GB2235819A - Gas discharge display device - Google Patents

Abstract

A gas discharge display device comprises an electrically insulating substrate 10 which bears at least one pair of discharge electrodes 12, 14 and an opposed electrically insulating face plate 16 bearing an electrically conductive low voltage phosphor 18, the substrate and face plate being assembled together with a gaseous discharge-sustaining atmosphere therebetween, wherein upon application of appropriate potentials to the conductive phosphor and discharge electrodes, electrons produced by the discharge between the discharge electrodes are attracted to and energise the conductive phosphor to produce a light output. The phosphor 18 is either formed on an electrode 20 on the faceplate, as shown, or is made inherently conductive, eg. by doping, mixing or coating the phosphor with an electrically conductive substance. The discharge electrodes may be arranged to produce a bar-graph display, a seven-segment display or a matrix crossed conductor addressed display, Figs. 4-6 respectively (not shown). <IMAGE>

Description

Title: Gas Discharge Display Field of the Invention This invention relates to gas discharge displays which comprise a primary discharge through a low pressure gas or gas mixture, the discharge energising a phosphor coated upon the interior surface of a face plate.

Background to the Invention Flat panel displays have been proposed which comprise a face plate bearing a non-conductive photo luminescent phosphor layer, and a substrate defining a volume which is filled with a gas, the substance carrying electrodes to establish and contain an electrical discharge to generate UV, to energise the phosphor. See Proc. SID Vol 29/2 1988 p.131 et seq.

Such devices offer considerable flexibility in layout but their life can be shortened by damage to the phosphor due to bombardment with ions from the discharge. Also the brightness level of such photoluminescent phosphors is often low.

It is also known from WO 88/01098 to form a flat panel display utilising a field emission cathode spaced from a cathodo-luminescent non-conductive cathodo-luminescent area on a faceplate, the intervening space being evacuated to enable electron emission from the cathode to energise the cathodo-luminescent material.

In matrix arrays of such devices. the cathode structure is very complex and subject to difficulty in manufacture (giving poor yields) and breakdowns in use, although the cathodo-luminescent phosphors tend to produce more light and a greater range of colours and in general luminescence can be more easily controlled than when a gas discharge energised phosphor is employed.

It is an object of the invention to provide an improved flat panel display which possesses the advangates of both types of device as described but which does not suffer from their disadvantages.

It is a further object to provide a display of simple construction.

Summary of the Invention According to the invention, there is provided a gas discharge display comprising: an electrically insulating substrate which bears at least one pair of discharge electrodes between which an electric field can be established; an electrically insulating face plate bearing an electrically conductive low voltage phosphor; the substrate and face plate being assembled together, with the discharge electrodes and the electrically-conducting phosphor in spaced facing relationship within a gaseous atmosphere capable of sustaining an electrical discharge between the discharge electrodes; and conductor means, for applying an electric potential to the conductive phosphor, and for applying different electric potentials to the discharge electrodes; in which, in use, electric potentials are applied to the discharge electrodes to produce an electric discharge therebetween and electrons produced by the discharge can be attracted to and energise the conductive phosphor when a suitable electric potential is applied to the phosphor.

The conductor means for applying potential to the phosphor may comprise an electrode on the faceplate which electrode is coated with the phosphor.

The electrode may be transparent or translucent or may comprise a grid or mesh.

The low voltage phosphor may, for example, be of the type used in vacuum fluorescent displays. Such phosphors may be energised by collision with electrons having kinetic energies in the order of electron volts or "hundreds-ofelectron-volts", rather than the "kiloelectron volts" such as are required to energise the phosphor types used in conventional Cathode Pay Tubes.

Preferably the substrate is substantially planar, the discharge electrodes consequently being substantially coplanar with each other.

The phosphor may be inherently conductive, or may be doped, coated or mixed with a conductive substance to give an electrically conductive coating on the face plate or on the electrode on the face plate.

By appropriate dimensioning and positioning of the electrodes, different display configurations may be achieved. Thus the size and shape of the display may be selected by appropriate selection of the dimensions and shape of the area of the phosphor (or phosphor coated electrode), in the faceplate.

Alternatively a plurality of separate phosphor areas (or phosphor coated electrodes) may be provided each with its associated discharge electrode pair or one phosphor area (or coated electrode) may have a plurality of separately addressable discharge electrode pairs associated herewith.

For example, there may be provided a linear array of a plurality of pairs of discharge electrodes in spaced facing relationship with a face plate which bears either a single electrode, or a plurality of electrodes each corresponding to a respective pair of discharge electrodes, the arrangement being such that the display is in the form of a bar-graph display.

In this case, the discharge electrodes are preferably arranged in interdigitated relationship with one another.

Alternatively, there may be provided a plurality of pairs of discharge electrodes, each being in spaced facing relationship with a respective electrode on the face plate, so arranged as to provide a multiple-bar character display.

As a further alternative, the display may include an array of a plurality of substantially parallel elongate discharge electrodes in spaced facing relationship with an array of elongate electrodes on the face plate which are non-parallel with, and preferably perpendicular to, the discharge electrodes, the arrangement being such as to provide a matrix display.

By varying the potential of the phosphor and/or electrode coated by the phosphor, the luminence (ie brightness) can be varied.

Brief Description of the Drawings In the drawings Figure 1 is an exploded perspective view of a single element display device illustrating the invention; Figure 2 is a section through the display device of Figure 1 to illustrate the principles of operation; Figure 3 is a graphical representation of Paschen's law; and Figures 4, 5 and 6 illustrate respectively bar graph, multi-bar character and matrix display layouts using the invention.

Detailed Description Referring to the drawings, Figure 1 shows a display element comprising substrate 10 carrying electrodes 12 and 14, and a face plate 16 carrying a conductive phosphor coating 18 upon a transparent electrode 20 laid down upon the surface 22 of face plate 16 facing substrate 10.

Substrate 10 and electrodes 12 and 14 may be covered by a dielectric layer 24.

The device is sealed, evacuated and back-filled with a Penning mixture of neon with an argon or xenon dopant, at a pressure of 100-600 torr.

A discharge established by applying a potential difference between two electrodes 12 and 14, produces electrons which may be withdrawn from the discharge, or plasma, by applying a positive potential to electrode 20 on faceplate 16. Electrons attracted to electrode 20 strike conductive phosphor coating 18 causing a visible emission characteristic of the phosphor type. The phosphor coating 18 may, for example, be green-emitting zinc oxide such as is used in currently available vacuum flourescent displays laid down upon a transparent electrode 20 of indium tin oxide. The phosphor coating 18 may either be inherently conductive or may be doped with an appropriate material eg zinc cadmium sulphide with a dopant of aluminium or mixed with conductive particles such as indium tin oxide.

Figure 2 illustrates diagrammatically the mechanisms involved in the generation of a visible emission from the element of Figure 1.

A discharge - shown as hatching 30 - established between electrodes 12 and 14 shown as C and Al and spaced a distance 's', releases electrons which are attracted to electrode 20, A2, held at a positive potential and spaced a distance 'g' from the plane of electrodes 12 and 14.

Any ions arising as a result of the discharge - for example neon ions from break-down of a typical neon-argon mixture surrounding the discharge - are repelled by the positive potential on A2 and attracted to electrode 12.

The voltage necessary to establish the discharge between electrodes 12 and 14 is dependent upon the gas or gas mixture contained within the display, and in accordance with Paschen's Law a function of gas pressure and electrode separation. A typical Paschen Law plot is illustrated in Figure 3.

By way of example, a display cell in accordance with the invention contained a gas mixture of neon and argon at a pressure of 100 torr. Electrodes 12 and 14 were separated by a distance of 0.30mm and the dimension 'g' between the substrate 10 and the lower face 22 of face-plate 16 was 0.6mm. Discharge was established and maintained by a potential difference of 180 volts applied to electrodes 12 and 14 and a positive potential of 150 volts was applied to electrode 20. It will be appreciated that the constructional details and operating conditions set out above are given by way of example only and that other constructions and operating conditions may selected by those skilled in the art within the scope of the invention and to achieve its objects.For example a display in accordance with the invention could incorporate one of more "keep alive" electrodes to maintain a low level ion source to ensure rapid ionisation to establish the discharge between electrodes 12 and 14 when a potential is applied.

Various physical configurations may be provided for displays in accordance with the invention. For example in Figure 4 interdigitated electrodes 42 and 44 laid down upon substrate 40 may, in conjunction with an appropriate conductive phosphor coating upon a transparent electrode carried by a face plate (not shown), provide, by suitable energisation of electrodes 42 and 44, a bar display.

Similarly pairs of electrodes 52 and 54 laid down as shown in Figure 5 upon a substrate 50 and combined with a face plate (not shown) with matching conductive phosphor stripes laid down upon transparent electrodes, may provide a seven element character display.

A matrix discharge display in accordance with the invention may be realised as shown in Figure 6.

A substrate 60 carries a number of pairs of electrodes 62, 64 disposed upon it overlaid at right angles by conductive phosphor stripes, 66, carried by respective conductive electrodes laid down upon a face plate not shown.

By energising an appropriate pair of electrodes 62, 64 to establish a discharge and applying a suitable positive potential to selected strip 66, light is emitted from the display only in the region 68 bounded by electrodes 62, 64 and overlaid by electrode 66. As no other electrode 66 is energised the remaining length of the discharge between 62 and 64 will be masked. A negative potential may be applied to other electrodes 66 to suppress any tendency to become energised due to stray electrons. In this way, a refreshed display of high resolution is possible with sufficient numbers of electrons and a phosphor voltage to overcome limitations of brightness for a refreshed mode at low duty cycle.

Cross talk problems, usually besetting dot-matrix flat panels, may be overcome by separating the technology of horizontal lines and vertical columns. Thus, there is little tendency for "off" state dots to become visible due to any charge leakages. Nor is there a need for the steep brightness - voltage characteristic usually needed for other displays such as electroluminescence.

Grey scale can be incorporated more simply than in gas discharges of prior art by variation of potential applied to the phosphor coated electrode 66.

Claims (14)

Claims

1. A gas discharge display comprising: an electrically insulating substrate which bears at least one pair of discharge electrodes between which an electric field can be established; an electrically insulating face plate bearing an electrically conductive low voltage phosphor; the substrate and face plate being assembled together, with the discharge electrodes and the electrically-conducting phosphor in spaced facing relationship within a gaseous atmosphere capable of sustaining an electrical discharge between the discharge electrodes, and conductor means for applying an electric potential to the conductive phosphor; and for applying different electric potentials to the discharge electrodes, in which in use, electric potentials are applied to the discharge electrodes to produce an electric discharge therebetween and electrons produced by the discharge can be attracted to and energise the conductive phosphor, when a suitable electric potential is applied to the phosphor.

2. A display according to claim 1 in which the conductor means for applying potential to the phosphor comprise an electrode on the faceplate which electrode is coated with the phosphor.

3. A display according to either claim 1 or claim 2 in which the discharge electrodes are substantially coplanar.

4. A display according to any of the preceding claims in which the phosphor is inherently conductive.

5. A display according to any of claims 1 to 3 in which the electrical conductivity of the phoshpor is achieved by means of doping, mixing or coating the phosphor with an electrically conductive substance.

6. A display according to any of the preceding claims in which there is provided a plurality of pairs of discharge electrodes in spaced-facing relationship with a single coated target electrode or with a plurality of electrodes on the faceplate, each corresponding to a respective pair of discharge electrodes.

7. A display according to claim 6 in which the discharge electrodes are arranged in an interdigitated linear array such that the display is in the form of a bar-graph display.

8. A display according to any of claims 1 to 6 in which there is provided an array of a plurality of substantially parallel elongate discharge electrodes in spaced-facing relationship with an array of substantially parallel elongate electrodes on the faceplate, the electrodes on the face plate being non-parallel with the discharge electrodes so as to provide a matrix display.

9. A display according to claim 8 in which the target electrodes are perpendicular to the discharge electrodes.

10. A display according to any of the preceding claims in which the gaseous atmosphere comprises a mixture of argon and neon gas.

11. A display as substantially described herein with reference to, and as illustrated in, Figures 1 to 3 of the accompanying drawings.

12. A display substantially as described herein with reference to, and as illustrated in, Figure 4 of the accompanying drawings.

13. A display substantially as described herein with reference to, and as illustrated in, Figure 5 of the accompanying drawings.

14. A display substantially as described herein with reference tog', and as illustrated in. Figure 6 of the accompanying drawings.