GB1594553A

GB1594553A - Electroluminescent display element

Info

Publication number: GB1594553A
Application number: GB1811/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-01-20
Filing date: 1978-01-17
Publication date: 1981-07-30
Also published as: IT1092278B; FR2378325B1; FR2378325A1; DE2800797A1; CA1126373A; IT7819324A0; JPS5392681A

Description

(54) ELECTROLUMINESCENT DISPLAY ELEMENT (71) We, N. V. PHILIPS' GLOEIL AMPENFABRIEKEN, a. limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a display element comprising an electroluminescent diode and a reflector which reflect the light emitted by the electroluminescent diode to form an oblong light image.

For displaying numerical or alphanumerical data, devices are used representing digits or letters in the form of assemblies of luminous strip-like elements which are simple and rectilinear. When the dimensions of the displayed characters are to be restricted and several of them are to be disposed quite closely to each other, mosaics of electroluminescent elements are used wherein semiconductor junction diodes formed in a monocrystalline material are excited selectively, for example, by the application of a voltage.

Said diodes are thus realised on crystals of small dimensions constituting light sources which are substantially punctiform. In order to transform each punctiform source into a luminous bar, various known devices are indifferently used. The basic principle in all cases is to add to the punctiform source a reflector so that to an observer the displayed character has a substantially rectangular shape. The diodes are usually situated at the bottom of cavities recessed in a block of insulating and light-reflecting material and are disposed in accordance with a determined configuration corresponding to the formation of a geometrical figure in the form of a parallelepiped constituted by seven light bars and forming a display device.

The said devices and their methods of manufacture present a certain number of disadvantages. In fact, the first difficulties encountered relate to the dimensions of the display device and its parallelepiped shape.

On the one hand, the number of cavities recessed in the block of insulating material limits any reduction in the dimensions of the said device but, on the other hand, it is not possible to greatly enlarge the said dimensions in order to preserve a good definition of each of the light bars.

Moreover, experience has taught that certain of the seven light bars which the display device comprises may never be used, their use depending in fact on the type of character to be displayed.

Besides, the disposition of the light bars in the block of insulating material and the parallelepiped shape of the display device do not permit variations of the displayed character using the same display element and so it is not possible to obtain a linear display which may be rectilinear or curvilinear.

In this latter case in which it is desired to obtain a linear display, the use of a method based on the presence of known reflecting cavities to obtain the said display proves to be without interest. In fact, thenecessity of maintaining a space between the said cavities to ensure the strength of the assembly does not permit the light bars to be juxtaposed and, during the illumination of the said bars, dark zones consequently remain between them.

In order to avoid these drawbacks, a proposed solution consists in manufacturing unitary display elements in the form of bars and obtained from punctiform sources and notably from electroluminescent diodes.

In particular United States patent No.

3,694,902 relates to such a display element.

In this patent the electroluminescent diode serving as a punctiform source is fixed at the end of a first conductive lamination constituting a first electrode and forming part of a stratified support of which a second lamination, which is also conductive and is insulated from the first, constitutes the second electrode of the said diode.

Moreover, the said diode is embedded in a block of a transparent curable material, notably a resin, the shape and the constitution of which permits reflection of the light emitted by the diode towards the substantially rectangular front face of the display element.

One of the drawbacks presented by said element is a lack of strength and mechanical rigidity. In fact, a block of plastic material having large mass is moulded around a stratified support having a small section. In this case, the useful part, that is to say the reflecting part, is heavy compared with the support which may lead to bending of the said support and, if the movement is repeated, to a deterioration of the moulding around the said support so that the moulding becomes loose.

Moreover, from a technological point of view the assembly of the crystal on the support and the moulding of the resin around the said crystal presents difficulties.

In fact it has been found that the centring of the crystal on the support and the centring of the block of resin around the crystal and around the support should be carried out very carefully and with precision. These requirements are even more difficult to meet the above operations are automated.

According to -the present invention there is provided display element comprising a semi-conductor crystal in which two regions of opposite conductivity type form an electrominescent diode, and a reflector which reflects the light emitted by the electroluminescent diode to form an oblong light image, the reflector having two halves which form the outer walls of said element and which define a cavity at the bottom of which the semiconductor crystal, is fixed, each half of the reflector comprising at least one metal connection pad electrically connected to one of the regions of the diode, and the cavity being filled with a transparent cured material.

This display element presents several advantages. In the first place it may be noted that its manufacture is simple and its cost-price is low. In addition, in accordance with the shape of the reflector, it is possible to obtain different characters by disposing the elements in various configurations.

The two halves of the reflector may be identical and disposed symmetrically with respect to the axis which is perpendicular to the surface of the crystal and which passes through the centre of the emissive zone of said crystal so that the reflected light forms a rectangular light image.

The display element is suitable for creating linear and rectilinear displays.

Alternatively the two halves of the reflector may be symmetrical with respect to a plane dividing the cavity which they define into two parts of equal volume, said plane being perpendicular-to the surface of the semiconductor crystal fixed on the bottom of the cavity and passing through the centre of the emissive zone of the said crystal so that the reflected light forms a trapezoidal light image.

This display element is suitable for circular or sectorial display devices.

In both of the above cases the resulting display has only a minimum occulation which is subtstantially negligible. In fact, the front or active surface of each display element, that is to say the outer surface of the cavity as viewed by the observer, corresponds to the surface defined by the two reflector halves diminished only by the thickness of the metal when the reflector halves are made from a metal sheet, or by the thickness of the edge of the wall of the reflector halves when they are made from insulating material. Moreover, the rectilinear definition of the edges of the cavity, no matter whether the said cavity has a rectangular or a trapezoidal section, permits juxtaposing the display elements.

Moreover, it is possible to create display devices comprising elements emitting in different wavelength ranges.

Also it is possible with the aid of display elements in accordance with the invention to create devices comprising only the necessary number of electroluminescent elements which is more rational and more economical.

The semiconductor crystal may be fixed with one of its faces on each of the two halves of the reflector at the bottom of the cavity. In order to facilitate the centring of the crystal at the bottom of the cavity, each of the reflector halves may comprise a recess to accommodate the crystal at the bottom of the cavity, the crystal being fixed to at least one wall of each recess.

Advantageously, the wall of the recess to which the crystal is fixed comprises a boss.

In accordance with the invention, the shape of the reflector halves may be different. In fact, the said reflector halves may be designed to form cavities having plane inner faces and/or cavities having parabolic inner faces. In the majority of cases, the inner side wall having. a large surface, termed principal wall, and the inner side wall having a small surface, termed secondary wall, of each of the half-shells are plane, but it is also possible that the halfshells have at least one plane principal wall and at least one parabolic secondary wall.

In accordance with the applications in view, the half-shells may be manufactured from metal strips which are comparatively malleable and electrically conductive, notably of copper or based on copper, and which are shaped by bending and stamping.

The metal strip is preferably covered with an electrically conductive layer having a high reflecting capacity.

However, the reflector halves may also be formed from an insulating and rigid synthetic material which may be transparent, covered at least partly with a metal layer having a high reflective capacity and electrically conductive for contacting the electroluminescent diode to an external control assembly.

The reflector halves of insulating and rigid synthetic material are notably used in those applications where it is necessary to leave only a minimum space between juxtaposed display elements. In fact, the reflector halves of synthetic material are usually moulded and it is hence easier to make a bevelled edge on the upper extremity of their lateral walls defining the perimeter of the cavity and the display element. Moreover, the symethetic material used may be chosen as a function of the nature of the material filling the cavity, on the one hand in such manner as to obtain a solid assembly and on the other hand to create, with the said material filling the cavity, a guide for the radiation emitted by the electroluminescent diode.

In the case in which the reflector halves are made of synthetic material, their surface constituting the inner walls of the cavity may be covered with a reflecting metal layer ensuring at the same time a contact with the two semiconductor regions of the electroluminescent diode and a better reflection of the light emitted by same. In this case the metal layer is advantageously based on silver.

In other applications, reflector halves which are entirely of metal are to be preferred and they present the advantage of constituting a reflector without the need for an additional reflecting layer.

The reflector halves may be obtained in a mechanized and automated manner from metal strips in the form of, what is known in the field of semiconductor products as, a "comb" The use of metal reflector halves permits manufacturing simultaneously a direct assembly of reflector halves and at least the soldering of the crystal which results in reducing the cost-price.

In this case, in fact, fixing the said crystal at the bottom of the cavity and making electrical connections to the two regions of the diode may be carried out in one operation.

It is in fact sufficient to directly adhere the two major faces of the crystal representing the two regions of the diode by means of a conductive adhesive to the metal faces of the two halves of the reflector defining the cavity: the radiation reflected by the reflector is then essentially that emitted by the semiconductor junction through the major surfaces or the lateral faces of the crystal.

The electroluminescent diode of the display element may be formed from - a crystal which is transparent at the wavelength of the light emitted by the diode, for example gallium phosphide, Ga P.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a half of a reflector of a display element, Figure 2 is a diagrammatic sectional view of the same half of a reflector taken on the line 11-11, Figure 3 shows a first embodiment of a display element comprising two identical reflector halves, one of the reflector halves being partially broken away for clarity of the drawing, Figure 4 shows another half of a reflector of a display element, Figure 5 shows a second embodiment of a display element comprising two identical reflector halves of Figure 4, Figure 6 shows a further half of a reflector of a display element, and Fig. 7 shows a third embodiment of a display element comprising two symmetical reflector halves of the type shown in Fig. 6.

It is to be noted that the figures are not drawn to scale for clarity.

It is also to be noted that certain portions of each display element are shown by means of broken lines when their presence is necessary to understand the drawing.

Taking into account the importance of semiconductor material in the optoelectronic industry,-- .the choice of the description relates to a crystal transparent at the wavelength of the emitted light, and in particular to a Ga P crystal, which explains in the figures the disposition of the said crystal with respect to the reflector.

As shown in Figs. 1 and 2, a half 1 of a reflector is manufactured from an opaque synthetic material, for example epoxy resin, starting from a parallelpiped block in which a cavity 2 has been made having a recess 3 to accommodate the electroluminescent diode. As shown in Figs. 1 and 2, the cavity 2 comprises, for example, two secondary walls 4 and 5 of a parabolic shape and one major wall 6 which is bevelled. Said plane wall 6 as well as the vertical wall 7 of the cavity 3 extending the said wall 6 are recovered by a layer of light-reflecting and readily electrically conducting metal. On the plane wall 6 a portion 8a of said metal layer has for its object to improve the efficacity of the reflector. Said portion 8a changes into the portion 8b on the wall 7 more especially destined to ensure an electric connection with one of the regions of the electroluminescent diode and for that purpose it terminates at the exterior of the parallelepiped block as the portion 8c forming the contact.

Fig. 3 shows a display element in accordance with the invention composed of two identical reflector halves as described above. By juxtaposing the two halves la and ib and adhering them with an appropriate resin the cavities 2a and 2b constitute the reflecting cavity 9 and the recess 3a, 3b which can accommodate a semiconductor crystal 10 having an electroluminescent diode. In the case, for example, in which the crystal is of Ga P, the two semiconductor regions of the diodes are disposed respectively adjacent the metal surfaces of the recess 3a, 3b and are adhered directly to -them using a conductive adhesive.

The cavity 9 is filled with transparent synthetic material 11, generally an epoxy resin chosen as a function of its index of refraction and of its adaptation with the material constituting the reflector halves la and lb.

In this first embodiment of the invention the reflector halves are identical and are disposed opposite to each other.

As shown in Fig. 4, the reflector half 21 is formed from a metal strip stamped in an appropriate manner to create the design shown. Said reflector half 21 comprises essentially two orthogonal walls 22 and 23.

The wall 22 is generally plane and the wall 23 has a configuration which it parabolic.

Said wall 23 is extended by a plane portion 24 comprising a boss 25 at the edge of which the semiconductor crystal is applied.

Usually the half of the reflector also comprises a plane wall 26 parallel to the wall 22, said wall being shaped at one of its parts so as to create a connection lug 27.

The metal chosen to form the reflector half must be a good electrical conductor and must have high reflecting power, so it is preferable that it is made from a copper strip covered afterwards by a thin silver layer.

Fig. 5 shows a display element composed of two identical reflector halves in accordance with the above description (Fig.

4) and disposed symmetrically with respect to a vertical axis passing through the centre of the electroluminescent diode formed in the semiconductor crystal 28. The juxtaposition of the two reflector halves defines a reflective cavity 29 which is then filled with a transparent material The crystal 28 is adhered to the bosses 25 of the two reflector halves by its two semiconductor regions. In this case it is adhered by means of a conductive resin known in the art but, consequently, during the assembly it should be ensured that the two reflector halves are not joined so that short-circuits are avoided.

Since the reflector halves are not joined, their filling with a transparent material should be carried out in a mould.

In another case, the crystal 28 may be adhered by means of an insulating adhesive, the contact being then provided, and in these conditions there is no inconvenience as regards the reflector halves being joined.

In Fig. 6, the reflector 31 is formed from a metal strip stamped in an appropriate manner to create a shape as shown.

Said reflector half 31 comprises two walls 32 and 33 which are, for example, plane, making an angle of approximately 90" with a third wall 34 which is also plane. Said walls have a trapezoidal shape and are chosen such that, by associating two reflector halves 31a and 31b of identical dimensions but symmetical, display element is obtained the section of the cavity 35 of which is itself trapezoidal (Fig. 7). Each of the reflector halves 31 comprises a plane portion 36 orthogonal to the wall 34 and provided with a boss 37 at the edge of which the semiconductor crystal 38 is to be adhered.

This embodiment is suitable for circular or sectorial display devices by juxtaposing the necessary number of elements having a trapezoidal configuration.

WHAT WE CLAIM IS:- 1. A display element comprising a semiconductor crystal in which two regions of opposite conductivity type form an electroluminescent diode, and a reflector which reflects the light emitted by said electroluminescent diode to form an oblong light image, the reflector having two halves which form the outer walls of said element and which define a cavity at the bottom of which the semiconductor crystal is fixed, each half of the reflector comprising at least one metal connection pad electrically connected to one of the regions of the diode, and the cavity being filled with a transparent cured material.

2. A display element as claimed in Claim 1, in which the two halves of the reflector are identical and are disposed symmetrically with respect to the axis which is perpendicular to the surface of the crystal and which passes through the centre of the emissive zone of said crystal so that the reflected light forms a rectangular light image.

3. A display element as claimed in Claim 1, in which the two halves of the reflector are symmetrical with respect to a plane dividing the cavity which they define into two portion of equal volume, said plane

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Claims

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more especially destined to ensure an electric connection with one of the regions of the electroluminescent diode and for that purpose it terminates at the exterior of the parallelepiped block as the portion 8c forming the contact.

Fig. 3 shows a display element in accordance with the invention composed of two identical reflector halves as described above. By juxtaposing the two halves la and ib and adhering them with an appropriate resin the cavities 2a and 2b constitute the reflecting cavity 9 and the recess 3a, 3b which can accommodate a semiconductor crystal 10 having an electroluminescent diode. In the case, for example, in which the crystal is of Ga P, the two semiconductor regions of the diodes are disposed respectively adjacent the metal surfaces of the recess 3a, 3b and are adhered directly to -them using a conductive adhesive.

The cavity 9 is filled with transparent synthetic material 11, generally an epoxy resin chosen as a function of its index of refraction and of its adaptation with the material constituting the reflector halves la and lb.

In this first embodiment of the invention the reflector halves are identical and are disposed opposite to each other.

As shown in Fig. 4, the reflector half 21 is formed from a metal strip stamped in an appropriate manner to create the design shown. Said reflector half 21 comprises essentially two orthogonal walls 22 and 23.

The wall 22 is generally plane and the wall 23 has a configuration which it parabolic.

Said wall 23 is extended by a plane portion 24 comprising a boss 25 at the edge of which the semiconductor crystal is applied.

Usually the half of the reflector also comprises a plane wall 26 parallel to the wall 22, said wall being shaped at one of its parts so as to create a connection lug 27.

The metal chosen to form the reflector half must be a good electrical conductor and must have high reflecting power, so it is preferable that it is made from a copper strip covered afterwards by a thin silver layer.

Fig. 5 shows a display element composed of two identical reflector halves in accordance with the above description (Fig.

4) and disposed symmetrically with respect to a vertical axis passing through the centre of the electroluminescent diode formed in the semiconductor crystal 28. The juxtaposition of the two reflector halves defines a reflective cavity 29 which is then filled with a transparent material The crystal 28 is adhered to the bosses 25 of the two reflector halves by its two semiconductor regions. In this case it is adhered by means of a conductive resin known in the art but, consequently, during the assembly it should be ensured that the two reflector halves are not joined so that short-circuits are avoided.

Since the reflector halves are not joined, their filling with a transparent material should be carried out in a mould.

In another case, the crystal 28 may be adhered by means of an insulating adhesive, the contact being then provided, and in these conditions there is no inconvenience as regards the reflector halves being joined.

In Fig. 6, the reflector 31 is formed from a metal strip stamped in an appropriate manner to create a shape as shown.

Said reflector half 31 comprises two walls 32 and 33 which are, for example, plane, making an angle of approximately 90" with a third wall 34 which is also plane. Said walls have a trapezoidal shape and are chosen such that, by associating two reflector halves 31a and 31b of identical dimensions but symmetical, à display element is obtained the section of the cavity 35 of which is itself trapezoidal (Fig. 7). Each of the reflector halves 31 comprises a plane portion 36 orthogonal to the wall 34 and provided with a boss 37 at the edge of which the semiconductor crystal 38 is to be adhered.

This embodiment is suitable for circular or sectorial display devices by juxtaposing the necessary number of elements having a trapezoidal configuration.

WHAT WE CLAIM IS:- 1. A display element comprising a semiconductor crystal in which two regions of opposite conductivity type form an electroluminescent diode, and a reflector which reflects the light emitted by said electroluminescent diode to form an oblong light image, the reflector having two halves which form the outer walls of said element and which define a cavity at the bottom of which the semiconductor crystal is fixed, each half of the reflector comprising at least one metal connection pad electrically connected to one of the regions of the diode, and the cavity being filled with a transparent cured material.
2. A display element as claimed in Claim 1, in which the two halves of the reflector are identical and are disposed symmetrically with respect to the axis which is perpendicular to the surface of the crystal and which passes through the centre of the emissive zone of said crystal so that the reflected light forms a rectangular light image.
3. A display element as claimed in Claim 1, in which the two halves of the reflector are symmetrical with respect to a plane dividing the cavity which they define into two portion of equal volume, said plane

being perpendicular to the surface of the semiconductor crystal fixed on the bottom of the cavity and passing through the centre of the emissive zone of the said crystal so that the reflected light forms a trapezoidal light image.
4. A display element as claimed in any of the Claims 1 to 3, in which the semiconductor crystal is fixed with one of its faces on each of the two halves of the reflector at the bottom of the cavity.
5. A display element as claimed in Claim 4, in which each half of the reflector has a recess to accommodate the crystal at the bottom of the cavity, the crystal being fixed to at least one wall of each recess.
6. A display element as claimed in Claim 5, in which the wall of the recess to which the crystal is fixed comprises a boss.
7. A display element as claimed in any of Claims 1 to 6, in which the inner lateral walls of the reflector halves define a cavity having plane faces.
8. A display element as claimed in any of Claims 1 to 6, in which the inner lateral walls of the reflector halves define a cavity having parabolic faces.
9. A display element as claimed in any of Claims 1 to 6, in which the inner lateral walls of the reflector halves define a cavity having at least one face which is parabolic.
10. A display element as claimed in any of Claims 1 to 9, in which each half of the reflector is formed from an electrically conductive metal strip.
11. A display element as claimed in any of Claims 1 to 9, in which each half of the reflector is formed from an insulating and rigid synthetic material.
12. A display element as claimed in either Claim 10 or Claim 11, in which the surface of each half of the reflector is at least partially covered by an electrically conductive layer which has a high reflecting capacity.
13. A display element as claimed in Claim 12, in which the electrically conductive layer having a high reflecting capacity is silver.
14. A display element as claimed in any of Claims 1 to 13, in which the cavity is filled with an epoxy resin.
15. A display element as claimed in any of Claims 1 to 14, in which the crystal is fixed at the bottom of the cavity by means of an insulating adhesive.
16. A display element as claimed in Claim 15, in which the balves of the reflector defining the cavity are joined.
17. A display element as claimed in any of Claims 1 to 14, in which the crystal is fixed respectively by its two semiconductor regions to the two halves of the reflector by means of a conductive adhesive.
18. A display element as claimed in Claim 17, in which the two halves of the reflector defining the cavity are insulated from each other.
19. A display element as claimed in any of Claims 1 to 18, in which the semiconductor crystal is transparent at the wavelength of the light emitted by the electroluminescent diode.
20. A display element as claimed in Claim 19, in which the semiconductor crystal is gallium phosphide.
21. A display element substantially as herein described with reference to Figures 1 to 3, or Figures 4 and 5, or Figures 6 and 7 of the accompanying drawings.