Electroluminescent device having a field-effect transistor addressing system

April 12, 1966 te ning chin 3,246,162

ELECTROLUMINESCENT DEVICE HAVING A FIELD-EFFECT

TRANSISTOR ADDRESSING SYSTEM

Filed March 24, 1965 3 Sheets-Sheet 1

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INVENTOR.

7f PP/hg CpP/pp

BY

April 12, 1966 _{TE N}ing _CHIN 3,246,162

ELECTROLUMINESCENT DEVICE HAVING A FIELD-EFFECT

Filed March 24, 1965 ™^SIS™ ^Ara™^SSI«^e SYSTEM

Sheets-Sheet 2

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INVENTOR.

7£ M/w Mm BY /rrrarMey

April 12, 1966 te ning chin 3,246,162

ELECTROLUMINESCENT DEVICE HAVING A FIELD-EFFECT

TRANSISTOR ADDRESSING SYSTEM

Filed March 24, 1965 3 Sheets-Sheet 3

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arroew/

3,246,162

Patented Apr. 12, 1966

United States Patent Office

3,246,162

ELECTROLUMINESCENT DEVICE HAVING A FIELD-EFFECT TRANSISTOR ADDRESSING SYSTEM

Te Ning Chin, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware

Filed Mar. 24,1965, Ser. No. 445,839

Claims. (CI. 250—211)

This invention relates to electroluminescent display devices. In particular, this invention relates to an improved electroluminescent device, including a novel addressing means or systems.

This application is a continuation of my copending application Serial No. 62,594, filed October 14, 1960, now abandoned, assigned to the same assignee.

In the prior art, there are many known types of electroluminescent devices. In general, an electroluminescent device comprises an electroluminescent phosphor positioned between two electrically conductive electrodes. When a potential of appropriate magnitude and frequency is applied between the electrodes, the electroluminescent phosphor is excited and produces light. In order to make a visible display of information from an electroluminescent lamp, or panel, some means must be utilized to electrically energize only certain selected areas of the electroluminescent phosphor so that the light emission is in the configuration of an image, a letter, a number or other desired shape.

In the prior art there are several known means for addressing or energizing the electroluminescent phosphor in the desired manner. Examples of these addressing means include photoconductive elements, ferro-electric elements and inductive type devices. The inductive type and ferro-electric type devices have been used in electrically addressed systems while the photoconductive type device has mainly been used in optically addressed systems.

It has been found that these prior systems or devices have certain limitations. Amongst these limitations is the problem of manufacturing an electroluminescent device including a sufficiently large number of addressing elements within a predetermined area so that the electroluminescent image will have proper picture definition. Another of the limitations on these prior devices is that, generally, the addressing systems can be energized either by electrical input or by optical input but not by both.

It is therefore an object of this invention to provide an improved electroluminescent device.

It is a further object of this invention to provide an improved electroluminescent display device having an addressing system which can be either optically or electrically energized and which is characterized in its relatively small volume for each elemental unit.

These and other objects are accomplished in accordance with this invention by providing an electroluminescent device each elemental unit of which includes an electroluminescent cell and a unipolar transistor. The unipolar transistor may be energized by either light or electrical signals. By energizing selected ones of the unipolar transistors, selected electroluminescent cells are turned on. By properly selecting the unipolar transistors to be energized, scanning lines or a complete image may be made to emit light.

The invention will be more clearly understood from the following specification when read in connection with the accompanying three sheets of drawings, wherein:

FIG. 1 is a partially schematic view of an elemental unit of an electroluminescent panel made in accordance with this invention;

FIG. 2 is a partially schematic sectional view of an embodiment of an electroluminescent panel made in accordance with this invention;

FIG. 3 is a partially schematic sectional view of another embodiment of this invention; _; .

FIG. 4 is an enlarged fragmentary sectional view of a further embodiment of an electroluminescent panel in accordance with this invention;

FIG. 5 is an enlarged fragmentary sectional view of still another embodiment of this invention;

FIG. 6 is a schematic circuit diaphragm of an embodiment of the invention in which electrical switching is employed.

FIG. 7 is an enlarged fragmentary sectional view of an electroluminescent panel and associated cells incorporating the invention shown schematically in FIG. 6; and

FIG. 8 is a plan view of the panel shown in FIG. 7.

Referring now to FIG. 1, there is shown a series circuit including an electroluminescent cell 10 and a unipolar transistor 12. The electroluminescent cell 10 comprises two spaced electrodes 14 and 16, having an electroluminescent phosphor 18 positioned therebetween. The electrodes 14 and 16 may take any desired shape, e.g. the cells may be in the shape of number or letter. At least one of the electrodes 14 or 16 is transparent. An example of materials which have been used as electrodes of electroluminescent cells is a transparent deposit of tin-oxide that is positioned on a transparent support plate (not shown in FIG. 1). The electroluminescent phosphor 18 may comprise any known electroluminescent phosphor material mixed with a suitable binding material. One example of an electroluminescent phosphor material is zinc sulfide while an example of a suitable binding material is a plastic such as an epoxy resin.

Connected electrically in series with the electroluminescent cell 10 and the unipolar transistor 12 is an alternating current, or pulsed direct current power supply 20. The unipolar transistor 12 comprises a layer of semiconducting material, such as silicon, having P and N regions similar to those shown and having a diffused junction illustrated by dotted line 22. Unipolar transistors are described in an article by W. Shockley, entitled, “A Unipolar ‘Field-Effect’ Transistor,” appearing in the Proc. IRE vol. 40, pp. 1365-1376 (1952). Generally, in a unipolar transistor, the conductivity of the layer of semi-conducting material from the source to the drain is modulated by a transverse electric field supplied by means of signal input or gate electrode 24. The unipolar transistor 12 is a relatively high resistance device, for example 20,000 ohms, when no gate signal is applied to the input or gating electrode 24, while it is a relatively low resistance device, for example 2,000 ohms, when a gate signal is applied to the input electrode 24. Since the amplifying action of the transistor involves currents carried predominantly by one kind of carrier, this device is known as a unipolar transistor. The change in conductance between the two N-type regions, i.e. the source to the drain, results from changing the number of carriers of this one type.

The elemental unit shown in FIG. 1 comprises a series connected arrangement of the electroluminescent cell 10, the unipolar transistor 12 and the power supply 20. When no gate signal is applied to the input or signal gate wire 24, the resistance of the transistor 12 is high as compared to that of the electroluminescent cell 10 so that substantially all of the voltage drop of the voltage from the power source 20 occurs in the unipolar transistor 12. Since most of the voltage drop occurs across the unipolar transistor 12, no light is produced by the electroluminescent cell 10.

When a positive (for this unipolar transistor) signal voltage is applied to the input terminal 24, the resistance

3,246,162 ³ of the unipolar transistor 12 is decreased substantially so that a large portion of the voltage drop of the voltage from the power source 20 now occurs across the electroluminescent cell 10. When the signal is applied, the phosphor is energized and light is produced or emitted by the phosphor 10.

The resistance of the unipolar transistor 12 may be varied by applying a signal voltage to the electrode 24 or, in the alternative, a constant potential may be applied to the electrode 24 and a light directed onto the unipolar transistor 12. The resistance of the unipolar transistor 12 will decrease in response to light somewhat similar to the well-known photoconductive action. Thus, in the elemental unit shown in FIG. 1, when a signal pulse is applied to the electrode 24, the electroluminescent element 10 will produce light. If the light from the electroluminescent element 10 lands on, or is fed back to, the unipolar transistor 12, the unipolar transistor 12 will remain in its low resistance condition after the signal pulse is removed and will thus, keep the electroluminescent cell energized. Thus, with light feedback between the electroluminescent cell 10 and the unipolar transistor 12, storage of the input information may be obtained.

Referring now to FIG. 2, there is shown a fragmentary section view of a single line of a light scanning device made in accordance with this invention. The device comprises a transparent support member 26 which may be made of a material such as glass. Since light is fed through the support member 26, the support member should be relatively thin, e.g. %e inch, in order to prevent loss of picture resolution. On one surface of the support member 26 there is provided a continuous electrical conductor 28. The conductor 28 is transparent to the wave-lengths of light which are emitted by the electroluminescent cells. The electroluminescent cells comprise a plurality of electroluminescent phosphor dots or areas 30 each covered by a different one of a plurality of conductors 32.

On the opposite surface of the transparent support plate 26 is provided a plurality of unipolar transistors 34. Each of the unipolar transistors 34 is in light receiving relationship with respect to a different one of the electroluminescent phosphor areas 30. The conductor 32 is electrically connected to the drain side of the unipolar transistor 34 that is positioned adjacent to, i.e. not in light feedback relation with, the particular electroluminescent cell. The source sides of all of the unipolar transistors 34 are connected to a common pulsating source of potential 36. The other side of the source 36 is connected to the transparent conductor 28.

During operation of the light scanning line shown in FIG. 2, when the first electroluminescent cell 30« is energized, light from this cell will fall on and decrease the resistance of the first unipolar transistor 34«. On the next pulse from the source 36, the electroluminescent cell 30Z&gt; will be energized, which will “make ready” the unipolar transistor 34/&gt; in that unit. Thus, each time a pulse is applied from the source 36, the spot of light will move down one elemental unit. Each of the elemental units in back of the elemental unit now emitting light will be continuously energized. It should be understood that a large plurality of lines of the type illustrated by the single line shown in FIG. 2, may be applied to one transparent support member and a complete moving spot panel device provided. The moving light spot may be stopped at any desired position, to indicate any particular information, by stopping the pulses from the source 36. Also, the moving spot may be started at any position along a line by shining a light on the selected unipolar transistor in any selected line. The direction of the light spot movement will always be from left to right, as illustrated in the drawings, since this is the only direction of light feedback.

The panel shown in FIG. 2 may be manufactured, for example, by spraying tin oxide, as the conductors 28, onto the support member 26 through a suitable mask. The electroluminescent phosphor areas 30 may also be deposited by spraying through a suitable mask. The array of unipolar transistor 34 may be made by the integrated circuit techniques through the known diffusion and lapping processes. The connections to the unipolar transistors, and between the unipolar transistor 34 and the electroluminescent cells may be made by printed circuit techniques. Using these techniques, elemental units of approximately 0.08 inch square have been made which produce a light output of approximately 0.13 lumen per square. These elemental electroluminescent cells can be controlled by unipolar transistors, for example, of properly doped silicon having a 90 ohm-cm. channel width of approximately 0.0016 cm. and a channel length of approximately 0.054 cm.

Referring now to FIG. 3, there is shown an .embodiment of this invention wherein a relatively thick support member 38 may be utilized. The support member 38 may be made of any transparent material, such as glass, and has a continuous line of transparent, electrically conductive· coating 40 on one surface thereof. On the transparent conductive coating 40 thre is provided a plurality of spaced apart electroluminescent phosphor areas or dots 42. Each of the phosphor areas 42 is covered by a different transparent electrically conductive coating 44. On each transparent conductive coating 44 there is provided a different light transparent layer of electrical insulator 46 which may be a material such as plastic. On each of the electrical insulators 46. there is provided a different unipolar transistor 48. A pulsating source of potenital 50 is connected 'between the transparent coating 40 and source side of each of the unipolar transistors 48. The drain side of each unipolar transistor 48 is connected to the next adjacent transparent conductor 44, i.e. to the conductor 44 of an adjacent electroluminescent cell. Thus, when a unipolar transistor 48 is in its low resistance state, the electroluminescent cell to its right, as shown in the drawings, produces light which decreases the resistance of its respective unipolar transistor 48.

The operation of the embodiment shown in FIG. 3 is substantially the same as that of the embodiment shown in FIG. 2. Also, a plurality of different coatings 40 can be provided on the same support member, arranged in rows, and a two dimensional display provided.

Referring now to FIG. 4, there is shown an embodiment of this invention wherein the unipolar transistors and the electroluminuescent cells are in a stacked array. The device shown in FIG. 4 comprises a support member 52 which is made of a transparent conducting glass having a relatively high resistivity, for example, 1,000 ohmcm. The transparent conducting glass may be any of the known transparent high resistance materials such as Corning x 857AJ commercially available from the Corning Glass Co. On one surface of the transparent conducting glass 52 is a transparent electrical conductor 54. The transparent electrical conductor 54 may be made of a material such as tin oxide. The other surface of the support member 52 includes a plurality of grooves 56 extending in opposite direction to form a plurality of spaced apart lands or hills. On each of the lands between the grooves 56 is deposited the gating side of a different unipolar transistor 58. On the drain side of each of the unipolar transistors 58 there is positioned a different electrically conducting plug 60. On each of the conducting plugs 60 there is provided a different electroluminescent cell 64. Each electroluminescent cell 64 comprises a transparent conductor 66, an electroluminescent phosphor 68 and a common conductor 70. On the source side of each unipolar transistor 58 there is provided a different electrical conductor 72. The electrical conductors 72 are all connected to a common conductor 74 which extends in a direction at right angles with respect to the conductors 70.

3,246 5

A source of potential is connected between the conductors 74 and the conductors 70 on each electroluminescent cell. By applying a potential to the unipolar transistor gate lead 54, selected unipolar transistors 58 in any line may be switched to their low resistance state 5 by means of a light spot on the selected unipolar transistor. This low resistance state permits the respective electroluminescent cell 64 to be energized and produce light. When an electroluminescent cell 64 is energized, the light therefrom will strike its respective unipolar transistor 58 which will “store” this bit of information. Thus, in the embodiment shown in FIG. 4, the electroluminescent panel will permit a display of information in two dimensions. This display may be originally energized by optical means such as a cathode ray tube or a flying spot 15 light source. It should be understood that the input “light” may be of wavelengths other than the visible spectrum such as infrared, X-ray or ultra violet.

It should be understood that the structure shown in FIG. 4 may be constructed in various forms. Thus, a 20 line of electroluminescent cells 64 and the conductors 70 may be provided on a support of apertured glass, e.g. Fotoform, with conducting plugs 60 and 72 in the glass. Also, a second support (not shown) may be provided with the electroluminescent cells on this support and this 25 structure positioned on top of the conductors 60.

Referring now to FIG. 5, there is shown an embodiment of this invention which may be utilized to produce an image of a scene. In this embodiment, an electroluminescent cell 76 is connected between a conductor 78, 30 that extends in one direction, and a unipolar transistor 80. The other side of the unipolar transistor 80 is connected to a conductor 82 that extends in the transverse direction. Thus, the conductors 78 and 82 extend at right angles with respect to each other. Any desired electrolumines- 35 cent element 76 may be turned on by selecting the proper conductors 78 and 82, and by energizing the respective unipolar transistor in the selected spot. The resistance of selected unipolar transistors may be decreased by means of a moving light spot. Once the unipolar tran- 40 sistor 80 is energized, the decrease in resistance is subtantial and the electroluminescent element 76 in that unit is turned on. As long as the current is applied to the selected conductors 78 and 82 the selected electroluminescent cell or cells 76 will remain on because of light feed- 45 back to the unipolar transistor 80 in each unit. Also, only the selected cell produces light because of the high resistance of the non-energized unipolar transistor.

Referring now to FIG. 6, there is shown an equivalent circuit diaphragm of an embodiment of this invention 50 which is energized electrically. Only one line is illustrated in FIG. 6, although a plurality of lines may be used as shown in FIG. 8, extending in a two-dimensional array. In this embodiment, a pair of resistors 84 and 86 are connected electrically in parallel with an electrolumi- 55 nescent cell 88. The parallel combination is connected to the drain side of a unipolar transistor 90. The source side of the unipolar transistor 90 is connected to a potential source 92 which is of sufficient magnitude to turn on the electroluminescent cell 88 when the unipolar transistor 90 60 is in its low impedance condition. The unipolar transistor 90 is switched to its low impedance condition by means of voltage pulses from the voltage generator 94.

Connected between resistors 84 and 86 is a gating lead 96 which is also connected to the gate electrode of the 65 next adjacent unipolar transistor 90. Thus, the first unipolar transistor 90 is turned on by a pulse from the source 94 which permits the first electroluminescent cell 88 to produce light. Part of the light from the electroluminescent cell 88 feeds back to the first unipolar tran- 70 sistor 90 and maintains the low impedance condition of the latter, as in the elemental unit shown in FIG. 1. The next pulse from source 94 will be fed through the parallel combination of the electroluminescent cell 88 and the resistors 84 and 86 to the gating lead 96 which

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will provide the low impedance condition to the next unipolar transistor 90. Thus, this action will switch on the next electroluminescent cell 88. The resistors 84 and 86 form a voltage divider which applies only a fraction of the voltage of the pulse source 94 to the next unipolar transistor 90.

Thus, in the scanning line shown in FIG. 6, any number of electroluminescent cells 88 may be turned on by continually pulsing the source 94. In this embodiment the information applied to the source 94 may be any desired information such as the output from a computer or other similar information. With a large plurality of such scanning lines, a two-dimensional display of information is provided.

Referring now to FIGS. 7 and 8, there is shown a twodimensional panel made in accordance with this invention and utilizing the circuit illustrated in FIG. 6. The device comprises a transparent support 98 having transparent conductive lines 100 deposited thereon. The conducting lines 100 are, during operation, connected to ground. On the transparent conducting lines 100 is a plurality of spaced apart electroluminescent phosphor areas 102. On the exposed side of each electroluminescent phosphor area 102 is a different electrical conductor 104. One side of each conductor 104 is electrically connected to, and butts against the drain end of a different unipolar transistor 106. The unipolar transistors 106 are supported on another glass support plate 99. The other side of the electroluminescent cell is an electrical contact with a resistance film 108, for example, nonactivated zinc sulfide. Substantially centrally located on each of the resistance films 108 is a different conductor 110, for example, of silver paste. Each of the conductors 110 extends from one of the resistance films 108 to the gating electrode of the next adjacent unipolar transistor 106. The other end of each of the resistance films 108 is connected back to a ground lead 109.

Connected to the other source side of each of the unipolar transistors 106 is a conductor 113 which is connected to source 114. Connected to the first unipolar transistor 106 in each row is a pulsating source 116. Thus, the embodiment shown in FIGS. 7 and 8 is a panel device having circuits which are the equivalent of that shown in FIG. 6.

When the pulsating source 116 is triggered, the first column of unipolar transistors 106 are actuated or switched to their low resistance state which permits the first column of electroluminescent cells 102 to produce light. The second pulse from the source 114 will automatically be applied to the second column of unipolar transistors 106 which will switch on the second column of electroluminescent cells 102. Then the first two columns emit light, etc.

All of the embodiments of this invention may be constructed by known evaporating, diffusion and lapping techniques as was explained in connection with FIG. 2.

In any of the embodiments of this invention, the electroluminescent cells may be elemental units of a composite panel. Also, in any of the embodiments of this invention, the electroluminescent cell or its area may be in the configuration of a number, letter, or other similar information for recording the output of computers and similar information reproduction.