PROCEEDINGS OF THE I.R.E.

we obtain

S/N= 2V\Tg(aT)where

(exp x)-1 [ x exp xg(x) - exp L (.\/x_ (exp x)-1

hence

efficiency= (S/N)/optimumn(S/N) = 2g(aT) = 2g(T/RC)... 00

.90 ~

.80

.70

350

.20PULSE WIDTH OF EXPOEiTTIALPULSE IS DEFILED TO BE TI _ 1TINE CONSTANT OF TOE 'XPON.'EITTIAL CURVE ¢

90 19lo so30 as at

Fig. 2--Noise suppressing efficiency of double RC circuit in the de-tection of a pulse superimposed on fluctuation noise having a flatfrequency distribution.

The functional relationship between efficiency andT/RC is shown in Fig. 2. Maximum efficiency is 93 percent and occurs for pulse width=2.98 RC (2.98 is anapproximation to double the root of

sinh z= V2).

z

Case 2: Exponentially Decaying Pulse

The problem is precisely that of Case 1, except thatnow

e(t) = exp (-bt)S-1(t).

Using Corollary: Optimum S/N=1-/V2b. Computingthe actual S/N, the output is given by

1 f jooE(t) = . a2 exp (pt)dp/[(p + b)(p + a)2];

2i7rj Joo

evaluating the integral, maximizing with respect to timeand dividing by N we obtain

whereS/N = 24t(a/b)/V/b

(y) = Y1 x e y- ]y-1 y

and where x is the solution of

y = (exp x - 1)/x.

Hence, efficiency= (S/N)/optimum (S/N)= 2v/2 Ai (a/b).The relationship is shown in Fig. 2. Maximum efficiencyin this case is 81 per cent and occurs when pulse width/RC= 2.18 (taking pulse width to be time constant of theexponential decay).

The Measurement of Contact Difference in Potentialon Certain Oxide-Coated Cathode Diodes *

I. E. LEVYt

Summary-Contact difference in potential for a conventional-type oxide-coated cathode diode is defined in terms of voltage re-quired to establish zero field between the anode and cathode. Con-tact difference in potential has been found to be relatively unaffectedby a wide fange of impurities added to the coating. Tests have shownthat the Ect measurement is a sensitive technique for observingchanges which take place on an initially clean anode.

fl HE PURPOSE of this study was to determine thejeffect on contact difference in potential of (1) vari-

ous impurities added to the conventional radio-tube cathode coating and (2) the anode deposit from thecathode due to tube processing techniques.

I. CONVENTIONAL DIODES

The diode structure chosen for the first part of thiswork is a diode of conventional receiving tube design,utilizing a cylindrical cathode and anode.

* Decimal classification: R331 XR262.9. Original manuscript re-ceived by the Institute, October 21, 1949; revised manuscript re-ceived February 27, 1950.

This work was sponsored by the Office of Naval Research as partof contract N7 ONR-389.

t Raytheon Manufacturing Co., Newton, Mass.

Definition of Contact Difference in Potential

Contact difference in potential for a diode of thiskind will be defined as the applied voltage required toestablish an average of zero electric field in the spacehalfway between the cathode and anode in the absenceof space charge. The contact difference in potential asdefined in this manner depends on the difference be-tween the average work function of the anode and theaverage work function of the cathode. Changes in workfunction resulting from impurity migrations and partsand processing changes would enable one to determinequalitatively to what extent the surface of the anodehas been affected.

Measutrement of Contact Difference in PotentialThe measurement of contact difference in potential

described below is based upon a so-called "low-field"temperature-limited emission test. The conditions forthis low-field test are Ef = 1.75 v, E =4. Since our testdiode has a 6.3-v heater, this test gives a means ofobserving temperature-limited direct-current emission

774 July

Levy: Contact Potential Difference for Oxide- Coated Cathode Diodes

and still keeps the anode voltage below the point wherepoisoning might occur due to breakdown of oxide andchlorides on the anode surface.

Fig. 1 shows Ep-Ip plots of three tubes, widely dif-ferent for low-field emission, but having essentially thesame contact difference in potential. Curve abcd inFig. 1, for example, shows that the true zero field islocated somewhere between points b and c. True zerofield for the other curves will be located also somewherein the vicinity of the bend of the E,-Ip curve. On theseEp-Ip curves, the electron emission is noted under thelow-field test, namely 4 v applied potential and cathode

Fig. 1-Curves of 3 diodes with widely varying low-field emission,but essentially identical contact difference in potential.

temperature of 643°K (Ef 1.75). Then the applied volt-age necessary to give 1 per cent of this low-field emis-sion current is determined. At this voltage which willbe defined as E.01, this great reduction in current hasresulted from the fact that only high-energy electronsare able to cross the space in such a retarding field. Inthe life history of this particular diode, it is anticipatedthat changes in E.o1 brought about by the condensa-tion of impurities on the anode, and other changes inthe anode and cathode surface will be a reasonably ac-curate measure of the changes of true contact differencein potential as previously defined. In the absence of the

effects due to surface inhomogeneity (anode and cath-ode) and space charge, zero field current should be ob-served at E.o1 plus 0.25 v. This added term represents

Fig. 2-Er-Ip curves for special diodes with sliding sleeve and con-trol after various time intervals on life.

Note: A lots are special diodes with sliding sleeve. B lots arecontrol, sleeve omitted. All during activation processing of A lots,the sleeve protected the anode. Tubes were then put on life so thatthe sleeve no longer protected the anode from any deposit.

the change in voltage required at a temperature of 643°Kto change the emission by a factor of 100.1 An inspec-tion of all curves in Fig. 1 shows that the observedchange in current for this range of voltage is far less thanthat predicted. This discrepancy is undoubtedly theresult of the cathode and anode inhomogeneity andspace charge enumerated above. A reasonable guess asto the influence of these factors brings us to the conclu-sion that the true contact difference in potential is, onthe average, approximately equal to E.o1+0.7; this willbe referred to as Et. Thus, E,,=E.ol+.7.

Correlation of Contact Difference in Potential with CathodeCoating ImpuritiesSeven lots of diodes, consisting of various impurities

added to the conventional double carbonate coating,"Radio Mixture #3," were checked for E,,. The resultssummarized in Table I show no significant difference inE,, between these lots both initially and after life.

I Computed byI

= e VIKT.100

wheree=electron charge 1.6X10-'9 coulombsK=Boltzman constant 1.38X10-1 joules per degreeT=absolute temperature 643°K.

J950 775

PROCEEDINGS OF THE I.R.E.

TABLE ICONTACT DIFFERENCE IN POTENTIAL ON LOTS WITH VARIOUS

IMPURITIES ADDED TO THE CATHODE COATING

Average Ea Average EctLIot. Volts Zero Volts After 500

Hours Hours Life

#735 Melt E (high Cu & Mg)#737-Melt I (,high per cent S & Si)#783-.35 per cent Ta#785-Control for .35 per cent Ta#747-Metallic chromium .15 per cent#745-Metallic chromium .05 per centd749-Control for metallic chromium

1.11.11.21.21.21.21.1

procedures. The control-lot tubes were identical to thespecial diodes, except that the sleeve was omitted. Onthe special tubes the sleeve protected the ano(le fromany deposit all during exhaust processing. After activa-tion was completed, the special diodes were inverted sothat the sliding sleeve no longer protecte(d the anode.In this position average E,=I, curves for the sixtubes from each lot were drawn at Ef= 1.75 for 0, 24,and 100 hours on life. The life-test conditions wereE = 6.3, Eb= 100 v dc, and RL=1,000 ohms.

IP was about 80 ma per square c-entimeter.

II. DIODES MADE WITH SLIDING SLEEVE OVERTHE CATHODE

In order to determine the effect of plate deposit dueto tube processing on contact difference in potential, adiode was developed with a sliding sleeve between thecathode and anode in such a way that the sleeve couldprotect the anode from any deposit, while the tube wasin the conventional upright position. When the tubewas inverted, the sliding sleeve would fall away andleave the anode exposed to any deposit from the cath-ode.

Constructional Features of Special Diode

L4

cAr PC

I(a) (b)

Fig. 3-(a) Constructional features of special diode. (b) Top view.

In Fig. 3, two half-diode plates A and B with cylin-drical barrels K and G are mounted on regular stem-plate supports H and I. The cathode C is only coatedon the lower end as indicated by the shaded portion. Thesliding sleeve D in the normal upright position of thetube rests on the bottom mica E and completely protectsthe inside of the barrel G from any cathode deposit. Thesleeve is large enough so that it can never touch thecathode. During processing and activation of the cath-ode any deposit from the cathode will land on the sleeveD and leave the plate barrel G clean. Then when thetube is to be tested, or a current is to be drawn toanode B the entire structure is inverted. The slidingsleeve D will then slide into plate barrel K and restagainst the top mica F.

Experiments Conducted on Special Diode

Six special diodes made with sliding sleeves and sixof a control lot were made with standard processing

Test ResultsThe life data for the special diode with sliding sleeve

and control are plotted in Fig. 2.It is seen that tubes made with the sliding sleeve have

considerably higher contact difference in potential (E,t)than the control at zero hours life. The reason for this isprobably due to the fact that diodes made with the slid-ing sleeve have a much higher anode work function be-cause of the clean anode. As life progressed, the contactdifference in potential (E,t) of the diodes made with thesliding sleeve decreased, until at the end of 100 hours,tubes made with the sliding sleeve had lower E,t thanthe control. This can be explained by the fact that onlife, the anode was not protected from the cathode andpicks up considerable Ba, Sr, and other impuritieswhich lowered the work function and decreased Et.When E,t was computed for these curves by the tech-nique already discussed, it was seen that, whereas thecontrol had an insignificant E,t change on life, the tubeswith the sliding sleeve showed a decrease in E,t of about87 per cent after 100 hours.

ConclusionsIt is evident that this Ea test is a sensitive technique

for observing changes which take place on the surfaceof an initially clean anode. Since with the conventionaldiode structure, it seems impossible to prevent the dep-osition of Ba, Sr, and other impurities onto the anodeduring normal processing, the low readings of E,t whichwere obtained on the diodes which served as a controlfor the sliding sleeve tubes (see Fig. 2) must be largelydue to this deposition and subsequent lowering of theanode work function. While it is true that the E6a meas-urements do not offer much information concerning thecathode, they are valuable in supplying informationconcerning the condition of the anode.

ACKNOWLEDGMENTSThe author is greatly indebted to W. B. Nottingham

of Massachusetts Institute of Technology for his helpin formulating the E.j testing technique; to J. Cardellof Raytheon Manufacturing Company who was re-sponsible for supplying many of the test lots; and toR. L. McCormack of Raytheon for his over-all super-vision and guidance.

I I

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