Patent Publication Number: US-2007109230-A1

Title: Electron emission display device and method of driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0086542, filed on Sep. 15, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
     BACKGROUND  
      1. Field of the Invention  
      The present invention relates to an electron emission display device and a method of driving the same, and more particularly, to an electron emission display device capable of reducing or preventing non-uniformity in color from being generated in processes of controlling a gate voltage and a cathode voltage and a method of driving the same.  
      2. Discussion of Related Art  
      In general, electron emission devices used for electron emission display devices can be classified into electron emission devices in which hot cathode rays are used as electron sources and electron emission devices in which cold cathode rays are used as electron sources. The electron emission devices in which the cold cathodes are used include field emitter array (FEA) type electron emission devices, surface conduction emitter (SCE) type electron emission devices, metal-insulator-metal (MIM) type electron emission devices, metal-insulator-semiconductor (MIS) type electron emission devices, and ballistic electron surface emitting (BSE) type electron emission devices.  
      The FEA type electron emission device uses material having a low work function or a high β function as an electron emission source so that electrons are emitted under vacuum due to difference in electric fields. A device in which an electron emission source is formed of a pointed tip structure, carbon material, or nano material has been developed.  
      In the SCE type electron emission device, a conductive thin film is provided between two electrodes arranged on substrates to face each other and minute cracks are provided in the conductive thin film so that an electron emission unit is formed. In the SCE type electron emission device, a voltage is applied to the electrodes so that current flows to the surface of the conductive thin film and that electrons are emitted from the electron emission unit that is a minute gap.  
      In the MIM type and MIS type electron emission devices, electron emission units having MIM and MIS structures are formed. When a voltage is applied between two metals or metal and semiconductor with an dielectric layer interposed, electrons are emitted while moving and being accelerated from the metal or semiconductor having high electron potential toward the metal having low electron potential.  
      In the BSE type electron emission device, an electron supply layer formed of metal or semiconductor is formed on an ohmic electrode and an insulating layer and a metal thin film are formed on the electron supply layer so that electrons are emitted by applying a power source to the ohmic electrode and the metal thin film in accordance with a principle in which electrons are not scattered but travel when the size of semiconductor is reduced to be smaller than the mean free patch of the electrons in the semiconductor.  
      The above-described electron emission devices can be used in various fields and have recently been actively studied due to their advantages in that they operate by emission of cathode electrode lines (self light sources, high efficiency, high brightness, wide brightness regions, natural colors, high color purity, and wide view angles) like the CRTs and that they have high operation speed and wide operation temperature regions.  
       FIG. 1  illustrates a structure of a conventional electron emission display device. Referring to  FIG. 1 , the electron emission display device includes a display portion  10 , a data driver  20 , and a scan driver  30 .  
      In the display portion  10 , pixels are located in regions defined by the crossings (or intersections) between cathode electrodes C 1 , C 2 , . . . , and Cm and gate electrodes G 1 , G 2 , . . . , and Gn. Each of the pixels includes an electron emission unit of an electron emission device. Electrons emitted from the electron emission units and the cathode electrodes collide with anode electrodes so that phosphors emit light to display gray scale images. The gray levels of the displayed images vary in accordance with the values of input digital image signals. To control the gray levels displayed in accordance with the values of the digital image signals, a pulse width modulation method or a pulse amplitude modulation method may be used.  
      Here, carbon nanotubes (CNT) having a high self emission efficiency are used as the electron emission unit.  
      The data driver  20  is connected to the cathode electrodes C 1 , C 2 , . . . , and Cm to generate data signals and to transmit the generated data signals to the display portion  10  so that the display portion  10  emits light corresponding to the data signals.  
      The scan driver  30  is connected to the gate electrodes G 1 , G 2 , . . . , and Gn to generate scan signals and to transmit the generated scan signals to the display portion  10  so that the display portion  10  sequentially emits light using a line scan method, in units of horizontal lines, with uniform time period to display an entire image on the display portion  10 . Therefore, the electron emission display device of  FIG. 1  is a light emitting display device.  
      Here, when an image of high brightness is displayed, a large amount of current flows through the display portion  10  so that a large amount of load is applied to the display portion  10 , thereby requiring a power source having high output. Therefore, the power consumption of the electron emission display device (or the light emitting display device) increases.  
      Also, when an image having low brightness is displayed, the brightness of the display portion  10  is reduced so that contrast may deteriorate.  
     SUMMARY OF THE INVENTION  
      Accordingly, an embodiment of the present invention provides an electron emission display device capable of reducing or preventing non-uniformity in color from being generated due to difference in brightness characteristics of red, blue, and green light (or color) components in processes of controlling a cathode voltage and a gate voltage to correspond to the entire brightness of a display portion, and a method of driving the same.  
      In an embodiment of the present invention, there is provided an electron emission display device including: a display portion having a plurality of pixels adapted to emit light in accordance with data signals and scan signals applying a voltage to first electrodes and second electrodes, the plurality of pixels including red pixels, blue pixels, and green pixels; a data driver adapted to receive image signals to generate the data signals and to transmit the data signals to the display portion; a scan driver adapted to generate the scan signals and to transmit the scan signals to the display portion; and a color controlling unit adapted to control a voltage of the first electrodes to correspond to the image signals and to correct the image signals to correspond to emission rates of the red pixels, the blue pixels, and the green pixels in accordance with a change in the voltage of the first electrodes so that the corrected image signals are transmitted to the data driver.  
      According to another embodiment of the present invention, there is provided an electron emission display device including: a display portion having a plurality of pixels adapted to emit light in accordance with data signals and scan signals applying a voltage to first electrodes and second electrodes, the plurality of pixels including red pixels, blue pixels, and green pixels; a color controlling unit adapted to correct image signals using red, blue, and green correction coefficients associated with data signals adapted to display gray scale images and to determine the red correction coefficient corresponding to the red pixels, the blue correction coefficient corresponding to the blue pixels, and the green correction coefficient corresponding to the green pixels to correspond to a voltage of the first electrodes; a data driver adapted to control emission times of the red pixels, the blue pixels, and the green pixels using the corrected image signals output from the color controlling unit to display the gray scale images; and a scan driver adapted to generate the scan signals to transmit the scan signals to the display portion.  
      According to yet another embodiment of the present invention, there is provided a method of driving an electron emission display device including pixels adapted to generate data signals using image signals and to emit red, blue, and green light components in accordance with difference in a voltage between first electrodes and second electrodes corresponding to the data signals, the pixels including red pixels, blue pixels, and green pixels. The method includes: adding the image signals with each other to control a voltage of the first electrodes to correspond to the image signals added with each other; determining a red correction coefficient corresponding to the red pixels, a blue correction coefficient corresponding to the blue pixels, and a green correction coefficient corresponding to the green pixels; and correcting the image signals by the red, blue, and green correction coefficients to generate the data signals using the corrected image signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.  
       FIG. 1  illustrates a structure of a conventional electron emission display device;  
       FIG. 2  illustrates a structure of an electron emission display device according to an embodiment of the present invention;  
       FIG. 3  is a graph illustrating a change in brightness of a pixel of the electron emission display device illustrated in  FIG. 2  in accordance with a voltage (or voltage level) of a gate electrode;  
       FIG. 4  illustrates a structure of a color controlling unit used for the electron emission display device of  FIG. 2 ;  
       FIG. 5  illustrates a structure of a voltage controlling unit illustrated in  FIG. 4 ;  
       FIG. 6  is a flowchart illustrating processes of generating data signals by an electron emission display device according to an embodiment of the present invention;  
       FIG. 7  is a perspective view illustrating a display portion of the electron emission display device illustrated in  FIG. 2 ; and  
       FIG. 8  is a sectional view illustrating a section of the display portion of the electron emission display device illustrated in  FIG. 2 .  
    
    
     DETAILED DESCRIPTION  
      In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments may be modified in various ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.  
       FIG. 2  illustrates a structure of an electron emission display device according to an embodiment of the present invention.  FIG. 3  is a graph illustrating a change in brightness of a pixel of the electron emission display device illustrated in  FIG. 2  in accordance with a voltage (or voltage level) of a gate electrode. Referring to  FIGS. 2 and 3 , the electron emission display device includes a display portion  100 , a data driver  200 , a scan driver  300 , and a color controlling unit  400 .  
      In the display portion  100 , a plurality of cathode electrodes C 1 , C 2 , . . . , and Cm are arranged to extend in a column direction, a plurality of gate electrodes G 1 , G 2 , . . . and Gn are arranged to extend in a row direction, and electron emission units are located in regions defined by the crossings (or the intersections) between the cathode electrodes C 1 , C 2 , . . . , and Cm and the gate electrodes G 1 , G 2 , . . . , and Gn to form pixels  101 . In other embodiments, the gate electrodes G 1 , G 2 , . . . , and Gn may be arranged to extend in the column direction and the cathode electrodes C 1 , C 2 , . . . , and Cm may be arranged to extend in the row direction. Hereinafter, it is assumed that the cathode electrodes C 1 , C 2 , . . . , and Cm are arranged to extend in the column direction and the gate electrodes G 1 , G 2 , . . . , and Gn are arranged to extend in the row direction. The display portion  100  reduces a difference in voltage between the gate electrodes G 1 , G 2 , . . . , and Gn and the cathode electrodes C 1 , C 2 , . . . , and Cm to reduce the brightness of each pixel when the number of pixels  101  that emit light with high brightness is relatively large and increases a difference in voltage between the gate electrodes G 1 , G 2 , . . . , and Gn and the cathode electrodes C 1 , C 2 , . . . , and Cm to increase the brightness of each pixel when the number of pixels  101  that emit light with high brightness is relatively small. Therefore, when the number of pixels  101  that emit with high brightness is relatively large, the brightness of the display portion  100  is reduced so that power consumption is reduced. When the number of pixels  101  that emit light with high brightness is relatively small, the brightness of the pixels that emit light with high brightness increases so that a difference in brightness between the pixels that emit light with high brightness and the pixels that emit light with low brightness is large, thereby improving contrast. Also, when difference in voltage between the gate electrodes G 1 , G 2 , . . . , and Gn and the cathode electrodes C 1 , C 2 , . . . and Cm changes, a change in brightness of each of red, blue, and green pixels varies in accordance with the deviation in emission efficiencies of the red, blue, and green pixels so that non-uniformity in color may be generated.  
      The data driver  200  generates data signals using image signals and is connected to the cathode electrodes C 1 , C 2 , . . . , and Cm to transmit the data signals to the cathode electrodes C 1 , C 2 , . . . , and Cm. The data driver  200  determines the emission time of the pixels  101  located in the regions defined by the crossings (or the intersections) between the selected gate electrodes G 1 , G 2 , . . . , and Gn and cathode electrodes C 1 , C 2 , . . . , and Cm by using the data signals.  
      The scan driver  300  is connected to the gate electrodes G 1 , G 2 , . . . , and Gn to select one or more of the gate electrodes G 1 , G 2 , . . . , and Gn arranged in the row direction so that scan signals are transmitted to the pixels  101  connected to the selected gate electrodes G 1 , G 2  .. . . , and Gn.  
      The color controlling unit  400  controls image data in accordance with the emission rates of the pixels that emit red, blue, and green light components so that the brightness compensation ranges of the red, blue, and green pixels vary to reduce or prevent non-uniformity in color. The brightness of the red, blue, and green pixels changes in accordance with a change in voltage difference between the cathode electrodes and the gate electrodes. Although the voltages of the cathode electrodes and the gate electrodes are applied to the red, blue, and green pixels, when a difference in voltage between the cathode electrodes and the gate electrodes varies, the ratio at which the brightness of each of the red, blue, and green pixels increases varies in accordance with the emission rates of the red, blue, and green pixels as illustrated in  FIG. 3 . Therefore, when the emission rates of the red, blue, and green pixels are controlled in accordance with a conventional white balance control method, white balance is not maintained. Therefore, in one embodiment of the present invention, the emission rates of the red, blue, and green pixels are controlled to correspond to a difference in voltage between the cathode electrodes and the gate electrodes of the red, blue, and green pixels so that the white balance is maintained. In  FIG. 3 , dotted lines are graphs illustrating initial brightness increase rates of the red, blue, and green pixels and solid lines are graphs illustrating brightness increase rates of the red, blue, and green pixels after the difference in voltage between the cathode electrodes and the gate electrodes is controlled.  
       FIG. 4  illustrates the structure of the color controlling unit  400  used for the electron emission display device of  FIG. 2 . Referring to  FIG. 4 , the color controlling unit  400  includes an image signal input and conversion unit  410 , a voltage controlling unit  420 , a coefficient look up table  430 , and an image signal operating unit  440 .  
      The image signal input and conversion unit  410  receives image signals and corrects the received image signals to output the corrected image signals. The red, blue, and green image signals are digital signals that are used to display gray scale values and are corrected by multiplying the image signals with correction coefficients for brightness deviation in accordance with a change in voltages of the cathode electrodes and the gate electrodes from the coefficient look up table  430 . The image signal input and conversion unit  410  corrects the input image signals to transmit the corrected image signals to the image signal operating unit  440 .  
      The voltage controlling unit  420  controls the voltage of the gate electrodes in accordance with the magnitude of the input image signals so that a difference in voltage between the gate electrodes and the cathode electrodes changes. The number of pixels that emit light with high brightness is relatively large when the magnitude of the image signals input to the display portion  100  is relatively large, and the number of pixels that emit light with high brightness is relatively small when the magnitude of the image signals input to the display portion  100  is relatively small. Therefore, after the voltage of the gate electrodes has been stored to correspond to the magnitude of the image signals and the magnitude of the image signals has been determined, a voltage control signal corresponding to the changed voltage of the gate electrodes is transmitted to the coefficient look up table  430 . Here, the magnitude of the image signals refers to the sum of the image signals input in the time period (one horizontal period) of one frame.  
      The coefficient look up table  430  stores red, blue, and green correction coefficients corresponding to each voltage of the gate electrodes, receives the voltage control signal from the voltage controlling unit  420 , selects a correction coefficient corresponding to the voltage control signal, and transmits the correction coefficient to the image signal input and conversion unit  410 . Therefore, when the voltage of the gate electrodes is changed, a correction coefficient corresponding to the changed voltage of the gate electrodes is transmitted to the image signal input and conversion unit  410 .  
      The image signal operating unit  440  corrects the red, blue, and green image signals using the correction coefficients and divides the corrected red, blue, and green image signals by the correction coefficients so that the red image signal is divided by the largest correction coefficient among the red correction coefficients, that the blue image signal is divided by the largest correction coefficient among the blue correction coefficients, and that the green image signal is divided by the largest correction coefficient among the green correction coefficients to generate red, blue, and green brightness change ratios.  
      Therefore, the red image signal is corrected in accordance with the red brightness change ratio, the blue image signal is corrected in accordance with the blue brightness change ratio, and the green image signal is corrected in accordance with the green brightness change ratio. The corrected image signals are transmitted to the data driver  200  so that the data driver  200  controls pulse width in accordance with the corrected image signals to display gray scale images.  
      Therefore, each of the red, blue, and green image signals corrects brightness that non-linearly increases in accordance with increase in voltages of the gate electrodes and the cathode electrodes by each of the red, blue, and green emission efficiencies to control the white balance.  
       FIG. 5  illustrates a structure of the voltage controlling unit  420  illustrated in  FIG. 4 . Referring to  FIG. 5 , the voltage controlling unit  420  includes a data summing unit  421  and a voltage look up table  422 .  
      The data summing unit  421  determines the sum of the image signals input in the time period of one frame. The magnitude of the image signals is large when high gray levels are displayed and is small when low gray levels are displayed. Therefore, it is determined that the number of pixels that emit light with high brightness is large when the sum of the image signals is large and that the number of pixels that emit light with high brightness is small when the sum of the image signals is small.  
      The voltage look up table  422  designates the voltage of the gate electrodes corresponding to the sum of the image signals so that the voltage of the gate electrodes corresponds to the sum of the image signals on a one-to-one basis. Therefore, when the sum of the image signals is calculated by the data summing unit, the voltage of the gate electrodes corresponding to the sum of the image signals is extracted and the extracted voltage of the gate electrodes is transmitted to the coefficient look up table  430 . The coefficient look up table  430  determines the correction coefficient corresponding to the voltage of the gate electrodes determined by the voltage look up table  422 .  
       FIG. 6  is a flowchart illustrating processes of generating data signals by an electron emission display device according to an embodiment of the present invention.  
      Referring to  FIG. 6 , in the first step (ST 100 ), image signals input during the time period of one frame are added with each other, and the voltage of the gate electrodes corresponds to the magnitude of the sum of the image signals. The voltage of the gate electrodes corresponding to the sum of the image signals input during the time period of one frame is stored in the voltage look up table  422  so that the voltage of the gate electrodes is determined by the voltage look up table  422  when the image signals are added with each other.  
      In the second step (ST 110 ), red, blue, and green correction coefficients corresponding to the voltage of the gate electrodes are determined by the coefficient look up table  430  so that the correction coefficients are applied to the image signals to correct the image signals. Here, the red, blue, and green correction coefficients corresponding to the voltage of the gate electrodes are stored in the coefficient look up table  430 . The image signals are corrected by multiplying the image signals by the respective correction coefficients and then, dividing the image signals corrected by the correction coefficients by the largest correction coefficients among the stored red, blue, and green correction coefficients so that the image signals are corrected in a uniform ratio.  
      In the third step (ST 120 ), the emission time of the red, blue, and green pixels is controlled by the image signals corrected by the multiplication and division operations so that the white balance is controlled in accordance with the emission time.  
       FIG. 7  is a perspective view illustrating a display portion of the electron emission display device illustrated in  FIG. 2 .  FIG. 8  is a section of the display portion of the electron emission display device illustrated in  FIG. 2 . Referring to  FIGS. 7 and 8 , the electron emission display device includes a bottom substrate  110 , a top substrate  190 , and spacers  180 . Cathode electrodes  120 , an insulating layer  130 , electron emission units  140 , and gate electrodes  150  are formed on the bottom substrate  110 . A front surface substrate, anode electrodes, and a fluorescent layer are formed on the top substrate  190 .  
      The cathode electrodes  120  are formed on the bottom substrate  110  in stripes and the insulating layer  130  has a plurality of first grooves  131  to expose parts of the cathode electrodes  120  and the emission units  140  positioned on the exposed parts of the cathode electrodes  120 . The gate electrodes  150  are formed on the insulating layer  130 . A plurality of second grooves  151  of a uniform size are formed in the gate electrodes  150  and the second grooves  151  are formed on the first grooves  131 . The electron emission units  140  are positioned on the cathode electrodes  120  in the regions where the first grooves  131  coincide with the second grooves  151 .  
      A glass or silicon substrate is used as the bottom substrate  110 . When the electron emission units  140  are formed using a carbon nanotube (CNT) paste through a rear surface light exposing process, the bottom substrate  110  may be formed by a transparent substrate such as the glass substrate.  
      The cathode electrodes  120  supply the data signals or the scan signals applied from a data driver (e.g., the data driver  200  of  FIG. 2 ) or a scan driver (e.g., the scan driver  300  of  FIG. 2 ) to the electron emission units  140 . The cathode electrodes  120  are formed of indium tin oxide (ITO).  
      The insulating layer  130  is formed on both the bottom substrate  110  and the cathode electrodes  120  to electrically insulate the cathode electrodes  120  from the gate electrodes  150 .  
      The gate electrodes  150  are arranged on the insulating layer  130  in a shape (e.g., a predetermined shape, such as stripes) to cross (or intersect) the cathode electrodes  120  and to supply the data signals or the scan signals applied from the data driver  200  or the scan driver  300  to the pixels. The gate electrodes  150  are formed of at least one conductive metal selected from metals having high conductivity such as Au, Ag, Pt, Al, and Cr and alloys thereof.  
      The electron emission units  140  are electrically connected to the cathode electrodes  120  exposed by the first grooves  131  of the insulating layer  130  and, in one embodiment, are formed of materials that emit electrons when an electrical field is applied, such as carbon based material or nanometer (nm) sized material (e.g., carbon nanotube, graphite, graphite nanofiber, diamond-like carbon, C 60 , silicon nanowire, and combinations thereof).  
      The top substrate  190  includes the fluorescent layer so that light is emitted when electrons collide with the fluorescent layer and includes the anode electrodes so that electrons emitted from the electron emission units  140  collide with the top substrate  190 .  
      The spacers  180  ensure that the bottom substrate  110  and the top substrate  190  are separated from each other by a uniform distance.  
      In view of the foregoing, an electron emission display device of an embodiment of the present invention and/or a method of driving the same can reduce or prevent non-uniformity in color from being generated in accordance with change in difference in voltage between the gate electrodes and the cathode electrodes and/or can reduce brightness when needed to reduce power consumption.  
      While the invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof.