. v .ri a

One general object of this invention is to improve the performance characteristics of selenium rectifiers. e i i .lVIore specifically, objects of this invention are to :reduce' the secondary barrier resistance in selenium rectifiers, to minimize variations in this .resistance with time and to realize these de- 11 Claims. (01. its-3'66) siderata concomitantly without the introduction of undesirable properties in-the rectifiers.

Another object of this invention is to enable and f cili e. he economic mass production of selenium rectifiers having stable and low secondary; barrier resistance. i

In accordancejw" h one feature of this invention, ina selenium rectifier ofthe general construction described hereinabove, a layer or film of tellurium is provided between the selenium layer and the backing member. Such aninterposed layer or film, it has been found, results not only in avery low secondary barrier 'resistance but also such a resistance which. is stable with time. Furthermore, rectifiers including such layers are readilyformed and substantially unaffected by atmospheric conditions.

A more specific feature of this invention resides in the-method of manufacturinga selenium rectifier which comprises roughing and cleaning an element of a material. suitable for aback electrode, vapor depositing a thin'layer of tellurium on the roughened surface and a layer of selenium on the tellu'rium, in a vacuum, then heat treating the unit to convert the selenium from the amorphous to the crystalline state, mounting a front contact onthe selenium surface, and electrically forming a rectifying barrier between the front contact and the selenium.

The foregoing and other objects and features of the invention will be understood more=fully and clearly from the following detailed description of an illustrative embodiment thereof taken in connectionfwith the appended drawings in which: M Fig. lis a longitudinal section of a vaporizing apparatus suitable, for'practicing a method in accordance with this, invention;

Fig. 2 is a cross-sectionalview of the apparatus of Fig. 1 taken along the line 2-1;

Fig. ,3 is a partial cross-section of t'heapparatus of Fig. 1 taken alongtheline 3-3 Fig. 4 is a plot of voltage versus current on a logarithmic scale showing a typical forward characteristic of a nickel-backed selenium unit, andits development during aging, curves A and Aa respectively, the subscript (a) being employed throughout this specificationflto identify the aged characteristics, including the three component resistances which makeup the overall characteristic: the resistance of the ohmic body curve B, the resistanceof the main'barrier, curves C and Ca, and that of the secondary barrier, curves D and De; i h

'Fig, 5 isa logarithmic plot of voltage versus current to the same scale as the curves of Figs. 4 and showing the forward characteristics and secondary barrier effects of a unit of this invenmeasured characteristic is such as to reducethe forward current at one volt, (the usual operating voltage) from 60 milliamps to milliamps for the particular unit described.

"The aluminum-selenium units also exhibit a large secondary barrier .loss, curves I and J of Fig. 5, which seriously'irnpairs' the operation of the device.

In order to more closely approach an ideal rectifier it is desirable to reduce the secondary barrier to an intimate, zero resistance, time stable contact between the selenium and the backing Y plate.

tion comprising a selenium film with atellurium interlayer on a backing of nickel before forming, curves Kr and Li respectively, the subscript f being employed hereafter to identify the characteristics of a unit which has not yet been formed, after forming, curves K and L, and after aging, curves Ka andls; I.

Fig. 7 shows a logarithmic plot of' the overall characteristics and thatof the secondary aluminum-tellurium-selenium barrier of a unit constructed in accordance with this invention and comprising a selenium film having an aluminum backing anda tellurium interlayer before forming curves Mr and Nr, after rormingcurvesM and N, and after aging-Curves M andN and Fig; '8 isa perspective View of a selenium rectifier embo-dying'this invention withpor-tions thereof broken away. 5

-It is'not'ed that the curves whichare shown are for an average of -arn-rmber of three-quarter inc'hdiarneter washer units of the general construction shownp in Fig; :8 havi-n-g a non-iodized seleniumlayer. is f Referring now'to theydrawings, -t-he device of Fig. 8"is illustrativepf those towvhichthis invention pertains; It comprises a washer ll having a roughened surface 2 0f some material such as nickel which provides the back electrodeand support for the remainder of the elements, a front electrode 3 andfa selenium layer 3. i It may include also anintermediate layer 5 between the selenium and the back electrode. The front electrode 3 is ofsome material such as an alloy of tin, bismuth and cadmium applied to theunit in intimate contactwith the underlying layer of selenium 4 to'form at the interface between the two the main "rectifying barrier. 'The secondary rectifying barrier in prior art devices is generally formed between the selenium and the back contact 2. Such barrier is "substantially eliminated inaccordance with this invention by making the layer 5 of tellurium as hereinafter described.

As evidenced by the-curves ofFi'g. 4, the secondary barrier contributes a considerable portion of the resistance in the characteristic. of the prior art selenium-nickel units. This secondary barrier, curves D and Da, is an undesirable part of the rectifier because it adds unwanted resistance to the unit. Furthermora'the continued development of this secondary barrier'during its use is largely responsible for the progressive increase in the forward resistance of the rectifier known as aging. Curves D and De. show how this secondary barrier develops during aging and how its v pment is reflected in an increasedforward resistance "for the rectifier characteristic curve Aa. During agingthe ohmic body resistance remains substantially unchanged, curve B and the main barrier, curves C and Ca, increases only slightly in resistance. The greatest change is in the reverse characteristic of the secondary barrier and the resulting increase in resistance of the Several methods have been employed in an attempt Ito-'reducethe undesirable secondary barrier effects; ,One' is to employ small quantities of iodine added to the selenium in the fabrication of the nickel-backed unit, and while this decreases, somewhat, both the magnitude of the secondary nickel-selenium barrier and the rapidity with which it changeswith time, these units are sensitive tov atmospheridmois'ture,and are difficult to form, Where aluminum "backings are employed, the surface of'the plate is covered with an evaporated,,coating of bisrn uth before applying the selenium. layer in" order to'reduce the secondary barrier-effects. Curve J of Fig. :5

shows the characteristic with".the bismuth inner layer. However, this still results in'a,rather large 'magnitude secondary barrier W'hichhha's a tendency to age inaboutthemannenof the nickel- .selenium barrier.

In accordance with. this.linventiomjextremely desirable characteristics; includingfa greatly reduced s'eccndary barrier resistance which .is substantially independentiof the, support plate material, has a stable'value unusually low aging, and eliminates the .needlof addingihalogen to the selenium, thus permitting. the'un'its to form more rapi-dlyfandlh'iakir'ig the-Ihn'its more stable istics of a numbler oi -typical nickel-backed units with the -tellurium i erlayerf. Curves are shown for the unit before .forming, after forming, and after aging. The body resistanoe and main barrier components-of the overall resistance have not been included in thesecurvessincethe .body resistance changes very little-andthe-mainbarrier ages in about the same wayas does that for the standard unit showrr in age," Th e secondary barrier characteristic is 'oflower-resistance than for units ho t hayingfihejtel lurium layer, and [it does not changesubstantiallyjm resistance during forming-and aging. r.Ihese; two features result in a composite characteristic z'ivhi'oh' is 'zoflow resistance in the one volt region and which does not exhibit substantial increase inzresist-anceicluring aging. The advantages :realized by the use of the tellurium protective layer-'on-the nickel backing are evident- When:thesevnharacteristics are compared with thosefzof Fig. '4, wherein the current at one volt after aging is about one-fifth of that for the telluriumffilm units? 1' The. effect refusing; the tellu-rium sprotective layer technique with aluminum backinglisshoxvn in Fig. 7. The curves for the-average forward characteristics. of. 1; typical 'iun'rts: before forming,

a after forming .and after :aging show again the low resistance secondary barriercharacteristic and teristic in the oneyolt operating region. They show also, as was the-case for'units' made with the tellurium coated nickel backings, that the characteristics in the operating region change onlvslightly with time.

Rectifiers may be constructed in accordance with this invention on a backing member of some suitable material such as nickel, nickel coated iron, aluminum, or magnesium which may be in any. convenient form, for example a disc f or washer. The preliminary step in the treatment of the backing member is to roughen the surface by sandblasting or an equivalent mechanical working so that the selenium layer is provided witha surface to which it will adhere during the subsequent processing and use. The residue from the roughening process is then removed by a preliminary washing after which the backing member is thoroughly cleaned of all dirt and grease. This may be done by dipping the washer in hydrochloric acid for about ten seconds, then washing inwater, dipping in nitric acid for about ten seconds and again washing in water, rinsing in acetone and then rinsing in ether and drying. The backing members may then be degassed and cleaned of residual grease and dirt, if any, by heating them in a vacuum of about millimeters of mercury at 600 or 700 C. for about thirty minutes. All of the foregoing preliminary steps may be performed on a large number of washers at the same time.

The cleaned units, washers H in Figs. 1 and 2 of the drawing, are then mounted on a holder, for example, the rack 12 having the spring fingers l3 depending therefrom and engaging the central apertures of the washers I I. The holder l2 and the mounted washers H are then placed within a vacuum chamber 14. The chamber I4 is then sealed by placing the cover I5 thereon with the gasket I6 intermediate the cover and chamber wall. Atmospheric pressure on the exterior of the cover forces it against the end of the chamber wall and compresses the gasket 16 to form an airtight joint when the chamber I4 is evacuated through the exhaust tube 18.

A pair of troughs l9 and 20 are attached to the cover l5 and extend along the bottom of the chamber l4 below the mounted discs. These troughs are supported from the cover at one end by metal straps 2| and 22 which are connected to the terminal bolts 23 and 24 secured in insulating

bushings

25 and 26 on the cover. The opposite ends of the troughs are supported by

metal arms

21 and 29 depending from the off-set end of the bar 29 which is connected to the terminal 30 extending through the cover [5. Additional support is provided for the troughs by the insulating bead 31 secured to the bar 29 and functioning as a leg therefor resting on the bottom wall of the chamber.

The troughs l9 and 20 are filled with the proper amount of tellurium and selenium, respectively, prior to mounting cover I5. After the chamber [4 has been evacuated to a pressure of about 10- millimeters of mercury, a coating of tellurium having a thickness of about 2 milligrams per square inch is vapor deposited on the washers by passing a sufficient current between the terminals 23 and 3|] to cause the trough I9 to heat up to the temperature at which tellurium vaporizes. When a sufficient layer of tellurium has been deposited upon the washers the current is removed from the trough l9 and applied to trough 29 to vaporize the selenium therein. The backing washers are sufiiciently spaced from the trough 6 heatersto-remaincool enough to allow condensationof the vaporized metals thereon. The: rate of deposition of the metals should be sufliciently rapid to avoid heating of the-washers and consequently melting of the condensed metals. Since the electrode surface must be relatively rough to retain the selenium layer, the tellurium layer must coat the backing member without smoothing the roughenedsurface, therefore, this layer is only about .0005 "millimeter. in thickness while the layer of deposited selenium must be thick enough to be continuous and present a smooth surface. A selenium layer approximately two mils in thickness has been found satisfactory.

'The seleniumas deposited is in the amorphous state which is of too high resistance to be practical for use in rectifiers. Therefore, it must be heat treated to convert it to the crystalline form. This treatment comprises a preliminary heating at about 1-10 0. for a period of about four hours in air and thena high temperature treatment also in air for about twenty minutes at a temperature in the range of about 214 C. to 217 C. A front'contact is now provided for each unit by one of well known methods; for example, the contact may be a spray deposited layer of soft metal suchas tin or an alloysuch as one of tin, bismuth and cadmium. Forming of the body is next accomplished by passing current through it in the high resistance direction, from the backing member through the selenium to the soft metal front contact, under progressively increasing voltages up to about 24 volts.

The operating characteristics, both initially and over the entire life of a unit constructed in accordance with this invention are considerably better than those of the prior art units as pointed out in the above discussion of the curves of Figs. 4, 5, 6, and '7. The tellurium interlayer enables a wide choice of backing materials to be employed for selenium rectifiers while maintaining a low stable secondary barrier resistance. Further the process of manufacturing these units is amenable to mass production and requires only a minimum of handling of the individual units.

What is claimed is:

1. An asymmetric conducting device comprising a backing electrode, a thin layer of tellurium thereon, a selenium layer on said tellurium layer, and a front electrode contacting said selenium layer.

2. An asymmetric conducting device comprising a backing electrode having a nickel surface, a thin layer of tellurium thereon, a selenium layer on said tellurium layerand a front electrode contacting the surface of said selenium.

3. An asymmetric conducting device compris ing a backing electrode having an aluminum surface, a thin layer of tellurium thereon, a selenium layer on said tellurium layer and a front electrode contacting the surface of said selenium.

4. An asymmetric conducting device comprising a backing electrode having a magnesium surface, a thin layer of tellurium thereon, a selenium layer on said tellurium layer and a front electrode contacting the surface of said selenium.

5. An asymmetric conducting device comprising a backing electrode, a layer of tellurium having a thickness of the order of .005 millimeter thereon, a selenium layer on said tellurium layer, and a front electrode contacting the surface of said selenium.

6. An asymmetric conducting device comprising a backing electrode, a vapor deposited layer of tellurium on a portion of said electrode, a seleni- '7 um layer on said tellurium layer and a front electrode contacting the surface of said selenium.

7. An asymmetric .conducting device comprising a backing electrode, a layer of tellurium thereon,-;a vapor deposited layer of selenium, and a front electrode contacting the surface of said selenium; p I

' 8. An asymmetric, conducting device-comprising a backing electrode; a vapor deposited film of tellurium thereon; a vapor deposited selenium layer on said tellurium layer, and a front electrode contacting the surface of said selenium.

9. The method of; making aselenium rectifier having a low, stable secondary rectifying barrier resistance that comprises coating a surface of an electrode element of suitable material with a thin layer of tellurium, coating the tellurium layer with a layer of selenium, heat treating the unit to convert the selenium to the crystalline state,v applying a front'electrode to the selenium surface and electrically forming the'unit.

10. The method of making a selenium rectifier unit which comprises rougheni-ng a surface of an electrode element-of suitable metal, cleaning the element, applying a thin coating of tellurium to the roughened surface of said element, coating the tellurium layer with a layer of selenium of sufiicient thickness to have .a relatively smooth external surface, heat treating the unit to conyert the selenium to a crystalline state, applying a front electrode to the selenium surface and electrically forming the unit.

11. The method of making selenium rectifier units that comprisesroughening a surface of each of a plurality of electrode elements of suitable material, vapor depositing tellurium on the roughened surfaces of said cleaned elements in a vacuum, vapor depositing selenium on the tellurium coating surfaces of said elements in a vacuum, the amount of selenium deposited being sufficient to produce a layer that is thick enough to have a relatively smooth external surface, heat treating the units to convert the selenium to the crystalline state, applying front electrodes to the selenium surface and electrically forming the units.

JOHN N. SHIVE.

REFERENCES CITED The following references are. of record in the file of this patent:

, UNITED STATES PATENTS Name Date 'Brunke Feb. 6, 1940

Claims

1. AN ASYMMETRIC CONDUCTING DEVICED COMPRISING A BACKING ELECTRODE, A THIN LAYER OF TELLURIUM THEREON, A SELENIUM LAYER ON SAID TELLURIUM LAYER, AND A FRONT ELECTRODE CONTACTING SAID SELENIUM LAYER.