Abstract:
Provided is a display device that performs addressing by discharging electrons and performs a sustain discharge according to gradation in an addressed discharge cell. The display device includes: first and second substrates facing each other; barrier ribs disposed between the first substrate and the second substrate and defining a plurality of discharge cells with the first and second substrates; first and second electrodes disposed in the barrier ribs; an electron emissive source formed on the second substrate and discharging a plurality of electrons into the discharge cells; a phosphor layer disposed in the discharge cells; and a gas stored in the discharge cells.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2006-0030133, filed on Apr. 3, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present embodiments relate to a display device that performs addressing by discharging electrons and performs a sustain discharge according to a gradation in the addressed discharge cells. 
         [0004]    2. Description of the Related Art 
         [0005]    Liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), etc. are display devices, and in particular, flat display devices. 
         [0006]    LCDs, which are non-light emitting display devices and require a separate back light, are distinguished from PDPs and FEDs, which are light emitting display devices and do not require a separate back light. 
         [0007]    PDPs form images using an electrical discharge, and have a good brightness and viewing angle, etc., and the use of PDPs has recently increased. PDPs display images using visible light emitted through a process of exciting a phosphor material with ultraviolet rays generated by a discharge of a discharge gas between electrodes when a direct current (DC) voltage or an alternating current (AC) voltage is applied to the electrodes. 
         [0008]    In FEDs, electron emission devices electrically connected to a cathode electrode discharge electrons due to a difference between voltages applied to a gate electrode and the cathode electrode, respectively, discharged electrons collide with phosphor substances, and visible light is emitted. 
         [0009]    To drive PDPs, a unit frame is divided into a plurality of sub fields to express gradation, and each sub field has an address period where a discharge cell is selected to be turned on and off from all the discharge cells and a sustain period where a sustain discharge is performed in discharge cells that are selected to be turned on according to a gradation allocated to each of the sub fields. In driving PDPS, an address discharge is performed to select a discharge cell to be turned on and off from all the discharge cells in the address periods of each sub field. However, since the address discharge is generally delayed in time, it is difficult to drive PDPs at high speed. Also, the address period of PDPs having high resolution increases to perform a stable address discharge, which reduces the sustain-discharge period. 
         [0010]    To drive FEDs, pulse amplitude modulation (PAM) or pulse width modulation (PWM) is used to express gradation. In driving FEDs, electron emissive characteristics of electron emissive devices must be identical to each other to express desired gradation. However, the electron emissive characteristics of electron emissive devices are different from each other due to a process, which deteriorates uniformity in expressing brightness. 
       SUMMARY OF THE INVENTION 
       [0011]    The present embodiments provide a display device that can reduce a driving voltage and improve light-emitting efficiency. 
         [0012]    According to an aspect of the present embodiments, there is provided a display device, comprising: first and second substrates facing each other; barrier ribs disposed between the first substrate and the second substrate and defining a plurality of discharge cells with the first and second substrates; first and second electrodes disposed in the barrier ribs; an electron emissive source formed on the second substrate and discharging a plurality of electrons into the discharge cells; a phosphor layer disposed in the discharge cells; and a gas stored in the discharge cells. 
         [0013]    The electron emissive source may comprise: third electrodes disposed on the second substrate; an electron emissive device formed on the third electrodes; and fourth electrodes disposed on the third electrodes. 
         [0014]    The electron emissive source further may comprise an insulation layer between the electron emissive device and the fourth electrodes. 
         [0015]    A unit frame used to display an image may be divided into a plurality of sub fields, and each sub field has an address period where a discharge cell is selected to be turned on and off from all the discharge cells and a sustain period where a sustain discharge is performed in discharge cells that are selected to be turned on according to a gradation allocated to each of the sub fields. 
         [0016]    In the address period, a scan pulse sequentially having a ground voltage or a negative voltage may be applied to the third electrodes, and a display data signal selectively having a positive voltage is applied to the fourth electrodes in accordance with the scan pulse. 
         [0017]    In the sustain period, a sustain pulse having a high level and a low level may be alternately applied to the first electrodes and the second electrodes. 
         [0018]    The sustain pulse having the high level and the low level may be applied to one of the first electrodes and the second electrodes, and an intermediate level between the high level and the low level of the sustain pulse is applied to the another electrodes. 
         [0019]    In the sustain period, the gas may be excited due to a difference in electric potentials between the first electrodes and the second electrodes and the discharged electrons and ultraviolet rays are generated, the phosphor layer is excited due to the ultraviolet rays, and visible light is generated. 
         [0020]    In the sustain period, the difference in electric potentials between the first electrodes and the second electrodes may be lower than a discharge start voltage. 
         [0021]    The first electrodes and the second electrodes may be parallel to each other. 
         [0022]    The third electrodes and the fourth electrodes may extend to cross each other. 
         [0023]    The display device may further comprise: fifth electrodes disposed on the first substrate and collecting the discharge electrons. 
         [0024]    The phosphor layer may be formed on the first substrate. 
         [0025]    The display device may further comprise: a protective layer formed on the sidewalls of the barrier ribs. 
         [0026]    The electron emissive device may be a carbon nanotube (CNT). 
         [0027]    The electron emissive device may be formed of oxidized porous silicon (OPS). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The above and other features and advantages of the present embodiments will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0029]      FIG. 1  is a cross-sectional view of a display device according to an embodiment; 
           [0030]      FIG. 2  is a cross-sectional view of an electron emissive source of the display device illustrated in  FIG. 1  according to an embodiment; 
           [0031]      FIG. 3  is a cross-sectional view of an electron emissive source of the display device illustrated in  FIG. 1  according to another embodiment; 
           [0032]      FIG. 4  is a cross-sectional view of an electron emissive source of the display device illustrated in  FIG. 1  according to another embodiment; 
           [0033]      FIG. 5  illustrates an arrangement of electrodes of a display device according to an embodiment; 
           [0034]      FIG. 6  is a timing diagram of driving signals applied to each of the electrodes illustrated in  FIG. 5  according to an embodiment; and 
           [0035]      FIG. 7  is a timing diagram of driving signals applied to each of the electrodes illustrated in  FIG. 5  according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    The present embodiments will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments are shown. 
         [0037]      FIG. 1  is a cross-sectional view of a display device according to an embodiment. 
         [0038]    Referring to  FIG. 1 , the display device of the current embodiment comprises a first substrate  10 , a second substrate  20 , an electron emissive source  22 , barrier ribs  15 , a first electrode  12 , a second electrode  14 , a phosphor layer  18 , and a gas. The display device can further comprise a fifth electrode  16 . 
         [0039]    The first substrate  10  and the second substrate  20  are spaced apart from each other and face each other. The first substrate  10  and the second substrate  20  can be formed of a transparent material such as glass, but the present embodiments are not restricted thereto. The first substrate  10  and the second substrate  20  may be formed of the same material or a different material. In some embodiments, the first substrate  10  and the second substrate  20  have the same coefficient of thermal expansion. 
         [0040]    The barrier ribs  15  can be integrally formed as shown, and be separately attached to the first substrate  10  and the second substrate  20  (a front barrier rib and a rear barrier rib). The barrier ribs  15  along with the first substrate  10  and the second substrate  20  define a discharge cell  25  in which a discharge is performed. The discharge cell  25  can be an aperture having a circular cross-section in the barrier ribs  15  but the present embodiments are not restricted thereto. The discharge cell  25  can have a hexagonal, octagonal, pentagonal, oval, etc. cross-section. The barrier ribs  15  partition the discharge cell  25  in the form of matrix but the present embodiments are not restricted thereto. The barrier ribs  15  can partition the discharge cell  25  in a variety of patterns such as waffle, delta, etc. as they can form a plurality of discharge spaces. 
         [0041]    The first electrode  12  and the second electrode  14  are disposed in the barrier ribs  15 . The first electrode  12  and the second electrode  14  may surround the discharge cells  25  forming the aperture having the circular cross-section, and extend in a direction. 
         [0042]    A protective layer  17  formed of MgO, for example, may be formed on the sidewalls of the barrier ribs  15  that define the discharge cell  25 . When a discharge is performed, the protective layer  17  protects the first electrode  12 , the second electrode  14 , and the barrier ribs  15  formed of a dielectric substance covering the first electrode  12  and the second electrode  14 , and discharges secondary electrons to facilitate the discharge. 
         [0043]    The electron emissive source  22  that discharges a plurality of electrons into the discharge cell  25  is disposed on the upper surface of the second substrate  20 . The electron emissive source  22  comprises a third electrode ( 32  in  FIG. 2 ,  42  in  FIG. 3 , and  52  in  FIG. 4 ), an electron emissive device ( 38  in  FIG. 2 ,  63  in  FIG. 3 , and  58  in  FIG. 4 ) electrically connected to the third electrode, and a fourth electrode ( 36  in  FIG. 2 ,  46  in  FIG. 3 , and  56  in  FIG. 4 ) disposed on the upper parts of the third electrode and the electron emissive device. The electron emissive device can be formed of, for example, oxidized porous silicon (OPS). The OPS can be, for example, oxidized porous poly silicon (OPPS) or oxidized porous amorphous silicon (OPAS). The electron emissive device can be, for example, a carbon nanotube (CNT), and have a boron nitride bamboo shoot (BNBS) structure. However, the structure of the electron emissive device is not restricted thereto and can have various modifications. 
         [0044]    The electron emissive device of the electron emissive source  22  uses a hot cathode or a cold cathode as an electron source. A field emitter array (FEA) type electron emissive device, a surface conduction emitter (SCE) type electron emissive device, a metal-insulator-metal (MIM) type electron emissive device, a metal-insulator-semiconductor (MIS) type electron emissive device, a ballistic electron surface emitting (BSE) type electron emissive device, etc. use the cold cathode as the electron source. 
         [0045]    When the FEA type electron emissive device uses a material having a low work function or a high β function, the FEA type electron emissive device easily discharges electrons due to a difference between electric fields in vacuum and uses a tip structure having a sharp leading edge and a main material such as, for example, Mo or Si, a carbon-circle material such as graphite, diamond-like carbon (DLC), and the like, and a nanomaterial such as a nanotube or a nanowire. 
         [0046]    The SCE type electron emissive device provides a conductive thin film between the third and fourth electrodes facing each other, and a hair crack to the conductive thin film to form the electron emissive source  22 . A voltage is applied to the third and fourth electrodes, a current flows to the surface of the conductive thin film, and electrons are discharged from the electron emissive source which is the hair crack. 
         [0047]    The MIM type and MIS type electron emissive devices form the electron emissive source  22  having a MIM structure and MIS structure, respectively, and discharge electrons by moving and accelerating the electrons from a metal or a semiconductor having a high electron potential to another metal having a low electron potential when a voltage is applied between two metals or the metal and the semiconductor, which are spaced apart from each other by a dielectric layer. 
         [0048]    The BSE type electron emissive device forms an electron supply layer (corresponding to the electron emissive device) forming a metal or a semiconductor on an ohmic electrode, and an insulation layer and a metal thin film on the electron supply layer, and discharge electrons by applying power to the ohmic electrode and the metal thin film using the principle that if the size of a semiconductor is reduced to lower than a mean free path of electrons of the semiconductor, the electrons do not scatter but instead travel. 
         [0049]    The phosphor layer  18  is formed on the first substrate  10 . When the fifth electrode  16  that collects electrons discharged into the discharge cell  25  is disposed on the first substrate  10 , the phosphor layer  18  can be formed on the fifth electrode  16  as illustrated in  FIG. 1 . Apart from  FIG. 1 , the fifth electrode  16  and the phosphor layer  18  can be disposed in a groove formed on the first substrate  10 . Also, the phosphor layer  18  can cover the sidewalls of the barrier ribs  15 . The location of the phosphor layer  18  is not restricted to that illustrated in  FIG. 1 . 
         [0050]    A gas is charged in the discharge cell  25 . The gas is used to perform a discharge in the discharge cell  25  and is hereinafter referred to as a discharge gas. The discharge gas can be a mixture with Xe gas at about 10% of the mixture, and one or more of Ne gas, He gas, and Ar gas each at about 10% of the mixture. 
         [0051]    The barrier ribs  15  can be formed of a dielectric substance that prevents the first and second electrodes  12  and  14  from sending a current therebetween, and prevents the first and second electrodes  12  and  14  from being damaged due to collisions between charge particles and the first and second electrodes  12  and  14 , thereby accumulating wall charges by inducing the charge particles. The dielectric substance may be, for example, PbO, B 2 O 3 , SiO 2 , and the like. 
         [0052]    Since a predetermined voltage is applied to the first and second electrodes  12  and  14 , respectively, to perform the discharge, the first and second electrodes  12  and  14  may be formed of, for example, Ag, Cu, Cr, etc. or other materials having a high electric conductivity. 
         [0053]    The phosphor layer  18  is formed by coating a phosphor paste that is a mixture of one of a red light emitting phosphor substance, a green light emitting phosphor substance, and a blue light emitting phosphor substance, a solvent, and a binder to the groove of the first substrate  10 , and drying and baking the coated groove. The red light emitting phosphor substance can be, for example, Y(V,P)O 4 :Eu, etc, the green light emitting phosphor substance can be, for example, Zn 2 SiO 4 :Mn, YBO 3 :Tb, etc., and the blue light emitting phosphor substance can be, for example, BAM:Eu, etc. 
         [0054]    A second protective layer (not shown) formed of, for example, MgO can be formed on the entire surface of the phosphor layer  18 . When the discharge is performed in the discharge cell  25 , the second protective layer prevents the phosphor layer  18  from being deteriorated due to collisions of discharge particles, and discharges secondary electrons to facilitate the discharge. 
         [0055]      FIG. 2  is a cross-sectional view of the electron emissive source  22  of the display device illustrated in  FIG. 1  according to an embodiment. 
         [0056]    Referring to  FIG. 2 , the electron emissive source  22  is a FEA type electron emissive source. The electron emissive source  22  comprises the third electrode  32 , the electron emissive device  38 , the insulation layer  34 , and the fourth electrode  36 . The third electrode  32  is disposed on the second substrate  20 . The electron emissive device  38  electrically connected to the third electrode  32  is disposed on the third electrode  32 . The fourth electrode  36  is disposed on the upper part of the insulation layer  34  so that the fourth electrode  36  can be insulated from the third electrode  32  and the electron emissive device  38 . In detail, the insulation layer  34  and the fourth electrode  36  are disposed on the second substrate  20  and a groove is formed in the second substrate  20 , so that the third electrode  32  and the electron emissive device  38  are sequentially disposed therebetween. The electron emissive device  38  can be a Spindt type micro tip. 
         [0057]    Since the third electrode  32  and the fourth electrode  36  perform addressing, they may extend to cross each other. The electron emissive device  38  discharges a plurality of electrons into the discharge cell  25  due to a difference between electric potentials applied to the third electrode  32  and the fourth electrode  36 . 
         [0058]    The third electrode  32  serves as a cathode electrode, and the fourth electrode  36  serves as a gate electrode, which is called an under-gate structure. Although not shown, another under-gate structure in which the cathode electrode and the electron emissive device  38  electrically connected to the cathode electrode can be disposed on the upper parts of the gate electrode and the insulation layer can be realized as the electron emissive source  22 . 
         [0059]      FIG. 3  is a cross-sectional view of an electron emissive source of the display device illustrated in  FIG. 1  according to another embodiment. 
         [0060]    Referring to  FIG. 3 , the electron emissive source of the current embodiment is a BSE type electron emissive source. The electron emissive source comprises the third electrode  42  and an electron supply layer formed on the third electrode  42 , e.g., the electron emissive device  62 , and the fourth electrode  46  disposed on the upper part of the electron emissive device  63 . 
         [0061]    More specifically, pillar-shaped polysilicon structures  65  are formed on the upper part of the third electrode  42 , and porous nanocrystal structures  63  are disposed between the pillar-shaped polysilicons  65 . The nanocrystal structures  63  can be formed of OPS. The OPS can be OPPS or OPAS. The fourth electrode  46  is formed on the nanocrystal structures  63 . If a voltage having a predetermined difference between electric potentials is applied to the third electrode  42  and the fourth electrode  46 , respectively, electrons entering into the nanocrystal structures  63  are accelerated without collisions and discharged into the discharge cell  25  (ballistic electron emission). The fourth electrode  46  may be formed of a thin film to discharge electrons. 
         [0062]      FIG. 4  is a cross-sectional view of an electron emissive source of the display device illustrated in  FIG. 1  according to another embodiment. 
         [0063]    Referring to  FIG. 4 , a plurality of perpendicularly arranged carbon nanotubes  58  are disposed on the third electrode  52  as an electron emissive device, and the fourth electrode  56  is formed on the upper part of the carbon nanotubes  58 . When the carbon nanotubes  58  are formed of a metal, an insulation layer (not shown) may be further formed on the surfaces of the carbon nanotubes  58  so that the carbon nanotubes  58  can be insulated from the fourth electrode  56 . If a voltage having a predetermined difference between electric potentials is applied to the third electrode  52  and the fourth electrode  56 , respectively, the carbon nanotubes  58  transfer ballistic electrons to discharge electrons into the discharge cell  25 . The fourth electrode  56  may be formed of a thin film to discharge electrons. 
         [0064]      FIG. 5  illustrates an arrangement of electrodes of a display device according to an embodiment. 
         [0065]    Referring to  FIG. 5 , the display device of the current embodiment comprises third electrodes and fourth electrodes that cross each other in an electron emissive source to perform addressing, and first electrodes and second electrodes in which a sustain discharge is performed in an addressed discharge cell. 
         [0066]    Hereinafter, the first, second, third, and fourth electrodes are Y, X, C (cathode), and G (gate) electrodes, respectively. Since a sustain pulse may be applied to at least one of the Y and X electrodes to perform the sustain discharge, the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n  are parallel to each other. Since a scan pulse is applied to the C electrodes C 1  through Cn and a display data signal is applied to the G electrodes G 1  through G m  to perform addressing, the C electrodes C 1  through C n  may be parallel to the X electrodes X 1  through X n  and the Y electrodes Y 1  through Y n , and the G electrodes G 1  through G m  may extend to cross the X electrodes X 1  through X n , the Y electrodes Y 1  through Y n , and the C electrodes C 1  through C n . The X electrodes X 1  through X n , the Y electrodes Y 1  through Y n , and the C electrodes C 1  through C n  are spaced apart from one another in order to express that the X electrodes X 1  through X n , the Y electrodes Y 1  through Y n , and the C electrodes C 1  through C n  are disposed between the first substrate  10  and the second substrate  20 . Apart from the drawing, the G electrodes G 1  through Gm can be parallel to the X electrodes X 1  through X n  and the Y electrodes Y 1  through Y n , and the C electrodes C 1  through C n  can extend to cross the X electrodes X 1  through X n  and the Y electrodes Y 1  through Y n , and G electrodes G 1  through G m . 
         [0067]      FIG. 6  is a timing diagram of driving signals applied to each of the electrodes illustrated in  FIG. 5  according to an embodiment. 
         [0068]    To drive the display device of the present embodiments, a unit frame (60 Hz under the national television system committee (NTSC) and 50 Hz under the phase alternation by line system (PAL)) to express an image is divided into a plurality of sub fields, and a gradation is allocated to each sub field. Each sub field is divided into an address period PA and a sustain period PS. In the address period PA, a discharge cell is selected to be turned on and off from all the discharge cells to express the gradation. In the sustain period PS, a sustain discharge is performed in discharge cells that are selected to be turned on according to a gradation allocated to each of the sub fields. 
         [0069]    Referring to  FIG. 6 , in the address period PA, a scan pulse is sequentially applied to the C electrodes C 1  through C n . The scan pulse sequentially having a high level and a low level is applied to the C electrodes C 1  through C n . A display data signal used to select a discharge cell is applied to the G electrodes G 1  through G m  in accordance with the scan pulses. The display data signal has a positive address voltage. In detail, the display data signal has an address voltage having the high level when the discharge cell is selected, and an address voltage having the low level (a ground voltage) when the discharge cell is not selected. An electron emissive source of a discharge cell to be turned on discharges a plurality of electrons into the discharge cell due to the application of the scan pulse and the display data signal. Also, in the address period PA, a bias voltage is applied to the X electrodes X 1  through X n  to hold electrons as wall charges. 
         [0070]    In the sustain period PS, a sustain pulse sequentially having a high level and a low level is applied to the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n . The length of the sustain pulse is determined according to the gradation weight. The gradation is expressed by performing a sustain discharge due to the electrons discharged in the address period PA and a difference in electric potentials between the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n . Although a difference in electric potentials between the high level and the low level of the sustain pulse, e.g., the difference in electric potentials between the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n , is smaller than a discharge start voltage, the sustain discharge is performed due to the electrons discharged into the discharge cell of the electron emissive source in the address period PA. 
         [0071]      FIG. 7  is a timing diagram of driving signals applied to each of the electrodes illustrated in  FIG. 5  according to another embodiment. In comparison with the previous embodiment of  FIG. 6  and the current embodiment of  FIG. 7 , both embodiments are identical to each other except that different driving signals are applied to the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n  in the sustain period PS and the bias voltage is applied to the Y electrodes Y 1  through Y 1  in the address period PA to hold electrons as wall charges. 
         [0072]    Since it is sufficient that a predetermined difference in electric potentials between the Y electrodes Y 1  through Y n  and the X electrodes X 1  through X n  is lower than the discharge start voltage to perform the sustain discharge, a sustain pulse sequentially having a high level and a low level is applied to the Y electrodes Y 1  through Y n . The sustain pulse may sequentially have a positive voltage and a negative voltage. An intermediate level of the high level and the low level of the sustain pulse applied to the Y electrodes Y 1  through Y n  is applied to the X electrodes X 1  through X n . That is, a ground voltage may be applied to the X electrodes X 1  through X n . 
         [0073]    Although not shown in  FIGS. 7 and 8 , when the display device is driven, it is possible to further perform a reset period where all the discharge cells are initialized before the address period PA of each sub field. A variety of driving signals can be applied in the reset period. For example, a reset pulse having a rising ramp and a falling ramp can be applied to the scan electrodes Y 1  through Y n . It is possible to use a selective reset method (application of a falling ramp pulse) of initializing a discharge cell in which the sustain discharge is performed in the sustain period of a previous sub field. 
         [0074]    As described above, the display device of the present embodiments uses an electric field emissive principle to perform addressing, and applies a sustain pulse to perform a sustain discharge, it is possible to address problems of a brightness deterioration caused by non-uniform characteristics in a manufacturing process of an electron emissive source of a FED and an address discharge delay of the PDP. That is, an address period is reduced and brightness uniformity is improved regardless of the manufacturing process. 
         [0075]    While the present embodiments have been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present embodiments as defined by the following claims.