Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-07-22
2004-11-09
Lee, Wilson (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169400
Reexamination Certificate
active
06815902
ABSTRACT:
TECHNICAL FIELD
The present invention concerns a device to produce a modulated electric field for an electrode. This applies in particular to flat field emission screens.
STATE OF THE ART
The devices for visualization by cathodoluminescence excited by field emission are well known. Such a device comprises a cathode arranged facing an anode. The cathode is a flat structure emitting electrons and the anode is another flat structure covered with a luminescent film. These structures are separated by a space in which a vacuum is created.
The cathode can be a source of microtips or a source with a low threshold field emissive material (the threshold field being the electric field needed to extract electrons from a material), for example nanostructures or carbon. The sources with an emissive material used in screen devices are usually shown in two forms—a diode type structure or a triode type structure.
FIG. 1
shows in a transversal cross-section view, a flat field emission screen operating according to a diode type structure. The cathode
1
is made up of a plate of insulating material
3
supporting parallel metallic tracks
4
and covered with layers of an emissive material
5
. The anode
2
is an insulating and transparent plate
6
, for example in glass, supporting parallel conductor tracks
7
and at right angles to the cathode tracks
4
. The tracks
7
are made by the etching of a layer of a transparent conducting material such as tin and indium mixed oxide (ITO). The tracks
7
are covered with films of phosphor
8
.
The cathode plate and anode plate are placed facing one another, the tracks being opposite to make up a matrix structure. The crossing of the track networks forms image elements or pixels. By applying an adequate potential difference between one track
4
of the cathode and one track
7
of the anode, an emission of electrons occurs on the zone of the track
4
corresponding to the considered pixel, and the zone of the phosphor
8
facing is excited. A complete image can be obtained on the screen by successively supplying each line of the screen and by sweeping.
So that electrode emission occurs, an emissive material with low threshold field such as carbon needs a minimum electric field of several V/&mgr;m between an anode track and a facing cathode track. If the space between these tracks is 1 mm, a potential difference of several kV must therefore be applied, usually between 5,000 and 10,000 V. This leads to two main problems. The first is the resistance in voltage—there is danger of breakdown between anode and cathode and above all between two adjacent tracks. The second problem results from the need to switch a voltage of several kV when sweeping the screen. This problem can be resolved by reducing the space between anode and cathode which facilitates reducing in the same way the potential difference between them while maintaining the same electric field. The disadvantage of this solution is that this decrease in potential causes a decrease in the output of the phosphors and less brilliance in the screen.
The triode type structure has been suggested in order to try and remedy these problems.
FIG. 2
shows in transversal cross-section a flat field emission screen implementing such a structure. The cathode
11
is made up of a glass plate
13
supporting parallel metallic tracks
14
and covered with layers
15
of an emissive material, carbon for example.
The tracks
14
are placed on the bottom of trenches etched in a layer of insulating material
10
, the layer
10
being covered with a metal layer
19
serving as extracting gate. The anode
12
can be made up of a transparent plate
16
with for example a transparent and conductive film
17
covered by a film of luminescent material
18
.
An emission of electrons by the emissive material can be obtained by applying, between the extraction gate
19
and track
14
, a potential difference so that the resulting electric field on the emissive material is greater than the threshold field of this material, usually several V/&mgr;m. As the distance separating the extraction gate from the tracks is very much smaller than the distance separating the anode from the cathode, the potential difference to be applied is reduced in the same way.
As the lines of electric field go from tracks
14
to the extraction gate
19
, a large part of the electrons emitted is going to be trapped by the gate. The triode type structure therefore has the disadvantage resulting from the fact that very few of the electrons emitted reach the phosphor layer.
Such a visualization device of triode type structure therefore enables avoiding the risk of electric breakdown and the problems of high voltage switching. However, these improvements are obtained to the detriment of electron density emitted which reach the luminophore or phospor layer. Moreover, this type of structure needs the realization of a deposit of emissive material solely on the bottom of trenches which presents considerable difficulties.
SUMMARY OF THE INVENTION
The present invention provides for solving the problems set forth above. The solution consists in applying a modulation electric field near to an electrode in the vicinity of which one wishes to obtain an electric field of specified value. Depending on the case, the modulation electric field will have the effect of decreasing or increasing the value of the electric field in the vicinity of the electrode in question.
The first object of the invention concerns a device which permits producing an electric field between a first and a second electrode, comprising:
means for applying a potential difference between these two electrodes, allowing to obtain, if this potential difference is applied alone, a predetermined value of electric field in the vicinity of the first electrode,
means forming modulation electrode located near the first electrode, either on the same plane or so that the first electrode is inserted between the second electrode and said means forming modulation electrode,
control means for applying a potential difference between the means forming modulation electrode and the first electrode in order to obtain through the contribution of said potential differences another predetermined value of electric field in said vicinity of the first electrode.
In a first case, the means for applying a potential difference between the first and the second electrode and the control means, supply potential differences such that the value of the electric field in said vicinity of the first electrode is greater than the value which would be due to the potential difference alone between the first and the second electrode.
In a second case, the means for applying a potential difference between the first and the second electrode and the control means, supply potential differences so that the value of the electric field in said vicinity of the first electrode is lower than the value which would be due to the potential difference alone between the first and the second electrode.
Conveniently, the first and the second electrode and the means forming modulation electrode are arranged parallel.
The means forming modulation electrode can comprise two electrodes surrounding the first electrode.
If the first electrode is inserted between the second electrode and the means forming modulation electrode, the means forming modulation electrode can be made up by a single electrode.
The second object of the invention concerns a process for producing an electric field between a first and a second electrode comprising:
the application of a potential difference between the first and the second electrode so as to obtain, if this potential difference was applied alone, a predetermined value of the electric field in the vicinity of the first electrode,
the application of a potential difference between the first electrode and means forming modulation electrode and located near to the first electrode, either in the same plane or so that the first electrode is inserted between the second electrode and said means forming modulation electrode, in orde
Fournier Adeline
Montmayeul Brigitte
Perrin Aime
Alemu Ephrem
Commissariat a l'Energie Atomique
Lee Wilson
Thelen Reid & Priest LLP
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