US2631191A - Electric rectifier of the contact type - Google Patents

Description

'Mardi 1953' A. H. w. BECK ETAl 2,631,191

ELECTRIC RECTIFIER OF THE CONTACT-TYPE.

Filed Nov. 25. 1949 2 sl-IEE'rs-sEEETd I nventors ARNOLD H. W. BECK ALAN B. CUTTING ALA/I D. BRISBANE Attorney March 10, 1 53 A. H. w. BECK ETAL ELECTRIC RECTIFIER OF THE CONTACT TYPE 2 SHEETS-SHEET 2 Filed Nov. 23. 1949 CUTTING ALAN D. BRISBANE y Inventors w. BECK A tiorn e y Patented Mar. 10, 1953 Beck,;Aian'13utlenC1ifting,

an Douglas Brisbane, London, Er n-sin.

Tits 1g "rs 't'o Internationalfstandartl me-ewe "Del aware oration, New York, N. Y., a ccrp6fation bf Applicationl lovember 2 3, 1949, Serial -llo.- 1-29;0 34 In Great Britain November 26, 1948 itclaims. (orific 366) ,7 The prese t invention relates igfrectifiers oi t e con typ an is. e ri e t y c ncert w t u s en o ms i-e ndfic Phe mena asexhibited bythermioniccat'hode of the oxide-coated type. w v q r r r v I The theory of electron emission fromoxidecoa d t o s 0? 3562 1 8 s Q d thoughit is becoming more generally established t o s ved e s i n hene ae e lihos to be expectedif the cathode coating be considered as a semi-conductor of analagous type to the surface layers of somepfl thef morefamiliar'reciifi r.e e nt,- The e'enis, tu o em sion from oxide-coated cathodes have lead'them to the conclusion that such a cathode, when heated, should exhibit 59. 33 of the Contact phenomena observed with rectifiers, and, in particular, that it should beha've in analogous manner to crystal rectifiers such as are used in high frequency radio circuits. 1

We have found that pur predictions are, at least in part, confirmedby experiment. Accordingly the present invention provides an electric circuit element of the semhconductor type'c'omprising a conducting mernber having an elecs e. idercoe ne m an s h. as an internal heater element, for heating the said coating to render it thermionicallyemissive and one or more conducting members in contact with the said coating. e U I V r The invention willbe described with reference to the following drawings in which:

Fig. 1 shows "graphs illustrating the voltagecurrent characteristics of a typicalcrystalrectifier-and of a thermionic; contact rectifier according to the present invention. I

Fig. 2 shows an embodiment of a rectifier according to the present invention.

Fig. 3 shows arectifier characteristic obtained by heating the probe'element in a device according to the invention. in

Figs. 4 to 6 show an embodiment of a modified circuitelement according to the invention.

Fig. 7 shows a modification of the embodiment of Figs. 4 to 6.

Fig. 8 is an enlarged, diagrammatic, crosssectional view of certain of "the elements illustratedinFig.2.

7 If a piece of, say, germanium'crystal be held in a holder which provides a terminal for connection to an external circuit and a fine-probe or cats whisker providing a secondterminal .be pressed against-the surface of the crystal,

th it s e kn wn that he..cu r e i" through hexc cxste fl s .e non i ar ffindfionbf the voltage applied to these terminals.

dictate t'ype'i recent-rs Jena ecdh empnarme The curve 1 6r itbwriicn the left 'hand scale er ordinates-refers, fhowse characteristic curveror'a typiarcrystal. When the probe is polarised pesieven "with *"r'spect "to the hoIder,

the current is approx mately linear for'potential differences i'n eiicss" gr new 0;3 "volt,the current being 20 "ma." at 2 voltsf'forfpositive.probe voltages 'below 0.3 'volttheciirrent is "approximately proportional to "the'ftliree-lialvesjpower of the 10 applied P. D., while i r "negative probe voltages a small reverse current will'fldw. "For small negative voltages the 'rversefic'urrentmay be imperce'ptable compared to *the 'forward current at ture of about idccf'igfahw the "probe is maintained at an appreciably newer temperature, the

fier characteii'stic'as'"slidwn' in curve 2 of Fig. 1 to which the righthand scaleQof'ordinatesre- 'fe'rs. With tliem'rc'ibev Polarised "with"a small positive voltage with "respect to "the cathode sleeve, a normarthreemalves, power law relationship between bliiiiithndiiifiblid voltage is obtained, "similar to "thatfcrthecrystal, but showing a higher im edance. Fof'neg'ative probe polarities no "appreciable" reverse current is obtairied-e. g. -at'a"probevoltage ffof volts the reverse current isoigf meorder 'of one microaihp'ere as odmfiaidwiiith som three mi11ia'mperes for a good germanium crystal.

It: appears, -tlierefoie," ti'iat the usiof an inqir enyheated' de atedelthode fin'con- I th a p ob e1eetroae'eontacting' the coated surfac' rovidesf'an-alternativei'to known h assfiliconand germanium crystals? and"'has "a better reverse "'cu'r're'nt c aracteristic.

'An'embodiinntfof a'fbt'ifier acee'rdmg to the invention is muses-medddiaerammaticainim Fig.

i y"'heatdi'cathode" 3 is mg e ted' wnh the aid or pacingT'insu1ators1 V i jeriiinon ahresss within'tnenveleise' To'f tliedeiiicef'ih' cathode 3 comprises a nickel sleeve 8 housing a heater 9 and having a coating 10 of the mixed oxides mentioned above. One of the support rods H carrying the insulators 4 and has welded to it a fine wire probe l2 of tungsten or other suitably rigid material. The probe is conveniently provided with a loop l3 so that it may press against the oxide layer ID. We have found a tungsten wire of 0.13 mm. diameter to have the necessary rigidity while maintaining a constant pressure upon the oxide layer. In order that the emissive coating may more easily be activated during manufacture, an anode I4 is provided having an aperture for passage of the probe 12 therethrough, as shown in Fig. 8. If desired a control grid, indicated at l5, may also be positioned between the cathode and anode electrodes. It is pointed out that the grid I5 is indicated purely diagrammatically on the drawing and alternative electrode arrangements such as will be evident to those skilled in the art may be provided for utilising the thermionic emission of cathode i0. Thus separate emissive portions may readily be provided upon the cathode sleeve.

It is to be noted that the curve i of Fig. 2 was obtained when the cathode was heated to the normal emission temperature of 1000" K.; satisfactory results may be obtained however with lower cathode temperatures, when merely the slope of the forward characteristic is affected, as shown in Fig. 1 by the curve 2.

With the type of construction illustrated in Fig. 2, it is found that the maximum forward current that may be passed through the device is limited by the cathode loading, which should be restricted to the same order as obtainable for normal thermionic emission. Thus with a 0.13 mm. diameter probe, currents of the order of 25 ma. 1). C. may be passed. The probe diameter should not be decreased to such a value that the probe becomes appreciably heated, since, as will be explained later, new phenomena then become apparent The reverse voltage limit in a rectifier according to the invention depends upon the cathode temperature (and hence the reverse current at the probe contact) as well as upon the breakdown voltage of the oxide layer. With the small reverse currents of the order of l, A obtainable with embodiments of the invention, we have applied reverse voltages in excess of 150 volts without damage.

It will be evident to those skilled in the art that there are many possible uses for rectifiers according to the invention. One such use is as a microwave detector in place of a crystal, while ,another is as a detector in a radio receiver, the .oxide coated electrode also serving as a cathode for a normal thermionic valve, the circuit being ,similar to that utilising a thermionic diode-triode valve, the thermionic diode detector being replaced by the thermionic contact rectifier of the present invention.

Returning to the discussion of crystals, more recently it has been found that if a germanium crystal be operated in the reverse current region and a second probe (referred to as the emitter electrode), polarised slightly positively with respect to the crystal holder, be held in contact with the crystal close to the first probe (referred to as the collector electrode), a change in the voltage applied to the emitter electrode profoundly alters the conductivity of the crystal path in the collector electrode circuit. Thus if an alternating E. M. F. be injected into the emitter circuit, a corresponding alternating E. M. F. ap-

pears in the collector circuit, the A. C. power in the collector circuit being greater, under suitable conditions, than that injected in the emitter circuit. This type of crystal circuit element has been named a transistor and may be used in amplifier and oscillator circuits in place of a thermionic triode valve, the crystal and holder being regarded as the cathode, the positively polarised probe or emitter electrode being the grid and the negatively polarised probe or collector electrode the anode of the valve.

We have found that analogous phenomena to those described above in respect of the transistor effect may also be evoked from an oxide coated cathode.

In our experiments it was observed that if, in a rectifier according to the invention, a second probe were placed near the first negatively polarised probe, no significant change in the conductivity of the path associated with the first probe occurred when the positive bias on the second probe was variedi. e. the transistor effect was absent or negligible. This was understandable in view 01 the negligible reverse current obtained; the absence of reverse current, however, was unexpected. It appeared probable that the work function for electrons entering the semi-conductor surface from the metal probe was effective in establishing a barrier layer higher than the thermal energies of the electrons in the probe at the prevailing probe temperature; by raising the probe temperature the electrons should be given sufficient energy to surmount the barrier layer. This was found to be the case.

We observed that if, in place of the negatively polarised point probe, a tungsten wire loop, heated to about 1030 K, were substituted, then the voltage current characteristic of this probe circuit became similar to that of a germanium crystal, a reverse current characteristic of the type normally associated with a barrier layer being obtained. A typical. characteristic curve for the modified rectifier is shown in Fig. 3. By working in the region of the knee of the voltage characteristic, i. e. in the region of the point A, the conductivity of this circuit is influenced by the voltage injected in the second probe circuit in similar manner to that obtained in the crystal transistor and the possibility of using analogous amplifier and oscillator circuit is established.

According to a further aspect of the invention there is provided an electric circuit element comprising an oxide coated cathode, heater means for heating the oxide coating to render it thermionically emissive, two electrodes in contact with the said coating, one being adapted to be heated to the same order of temperature as the said coating to reduce the work function at the surface between it and the coating, and the other contacting said coating at such close distance from the region of contact of the said heated electrode, that potentials applied between the said other electrode and the coating may control the current passing between the heated electrode and the coating.

An embodiment of the invention according to this further aspect is shown in Figs. 4 to 6 in which the cathode and probe assembly I6 comprises a nickel cathode sleeve ll housing a heater it. A pair of ceramic sleeves l9 and 20 (Figs. 5 and 6) pass through apertures in the cathode sleeve, corresponding ends of the ceramic sleeves being flush with or slightly below the outer surface of the metal cathode sleeve. The other ends or the ceramic sleeves are supported by a bracket 2| welded to the cathode sleeve. A pair of fine tungsten wires 22 and 23 are threaded through respective holes in the ceramic sleeves to form a pair of loops parallel with the surface of the cathode sleeve. The cathode sleeve in the neighbourhood of the tungsten wire loops, which serve as probes, is covered with a standard mixed oxide coating 24 so that the loops are embedded in it. The tungsten wires we have used were of 0.13 mm. diameter spaced 0.13 mm. apart and about one half this distance away from the metal surface of the cathode sleeve. The cathode assembly is supported by means of mica insulators 25 and 26 mounted on support rods 27 and 28 sealed in the press 29 of a standard valve envelope 39. For ease of activation of the cathode, an anode 3| is also provided on the electrode structure. Since the wire loops are embedded in the oxide coating they attain the temperature of the cathode without separate heating.

It is found that having both probes presenting extended surface areas result in low circuit impedances. For this reason when a higher output impedance is required, it is preferred to modify the structure, as shown in Fig. '7, by making the emitter electrode in the form of a pointed probe 32. This probe may be made of thinner wire than the loop and is carburised at its tip to increase its contact resistance with the oxide layer.

In order still further to increase the input impedance of the device, using the modification of Fig. 7, we have found it advisable to keep the temperature of the emitter probe as low as possible; we therefore reduce the cathode heater current and pass current through the emitter loop 23 to compensate for the reduction of cathode temperature.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. An electric circuit element of the semi-conductor type comprising a dry contact rectifier structure comprising a cathode electrode having an electron emissive oxide coating, a heater for heating said coating to render it thermionically emissive and an anode electrode comprising a metallic wire having a relatively small area only thereof galvanically coupled to said layer.

2. An electric circuit element comprising a pair of insulating spacers mounted on support rods sealed in the glass envelope of the device; a metal cathode sleeve mounted between the said spacers and surrounding a cathode heating element; an oxide layer upon at least a part of the exterior surface of the cathode sleeve, the said layer being formed from a mixture of barium, strontium and calcium carbonates; and a resilient probe of high melting point wire mounted with an end pressing against the said layer.

3. An electron discharge device comprising an electric circuit element according to claim 2 and further comprising an anode electrode spaced from the said oxide layer and a control grid interposed between the cathode and the said anode electrode.

4. An electric circuit element according to claim 1 comprising a second electrode in intimate contact with the said coating, means for heating said second electrode to the same order of temperature as the said coating to reduce the work function at the surface between it and the coating said second electrode contacting the said coating at a close distance from the region of contact of the said anode electrode whereby the current passing between it and the coating may be controlled by the potential applied to the heated electrode.

5. An electric circuit element comprising an indirectly heated cathode coated with an electronemissive oxide layer, a collector electrode in contact with the said layer, means for heating said collector electrode at the region of its contact to the temperature of the cathode; and an emitter electrode contacting the said layer adjacent the said region so that the current to the said collector electrode may be controlled by the potential applied to the said emitter electrode.

6. An electric circuit element according to claim 5 in which the said control and collector electrodes are metal wires having portions embedded in the said coating.

'7. An electric circuit element according to claim 6 in which the said collector electrode comprises a metallic loop and said means for heating comprises terminals for applying current therethrough.

8. An electric circuit element according to claim '7 in which the said emitter electrode comprises a wire, one end of which is carburised embedded in the said layer substantially perpendicularly to the cathode surface.

9. An electric circuit element according to claim 5 comprising a pair of ceramic sleeves positioned in apertures in the cathode sleeve of the said cathode, a wire forming the said collector electrode threaded through the said sleeves so as to be held substantially parallel to the surface of the cathode sleeve; and a wire forming the said emitter electrode threaded through at least one of the said sleeves so as to be held in spaced relationship with respect to the said collector electrode, the oxide coating being applied to cover the cathode sleeve and the exposed portions of the control and collector electrode wires.

10. An electric circuit element according to claim 9 further comprising a glass envelope, a pair of insulating spacers secured to support rods sealed in the base of the said envelope, said cathode and electrode structure being mounted between said spacers and an anode electrode is mounted in spaced relationship with the cathode coating to receive electrons emitted therefrom.

ARNOLD HUGH WILLIAM BECK. ALAN BUTLER CUTTING. ALAN DOUGLAS BRISBANE.

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

UNITED STATES PATENTS 115,700 Great Britain May 21, 1918

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