Abstract:
The display device comprises at least one compartment ( 30, 30′, 30″ ) which contains an ionizable gas ( 33 ). Walls of the compartment ( 30, 30′, 30″ ) are provided with electrodes ( 31, 32 ) for selectively ionizing the gas ( 33 ), during operation, by means of a direct current. The display device is characterized in that the walls of the compartment ( 30, 30′, 30″ ) are provided with a layer ( 20 ) of a secondary electron-emitting material, the electrons ( 31, 32 ) remaining completely or partly uncovered. Preferably, the thickness of the layer ( 20 ) exceeds 20 nm, and the secondary electron-emitting material comprises a material of the group formed by magnesium oxide, chromium oxide, silicon nitride and yttrium oxide. Preferably, the display device further comprises an electro-optical layer ( 35 ), and a further layer ( 36 ) of a dielectric material is situated between the compartment ( 30, 30′, 30″ ) and the electro-optical layer ( 35 ), which further layer ( 36 ) is provided with the secondary electron-emitting layer ( 21 ) on a side facing away from the electro-optical layer ( 35 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a display device comprising at least one compartment, which compartment contains an ionizable gas, walls of the compartment being provided with electrodes for selectively ionizing the ionizable gas during operation, and a voltage being applied across the electrodes during operation. 
     Display devices for displaying monochromatic or color images comprise, inter alia, direct-current plasma-display panels (dc PDPs) and direct-current plasma-addressed liquid-crystal display devices (dc PALC-displays), both types of display devices preferably being of the thin type. 
     A display device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,596,431 (PHA 60 092). The thin-type display device described in said document comprises a display screen having a pattern of (identical) so-called data-storage or display elements and a plurality of compartments. Said compartments are filled with an ionizable gas and provided with electrodes for (selectively) ionizing the ionizable gas during operation. In the known display device, the compartments are mutually parallel, elongated channels (shaped in a so-called channel plate), which serve as selection means for the display device (the so-called plasma-addressed row electrodes). By applying a voltage difference across the electrodes in one of the channels of the channel plate, electrons are emitted (from the cathode) which ionize the ionizable gas, thus forming a plasma. If the voltage across the electrodes in one channel is switched off and the gas is de-ionized, the following channel is energized. On the display-screen side of the display device, the compartments are closed by a (thin) dielectric layer (“microsheet”) provided with a layer of an electro-optical material and further electrodes which serve as so-called data electrodes or column electrodes of the display device. The display device is formed by the assembly of the channel plate with the electrodes and the ionizable gas, the dielectric layer, the layer of the electro-optical material and the further electrodes. 
     In a plasma-display panel, a plasma-discharge is used, on the one hand, to directly excite phosphors of desirable display elements or, on the other hand, to generate light (for example UV light) which is used to excite phosphors of desirable display elements. 
     A disadvantage of the known display device is that such display devices exhibit a relatively high energy consumption. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the invention to provide, inter alia, a display device exhibiting a lower energy consumption. 
     To achieve this, the display device in accordance with the invention is characterized in that at least a part of the surface of the walls of the compartment is provided with a layer of a material for emitting secondary electrons, the electrodes being at least partly uncovered. 
     During the time that the gas is ionized, the plasma-discharge is maintained by applying a specific sustain current and a sustain voltage across the electrodes in the compartment. In a display device with a plasma-discharge in an atmosphere comprising an ionizable gas, this sustain current is much higher, on average, than in the case of a (pure) discharge between two electrodes under vacuum conditions. In the known display device, a (considerable) part of the electrons and ions in the plasma discharge are lost at the walls of the compartment. By covering, in accordance with the invention, at least a part of the surface of the walls of the compartment with a layer of a material having a (high) secondary electron-emission coefficient, the losses at the walls are reduced and the sustain current for maintaining the plasma in display devices in accordance with the invention is lower than in the known display devices. Said lower sustain current causes the energy consumption of the display device to be reduced. As the plasma-discharge is maintained by a direct current, the secondary electron-emitting material leaves the electrodes at least partly uncovered. 
     A material for emitting secondary electrons is taken to mean in this application, a material which emits one or more secondary electrons in response to a (primary) electron which is incident on (the surface of) the material. The yield δ of secondary electrons is a function of the energy of the primary electrons E p , the maximum yield δ max  being achieved at a value of the primary electron energy of E p   max . The two primary-electron energy values corresponding to a yield of δ=1 are commonly referred to as the first and the second crossover and are referenced E I  and E II , respectively. Preferably, the secondary electron-emitting materials covering parts of the surface of the walls of the compartment of the display device have a relatively low first crossover energy value E I . As a result of the high density and a plurality of intercollisions, the primary energy values of the drifting electrons reaching the wall of the compartment are relatively low (in the range between 2 and 5 eV). The above-mentioned choice of E I  enables a relatively high secondary-electron yield to be achieved. 
     In general, the display device comprises a number of compartments, each compartment comprising at least two electrodes for ionizing the gas. 
     An additional advantage of covering the walls of the compartment of the display device with the secondary emitting material is that the depth of the compartment can be reduced (for example by 10-20%). Such a reduction is possible because the walls contribute to the provision of (secondary) electrons to the plasma discharge, so that a compartment having a smaller depth is sufficient. 
     To bring about a uniform ignition of the plasma-discharge, it is desirable that the effective surface area of the electrodes should be as large as possible. To achieve this, the electrodes are preferably completely uncovered, that is, they are not provided with a layer of the secondary emitting material. 
     It is also desirable that the layer of the material provided on the walls of the compartment should not only have a high secondary-electron yield but also a high stability against ion and electron bombardment. To achieve this, an embodiment of the display device in accordance with the invention is characterized in that the material includes a material of the group formed by magnesium oxide (MgO), chromium trioxide (Cr 2 O 3 ), silicon nitride (Si 3 N 4 ) and yttrium trioxide (Y 2 O 3 ). A material which can very suitably be used is magnesium oxide MgO (high stability against ion and electron bombardment) which has a measured secondary-electron emission coefficient δ of approximately 5 to 11. Other suitable materials are TiO 2 , Ta 2 O 5 , AlN and Al 2 O 3 . Particularly Al 2 O 3  has a high secondary electron-emission coefficient (δ≈10). The use of mixtures of said materials enables the desired properties of the layer of the secondary emitting material to be achieved. 
     A preferred embodiment of the display device in accordance with the invention is characterized in that the thickness of the secondary electron-emitting layer is above 20 nm. Too thin a layer (&lt;5 nm) contributes only little to the secondary electron emission. Secondary electron-emission measurements carried out on said materials show that the yield of secondary electrons increases rapidly up to a layer thickness of approximately 20 nm, between 20 and 40 nm a further improvement of the yield is obtained and, at layer thicknesses≧40 nm, the yield of secondary electrons remains stable. 
     An embodiment of the display device in accordance with the invention is characterized in that the compartment comprises an elongated channel. If the display device is composed of a number of channel-shaped compartments, these channels are arranged so as to be mutually parallel. The secondary electron-emitting layer can be provided more readily in such elongated channels than in the plurality of compartments. In addition, in the case of channels, the ratio of the surface of the walls to the volume of the compartments is relatively favorable. 
     Further preferred embodiments of the display device in accordance with the invention are characterized in that the compartment comprises phosphors. In this case, two types of phosphors can be distinguished, that is, so-called electroluminescent phosphors, in which the ionized gas in the compartment itself excites the phosphors, and so-called photoluminescent phosphors, in which the ionized gas emits light (for example UV light) which excites the phosphors. The above-mentioned embodiments generally relate to plasma-display panels. 
     Other preferred embodiments of the display device in accordance with the invention are characterized in that the display device comprises a layer of an electro-optical material, for example a liquid-crystal material. In this case, the ionized gas acts as a virtual switch for the electro-optical material. For this reason, such display devices are referred to as plasma-addressed (liquid-crystal) display panels. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 schematically shows a block diagram of a display device; 
     FIG. 2 is a schematical, perspective view, partly cut away, of a part of a construction of a plasma-addressed liquid-crystal display device (PALC) in accordance with the invention; 
     FIG. 3A is a schematic, sectional view of an embodiment of a compartment of a monochrome plasma-display panel, in which visible light is produced in a discharge of an ionized gas; 
     FIG. 3B is a schematic, sectional view of an embodiment of a compartment of a color-plasma display panel, in which the plasma-discharge emits visible or UV-light which excites the phosphors, and 
     FIG. 4 is a schematic, perspective view, partly cut away, of a part of a construction of a plasma-display panel (PDP) in accordance with the invention. 
    
    
     The Figures are purely schematic and not drawn to scale. In particular for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts whenever possible. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 very schematically shows a block diagram of a conventional display device. Said display device comprises a substrate  1  with a surface  2  which is provided with a pattern of pixels which are separated from each other in the vertical and horizontal directions (the space between the pixels being predetermined). Each pixel  3  comprises overlapping portions of (thin, narrow) electrodes  4  of a group of electrodes arranged in vertical columns and (thin, narrow) electrodes  5  of a further group of electrodes arranged in horizontal rows. The electrodes  4  of the group of electrodes are also referred to as column electrodes, and electrodes  5  of the further group of electrodes are also referred to as row electrodes. In a plasma-addressed liquid-crystal display device (PALC), the rows are formed by long, narrow channels (the compartment). The pixels  3  in each of the rows of electrodes (channels)  5  represent one data line. 
     The width of the electrodes  4 ,  5  determines the dimensions of the pixels  3 , which are typically rectangular in shape. Electrodes  4  receive (analog) drive signals (“data drive signals”) from a drive circuit  8  via parallel conductors  6 , and electrodes  5  receive (analog) drive signals (“data drive signals”) from a drive circuit  8 ′ via parallel conductors  7 . 
     To realize an image or a data-graphic display on a relevant area of the surface  2  of substrate  1 , the display device employs a control circuit  8 ″ (“scan control circuit”), which controls the drive circuits  8 ,  8 ′. In the display device, use can be made of various types of electro-optical materials. Examples of electro-optical materials include nematic or ferro-electric liquid-crystal materials. In general, the electro-optical materials weaken the passed or reflected light in response to a voltage applied across the material. 
     FIG. 2 shows a schematic, perspective view, partly cut away, of a part of a construction of a plasma-addressed liquid-crystal display device (PALC) in accordance with the invention, which comprises a first substrate  38  and a second substrate  39 . In FIG. 2, only three column electrodes  29 ,  29 ′,  29 ″ are shown. The row electrodes  30 ,  30 ′,  30 ″ which serve as selection means are formed by a number of mutually parallel, elongated channels (compartments) under a layer  35  of an electro-optical material. The panel is provided with electric connections to the column electrodes  29 ,  29 ′,  29 ″ and to the plasma electrodes  31 ,  32 , said column electrodes  29 ,  29 ′,  29 ″ receiving (analog) drive signals from output amplifiers  27 ,  27 ′,  27 ″, and the anode electrodes  32  in the (plasma) channels  30 ,  30 ′,  30 ″ receiving drive signals from output amplifiers  26 ,  26 ′. Each of the (plasma) channels  30 ,  30 ′,  30 ″ is filled with an ionizable gas  33  (for example a low-pressure ionizable gas, such as helium, neon and, if desirable, argon) and is sealed with a thin dielectric layer (“microsheet”)  36 , which is made, for example, of glass. Each of the compartments (the channels) is provided, at an inner surface (wall), with first and second elongated electrodes  31 ,  32  extending throughout the length of the channel. The second electrode  32  is referred to as the anode and a direct-current pulse, a so-called “dc strobe pulse”, is applied to said anode, causing electrons emitted from the cathode  31  to ionize the gas, thereby forming a plasma. In an alternative embodiment, a negative direct current pulse is applied to the cathode. The next channel is not energized until after the “dc strobe pulse” has ended and the gas has been de-ionized. To reduce the duration of the cycle, the subsequent channel is generally ionized already before the previous channel has been (completely) de-ionized. The column electrodes  29 ,  29 ′,  29 ″ each cross an entire column of pixels, so that, in order to preclude crosstalk, the number of possible plasma row connections per unit of time is limited to only one. The output amplifiers  26 ,  26 ′ in combination with the drive circuits  8 ,  8 ′ and the control circuit  8 ″ form the means for applying a dc voltage across the electrodes ( 31 ,  32 ) for selectively ionizing the gas ( 33 ) during operation. 
     With the exception of the electrodes  31 ,  32 , the walls of the channels (the compartments) are provided with a layer  20  of a material which emits secondary electrons when the gas is ionized. In the example associated with FIG. 2, also the lower side of the dielectric layer  36  is provided, at the location of the channels, with a layer  21  of a material which emits secondary electrons when the gas is ionized. By virtue of said coating, the (ion and electron) losses at the walls of the channels of the display device are reduced considerably, so that the sustain current necessary for maintaining the plasma in the channels is lower and hence the energy consumption of the display device in accordance with the invention is much lower than that of the known display device. 
     In the manufacture of the PALC display device, the thin layer of MgO is applied to the walls of the channels, for example, by vapor deposition or spray pyrolysis (typical thickness approximately 50-100 nm), during which operation the electrodes are shielded. In an alternative embodiment, the electrodes are not provided until after MgO has been applied to the walls. Preferably, also the thin, dielectric (glass) “microsheet”  36  is provided with an MgO layer  21  on the side facing away from the electro-optical layer  35 . 
     FIG. 3A is a schematic, sectional view of an embodiment of one compartment of a monochrome plasma-display panel, in which visible light L vis  is produced in the negative glow D of a (weakly) ionized gas. In this example, the PDP comprises a rear wall  50 , for example of glass, on which a cathode  51  is provided. The anode  52  is situated on the light-transmitting front wall  53  which is located opposite the rear wall  50  and which is for example also made of glass. The anode  52  is preferably made of a light-transmitting material (for example indium tin oxide (ITO)). The compartments are separated from each other by walls  54 . The color of the emitted light L vis  depends on the gas composition. For example, neon (Ne) produces orange-red light, xenon (Xe) produces white light. Also gas mixtures of neon-argon (Ne—Ar) and of neon-xenon (Ne—Xe) are used. 
     FIG. 3B is a schematic, sectional view of an embodiment of one compartment of a color plasma-display panel, in which the plasma-discharge D emits visible or UV-light L vuv  which excites the phosphors. In this example, the PDP comprises a rear wall  60 , for example of glass, on which a cathode  61  is provided. The anode  62  is situated on the light-transmitting front wall  63  which is located opposite the rear wall  60  and which is for example also made of glass. The anode  62  is preferably made of a light-transmitting material (for example indium tin oxide (ITO)). The compartments are separated from each other by walls  64  which also ensure that light emitted by the plasma cannot reach adjoining compartments (limitation of crosstalk). Besides, in general, visible light originating directly from the plasma-discharge D must not reach the viewer. 
     In the example associated with FIG. 3B, photoluminescent or electroluminescent phosphors  65  are provided on the walls of the compartments, each compartment generally containing one of the basic colors red, green and blue. In principle, there is a regular pattern which is always comprised of three or four compartments, that is, a red, a blue and a green compartment with, possibly, one of the colors in a double configuration. A complete color palette can be obtained by adjusting the intensities of the light emitted by the individual display elements (sub-pixels) of the PDP independently of each other. The intensity of the plasma-discharge depends on the gas composition. In particular, xenon (Xe), helium (He) and mixtures of neon-xenon (Ne—Xe) or helium-xenon (He—Xe) can effectively be used to excite the phosphors. In the example associated with FIG. 3B, the phosphors are provided on the inner surface of the front wall  63 ; such a configuration is referred to as “transmissive view” PDP. In an alternative embodiment, the phosphors are located on the inner surface of the rear wall  60 ; such a configuration is referred to as “direct view” or “reflective view” PDP. 
     FIG. 4 is a schematic, perspective view, partly cut away, of a part of a construction of a plasma-display panel (PDP) in accordance with the invention. The embodiment shown comprises four compartments  78 ,  78 ′ of a color plasma-display panel, the plasma-discharge emitting visible or UV-light which excites the phosphors. In this example, the PDP comprises a rear wall  70 , for example of glass, on which cathodes  71 ,  71 ′ are provided. Anodes  72 ,  72 ′ are situated on the light-transmitting front wall  73  which is located opposite the rear wall  70  and which is for example also made of glass. (In FIG. 4, for clarity, the rear wall  70  is drawn so as to be detached from the front wall  73 ). The anodes  72 ,  72 ′ are preferably made of a light-transmitting material (for example indium tin oxide (ITO)). The compartments (pixels)  78 ,  78 ′ are separated from each other by walls  74  which also ensure that light emitted by the plasma does not reach adjoining compartments (limitation of crosstalk). In addition, visible light emitted by the plasma must not reach the viewer. FIG. 4 shows an example of a “transmissive view” PDP, in which phosphors  75  are provided on the inner surface of the front wall  73 . 
     In a dc PDP, the electrodes are exposed to the plasma-discharge, so that, if a voltage is applied in one of the compartments, electrons are ejected from the cathode. In a dc-type PDP, the discharge can continue as long as the voltage is applied. The voltage necessary to maintain the plasma discharge typically amounts to several hundred volts. An ionizable gas at a specific pressure (a typical gas pressure ranges from 2 to 20 kPa) is present between the electrodes, said gas being partly ionized by the electrons. In the electric field between the electrodes, the electrons and the ions formed are accelerated. This acceleration is limited by elastic collisions with neutral particles in the ionizable gas. From time to time, the charged particles lose speed as a result of elastic collisions, whereafter they gain speed again under the influence of the electric field. The macroscopic result is that the electrons and the ions are not accelerated, but instead move at a constant velocity, which is referred to as the “drift velocity”, at which the electrons gradually move towards the anode and the ions gradually move towards the cathode. In general, the drift velocity of the electrons is twice as high as the drift velocity of the ions. 
     In a dc-type plasma panel (see the example associated with FIG.  4 ), the primary discharge generally takes place in an auxiliary cell  98  which is situated next to the compartments  78 ,  78 ′ for the emission of light. The auxiliary cell  98  comprises an electrode  92 ,  92 ′. As the primary plasma discharge in the auxiliary cell  98  does not contribute to the light emitted, the display device has a high level of black color and a good contrast. 
     Each of the compartments (pixels) in a dc plasma-display panel generally also comprises a current-limiting resistance (not shown in FIG.  4 ), enabling a plurality of grey levels to be adjusted in a simple manner. 
     In accordance with the invention, the walls  74  at the inner surface of the compartments  78 ,  78 ′ are provided with a layer  80  of a material emitting secondary electrons when the gas is ionized. The anodes  72 ,  72 ′ are not covered with the secondary electron-emitting material. In the example associated with FIG. 4, also the side of the rear wall  70  facing the compartments  78 ,  78 ′ is provided with a layer  81  of a material emitting secondary electrons when the gas is ionized. The cathodes  71 ,  71 ′ are not covered with the secondary electron-emitting material. Said coating causes the (electron) losses at the walls of the channels of the display device to be reduced considerably, so that the sustain current necessary for maintaining the plasma in the channels is lower and hence the energy consumption of the display device in accordance with the invention is much lower than that of the known display device. 
     It will be obvious that within the scope of the invention many variations are possible to those skilled in the art.