310 PHILIPS TECHNICAL REVIEW VOLUME 23

A PHOTORECTIFYING LAYER FOR A READING MATRIX

by J. G. van SANTEN *) and G. DIEMER *). 621.383.44:621.319.2

Photoconductive materials have already proved their usefulness in many fields; we maymention the use ~rphotocells for flame monitoring in oil-heating systems and for crackle-freevolume control in radio receivers, and the use of photocathodes for pick-up tubes in teleuisioticameras. The use of photocoruluctiue ca,dmium sulphide for a "reading matrix" involves aspecial problem, which has been elegantly solved by the authors.

Automatic read-out with the aid of photoresistors

With the continued advances in the field ofelectronic computers and their uses, it is frequentlydesirable to have some means by which an opticalimage can automatically be recognized or "read".For this purpose the image can be projected on to aplate which is prepared in such a way that thequantity of light incident on it can be determinedpoint by point by electrical means. In cases ofimportancc in practice, only the contour of thefigure needs to be recognized, so that it is sufficientto determine which points are illuminated andwhich are not.

To this end, use can be made of photoresistors,e.g. of cadmium sulphide, whose electrical resist-ance is dependent on the intensity of illumination.A number of these photoresistors are arranged inthe manner of the elements in a matrix, the imageto bc identified is proj ected on to this array, and

Fig. 1. How a matrix, with which an optical image can beautoma tically "read" (identified), "sees" the figure proj ectedupon it.

*) Philips Research Lahoratories, Eindhoven.

a voltage is applied successively to each of thcphotoresistors. Depending on whether a large orsmall current begins to flow, one can concludewhether the photoresistor is illuminated or not.In order for the "reading matrix" to "see" thecontour in sufficient detail, the dimensions of thepicture elements, i.e. of the photoresistors employed,must be as small as possible (fig. I).

Fig, 2, Principle of automatic read-out wit.h the aid of amatrix. On the two sides of a layer of photoconductive mate-rial, strips a, b, c, d, 1, 2, 3, 4- are applied. The strips are scannedby selectors KJ and K2• Whenever Kl is in one of the positionsa" b ... , K2 traverses the positions 1, 2, .... When the voltageis on strips Ct and 2 and the cross-point of the strips is illumi-nated, a current I begins to flow (solid arrow). Since, as a rule,other Cross points are illuminated too, stray currents flow alongall sorts of paths. One of these paths is indicated in the figureby a dashed line.

The reading matrix IS most simply constructedby applying conductive strips to both sides of aflat, thin layer of cadmium sulphide, as illustratedin fig. 2. A picture element consists of that part ofthe layer where two strips cross each other. (Aswill be explained at the end of this article, theconstruction of the actual array (fig. 3) is slightlydifferent.) The figure to be examinedis projected onto the matrix, and the strips are scanned with theaid of two selectors Kl and K2• Every time Kl isin one of the positions a, b, c, ... , K2 traverses thepositions 1, 2, 3, .... When the position of the

1961/62, No. 10 PHOTORECTIFIER FOR READING MATRIX 311

Fig. 4. When the voltage is on strips a and 2, all other resistors are connected in certaincombinations parallel to a2, as shown here for a matrix with two sets of five strips. Thecurrent I' through the parallel circuits can make such a large contribution to the totalcurrent Ia2 as to give the impression that the element a2 is illuminated, although it is not.

Fig. 3. A matrix for reading optical images.

selectors is such that the voltage appears on theelement a2 (between strip a and strip 2), for example,a current la2 starts to flow, which is higher the morelight falls on a2. Ifthis current is higher than a presetvalue, a2 is counted as being inside the contour. Thetotal information about the elements which is thusobtained can be compared in a computer with thatfor a series of contours stored in the computermemory. If the computer finds a correspondence,it reports this and the figure is recognized.

The matrix made in the manner described isnot readily usable, however, for the followingreason. When the voltage is across strips a and 2,a current flows not only through the photoresistora2 but also along all sorts of other paths, e.g. throughthe series-connected photoresistors b2, b3 and a3;sce fig. 2. Fig. 4 shows the equivalent circuit for amatrix containing two sets of five strips; all otherresistors are seen to bein certain combinationsparallel with a2. If thereare a number of illumi-nated photoresistors in theparallel circuits, it is pos-sible that stray currentswill flow which will makea contribution Ï' to the

total current 102, Thus,even though a2 is notilluminated, the total

current la2 may be greater

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than the preset value referred to, and as a resultthe element a2 is interpreted as being illuminated.

This difficulty can be overcome by using amaterial whose resistance depends not only on thelight intensity but also on the direction in whichthe voltage is applied. In such a case we speak of aphotorectifier. When the matrix is made using amaterialof this kind, we have what amounts toa rectifier in series with each resistor. The path fora stray current will always contain at least onerectifier in the reverse direction, so that the totalcontribution from the stray currents remains suf-ficiently low.

Although there are already a few types of photo-rectifiers on the market, they cannot readily beused in a matrix. This is due for one thing to theirsize, and for another to the fact that the matrixwould have to be built up from a large n umber ofindividual elements, which would be cumbersomework compared with the application of strips to alayer. It has now proved possible, on the basis ofthe photoconductive material cadmium sulphide 1),to produce a new type of photorectifier with whichthe matrix can easily be formed with strips in themanner described.

The new photorectifier 2)

The new photorectifier IS prepared by mrxmgedS powder with a small quantity of powdered

1) The properties and possible applications of CdS have beendealt with at length in: N. A. de Gier, W. van Goal andJ. G. van Santen, Photo-resistors made of compressed andsintered cadmium sulphide, Phil ips tech. Rev. 20, 277-287,1958/59. It may be recalled here that the best photoconduc-tive properties are obtained not with pure CdS but withpowdered CdS which contains suitable additives in care-fully controlled amounts. This mixture is compressed intopellets and sintered. Extremely sensitive photoresistorscan be made in this way, e.g. types ORP 30, ORP 90 andLDR.B8.73104.

2) This photoreetifier is described in: .J. G. van Santen and G.Diemer, Photorectifier based on a combination of a photo-conductor and an electret, Solid-state electronics 2,149-156,1961 (No. 2/3), which goes into more detail.

312 PHILlPS TECHNICAL REVIEW VOLUME 23

glass enamel 3), and firing the mixture at the melt-ing point of the enamel (about 600°C). As aresult of the adhesive forces, the enamel spreads inthe form of a thin layer (thickness a few Á) betweenthe grains (diameter a few fL), thus, as it were,cementing the grains together (fig. 5). Although,owing to the insulating nature of the enamel, theresistance upon illumination is considerably morethan that of the sintered CdS powder, the essentially

Fig. 5. Schematic cross-section of the new photorectifyingmaterial, consisting of a layer of CdS grains with glass enamelbetween them.

new feature is that we can now, by a special treat-ment, make from the enamel an electret, i.e. apermanently polarized dielectric, which gives therequired rectification. The treatment consists inagain firing the edS and glass enamel, this time atabout 200°C, and cooling it in a strong electricfield. The polarization produced in the enamel by theaction of the field is thus "frozen in".We shall now consider in more detail the expla-

nation of the rectifying action of the combinationdescribed, and discuss its application in a matrix.

Conduction mechanism of photoconductor-electretcombination

We shall first consider the case of two ordinaryconductors (e.g. of copper) separated by an insu-lator, and discuss the conduction mechanism interms of the band scheme as represented infig. 6a, and explained in the caption to this figure ").

When a voltage is applied between the two con-ductors, the band schemes are mutually displaced(fig. 6b). Although there is now a potential differenceacross the insulator, no perceptible current flows,provided the insulator is not too thin. There islittle chance that an electron will "tunnel" throughthe potential barrier klmn, i.e. pass from one con-

3) Glass enamel is a type of glass which has a very shortmelting range (at about 600°C), unlike normal kinds ofglass, whose melting range is very long.

*) Editor's note: This subject will he dealt with in detail in aforthcoming article in tills journal, dealing with the prin-ciples of photoconduction.

ductor to the other without having sufficient energyto surmount the potential barrier; the probabilityis in fact smaller the greater are the "cross-section"klmn and the height kl. Ifwe raise the voltage, moreand more electrons will tunnel through the barrier,so that the current increases. This does not changethe picture in fig.6b, however, since the potential dropin the conductors is negligible. We note that theapplication of the voltage causes polarization inthe insulator (displacement of the positive andnegative charges). The accompanying additionalsurface charges at the contact faces between insu-lator and conductors are compensated by the supplyand removal of the free negative charge carriersabundantly present in a conductor.

We shall now turn to the case of two grains ofphotoconductive CdS between which there is avery thin layer of insulating glass enamel (fig. 7a).When a voltage is applied to the CdS grains, thepotential barrier due to the presence of the enamelundergoes a change similar to that in fig. 6b. Becausethe enamel layer is very thin, the potential barrierhas a relatively small cross-section even before the

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Fig. 6. a) Energy-level diagram showing the allowed and for-bidden bands of two conductors c (of the same material)separated by an insulator i. The single hatching relates to bandsof allowed levels, the cross-hatching indicates that the levelsare occupied by electrons. 1 is a completely occupied band, 2a forbidden band, 3 is the partly occnpied band typical of aconductor. Some of the electrons which cannot move in theoccupied levels below 4 are raised, upon the application of avoltage, to the allowed empty levels above 4, where they cantake part in the conduction. 5 and 7 are respectively a fullyoccupied and an empty band of the insulator, 6 is the broadforbidden band typical of an insulator: at voltages lower thanthe breakdown voltage the electrons cannot pass from 5 to 7,and condnction is precluded.

Bands 1 and 5 are not drawn in b.b) The application of a voltage to the conductors causesrelative displacement of the band schemes. A surface chargeis produced in the conductors, and partly serves to cornpcnsa tethe insulator surface charge, which is due to the polarizationproduced in the insulator by the voltage. The cross-section ofthe potential barrier klmn and the height kl are so large thatthere is hardly any chance for an electron to "tunnel" throughthe barrier.

1961/62, No. 10 PHOTORECTIFIER FOR READING MATRIX

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Fig. 7. a) The conductors in fig. 6 are replaced here by two grains s of photoconductive CdS;the insulator i is a very thin layer of glass enamel. Characteristic of a semiconductor likeCdS is the forbidden band 2, which separates the valence band 1 from the conduction band3. Upon absorption of a photon of sufficient energy, the electrons from the valence bandcan he raised to the conduction band. The relatively few electrons contained in the conduc-tion band upon illumination are denoted here by a small number of free charge carriers andnot by cross-hatching. Upon the transition of electrons from 1 to 3, holes appear in thevalence band which behave like positive charges, but which in CdS can only move slowlythrough the crystal lattice.b) The already small cross-section of the potential barrier (due to the extreme thinness ofthe glass enamel layer) is narrowed still further when a voltage is applied. The electronsnow have a greater chance to tunnel through the potential harrier, and a perceptiblecurrent flows. The holes, which are attracted towards the barrier under the action of thefield, move slowly. They give rise to a space charge, and hence to a potential distrihutionwhich corresponds to a curvature of the energy levels.c) After some time, most of the holes have reached the surface. Consequently, the poten-tial drop is much less localized in the CdS grain than in (b). There is now a greater voltageacross the barrier, whose cross-section is therefore still smaller. More electrons now tunnelthrough the harrier, as a result of which the electric current increases until a saturationvalue is reached. Holes constantly disappear as a result of recombination and are replacedby new holes, so that the space charge is never entirely zero.

voltage is applied. The application of the voltagenarrows it still further from klm'n to klmn (fig. 7b).So many electrons can now tunnel through thebarrier that a noticeable current starts to flow.During the first ten seconds after switching on, thiscurrent is found to be time-dependent (fig. 8). Thereason is that the conduction mechanism in thiscase differs in one point from the previous one.edS contains relatively few charge carriers, somepositive and some negative. The positive chargecarriers are the "holes" that are formed in thevalence band when an electron jumps to the con-duction band upon the absorption of a photon. Theseholes are not only few in number, they are also re-stricted in their freedom of movement by beingrelatively strongly bound to certain lattice sites.In the case under consideration, the holes in thepositive grain also take part in the charge transport.Owing to their low mobility the holes take some timeto travel to the insulator, so that there arises in thepositive grain, in addition to the surface charge, a

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space charge near theinsulator. The electronsmove very fast and causeno perceptible spacecharge in the negativegrain. Owing to the pres-ence of the space charge,part of the potentialdrop is localized inside thepositive grain, and this isrepresentedbya curvaturein the energy levels (fig.7b). This state is not,however, stationary; moreand more holes reach thesurface, where they addto the surface charge. Asa result the space chargedecreases and the poten·tial drop in the grain isreduced, while the poten-tial difference across theinsulator mercases. Thebarrier becomes narrowerand the current higher.After about 10 seconds astationary state is reached.A small space chargeremains (fig. 7c) as a resultof recombination (elec-trons dropping back tothe valence band): holeskeep disappearing and

fresh holes are supplied to replace them.Summarizing then, a voltage supplied to two CdS

grains gives rise to a current which rises in a time of10 seconds to a stationary value. Its magnitudedepends on the thickness of the insulator. It shouldbe borne in mind, incidentally, that edS is itself aninsulator in the dark, when it has no electrons in the

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Fig. 8. The current I through the CdS with glass enamel, asa function of time t for constant voltage and illumination.The voltage is switched on at the moment t = O. The curveconsists of two portions: 1 immediate response, 2 slow rise toa saturation value.

314

conduction band. Thecurrents mentioned arethus dependent on theillumination.

We now consider athird case, again with thetwo CdS grains, which arenow however embeddedin an electret, i.e. a thinlayer of glass enamel hav-ing the above-mentionedpermanent polarization.Fig.9a shows the relevantband scheme. The perma-nent polarization in theinsulator corresponds to anelectric field, denoted bythe arrow in the figure.The form of the potential

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PHILIPS TECHNICAL REVIEW VOLUME 23

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Fig. 9. a) The glass enamel is made into an electret by a special treatment, i.e. part of thepolarization remains after the external field is removed.b) An external voltage of the opposite polarity to that of the field of the electret is opposedby the la tter field, so that the potential banier is not significantly reduced.c) An external voltage in the direction of the electret field makes the barrier very narrow.Immediately after application of the voltage, therefore, a much higher current flows than

barrier is now asyrnmet.ri- in case (b).

cal. An external voltage, applied in such a directionthat the field between the grains acts in conjunctionwith that of the electrct (fig. 9c), reduces the widthof the barrier much more than a voltage of the samemagnitudc but in the opposite direction (fig. 9b).In the first case a current starts to flow immediately,whilst in the second case the current immediatelyafter switching on is still almost zero.Thc above applies only immediately after the

application of the voltage. After some time, sufficientchargc carriers have arrivcd at the surfaces toneutralizc the surface charge of the electret. Thepotential barrier in fig. 9c is then not quite so sharp,and that in fig. 9b is somewhat sharper, so that thetwo figures are each other's mirror image. Thecurrent from that moment onwards is of equalmagnitude in both cases. Fig.lO shows the differencebetween the currents in the two directions as afunction of time.

Now it follows from fig. 10 that the resistance ofilluminated CdS with glass enamel is equal in both

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Fig. ID. Photocurrent I in CdS with polarized enamel as a func-tion of time i, in the forward direction (broken curve) and in thereverse direction (solid curve). ln the forward direction part ofthe response is immediate, but not in the reverse direction.

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directions upon the application of direct voltage(at least after 10 seconds), but upon the applicationof alternating voltage it is greater in one directionthan in the other. For alternating currents, the systemis always in the state prevailing immediately afterswitching on. The combination CdS-elcctret-CdSis thus seen to havc a rectifying action for alter-nating current. A current-voltage characteristic ofthis new rectifier can he seen in fig. 11.

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Application of the photorectifier material in a readingmatrix

To give an impression of the effect of illumination,fig. 12 shows the relation between current and volt-age for CdS with unpolarized enamel, subjected to

1961/62, No. 10 PHOTORECTIFIER FOR READING MATRIX

various intensities of illumination. For comparison,the figure includes a curve relating to sintered CdSpowder 1). It should be remembered in this connec-tion that, apart from the photoconductive proper-ties of the two materials, a part is also played byother factors such as the interelectrode spacing and

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Fig. 12. Current-voltage characteristics of the photoconductiveCdS layer with unpolarized enamel, illuminated with 0.2, 1.3,10 and 90 lux respectively, and of sintered CdS powder (ORP30) illuminated with 0.17 lux. The dashed lines apply to nor-mal resistors of the value indicated. The relation between Iand V for the CdSlayer with enamelis approximately given byI.o: V5, and its resistance is roughly a factor of lOG greater thanthat of sintered CdS.

the surface area exposed to the light 2). Even so,it is r~asonable to deduce from fig. 12 that thesintered CdS powder has a small and constantresistance, whereas the resistance of CdS with enamelis much higher for the same illumination, and ismoreover dependent on the voltage. This is bound upwith the fact that the resistance is primarilygoverned by the form of the potential barrier and notby the CdS.

Fig. 12 shows that the illumination can easilyreduce the resistance of CdS with glass enamel by afactor of 1000. The material can thus be used for areading matrix as described in the introduction.The obstacle to the discrimination between lightand dark due to stray currents (fig. 2) is now over-come by the rectifying effect: when the voltage isapplied in the forward direction to one partienlarphotoresistor, there will then, as explained above,he rectifiers in the reverse direction in all otherpossible paths.

Fig. 13 represents a cross-section of a matrix asactually made. The grooves between the upperstrips are -applied for the following reason. Thephotons cannot penetrate deep into the material,so that the photoconduction takes place primarilynear the surface. When the photoconductive ma-terial is grooved in the manner illustrated, thecurrent is able to How from an upper strip to alower one via the surface layer. Without thisgrooving, the resistance in the forward directionwould he too high.

Fig. 13. Cross-section of an actual matrix. 1 ceramic base.2, 3 electrode strips. The photoconducting layer 4. of CdSembedded in glass enamel is deeply grooved.

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One last comment on this resistance. For practicalapplications, the relatively high resistance is adrawback. Because of the voltage dependence men-tioned (fig. 12), however, we can make the resistancequite a lot smaller by choosing a higher voltage. Ifthe matrix is used in conjunction with equipmentcontaining electron tubes, this presents no difficulty.Since the advent of the transistor, however, therehas been a tendency to work with low voltages(below 4,0 V), and in that case the resistance of eachelement of the matrix would be too high. Thisdifficulty can he met by choosing the highestfeasible value of illumination and by designing thematrix so that every picture element always con-tains a number of photorectifiers, e.g. four, inparallel, thus reducing the resistance per element bya factor of 4. (The matrix in fig. 3 was designedin this way, the upper strips being divided intothree.) This conflicts of course with the requirementthat the elements should be as small as possible inorder to allow accurate read-out, so on this point aoompromise has to he accepted.

,Summary. The simplest way of making a matrix with which anoptical image can automatically be read (identified) is byapplying parallel conducting strips to both sides of a thinlayer of photoconductive cadmium sulphide, in such a waythat the strips on one side cross those on the other side at rightangles. This "cross-bar" construction entails stray currents,which hinder the discrimination between light and dark. Anew photorectifying material is described, which is based on acombination of a photoconductor (CdS) and an electret (i.e. apermanently polarized glass enamel). When the ordinary CdSis replaced by this photorectifying material, the stray currentsare largely suppressed. The rectifying effect in this material canbesatisfactorily explainedwith the aidof'the energy-bandmodel.

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