Patent Publication Number: US-7221099-B2

Title: Electron emission device and driving method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0056825 filed on Jun. 29, 2005 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electron emission device, and in particular, to an electron emission device which prevents the luminance from being varied when displaying moving images by inhibiting the delay of emission. 
   2. Description of Related Art 
   Generally, electron emission devices are classified into those using hot cathodes as an electron emission source, and those using cold cathodes as the electron emission source. There are several types of cold cathode electron emission devices, including a field emitter array (FEA) type, a metal-insulator-metal (MIM) type, a metal-insulator-semiconductor (MIS) type, and a surface conduction emitter (SCE) type. 
   The FEA type of electron emission device is based on the principle that when a material having a low work function or a high aspect ratio is used as an electron emission source, electrons are easily emitted from the material due to the electric field in a vacuum atmosphere. A sharp-pointed tip structure based on molybdenum (Mo) or silicon (Si), or a carbonaceous material, such as carbon nanotube, graphite and diamond-like carbon, has been developed to be used as electron emission regions. 
   In common FEA type electron emission devices, cathode electrodes, an insulating layer and gate electrodes are sequentially formed on a first substrate, and openings are formed at the gate electrodes and the insulating layer. Electron emission regions are formed on the cathode electrodes within the openings. Phosphor layers and an anode electrode are formed on a surface of a second substrate facing the first substrate. 
   The cathode electrodes supply the electric current required for emitting electrons to the electron emission regions, and the gate electrodes control the electron emission using the voltage difference thereof from the cathode electrodes. The anode electrode receives a direct current (DC) voltage of several hundred to several thousand volts, and keeps the phosphor layers in a high potential state, thereby effectively accelerating the electrons emitted from the electron emission regions toward the phosphor layers. 
   Commonly, the gate electrodes are used as scan electrodes, and the cathode electrodes are used as data electrodes for carrying the image data. 
   Scan pulses are sequentially applied to the gate electrodes, and data pulses are selectively applied to the cathode electrodes corresponding to the gate electrodes receiving the scan pulses. Electric fields are formed around the electron emission regions at the pixels where the voltage difference between the two electrodes exceeds a threshold value, and electrons are emitted from the electron emission regions. The emitted electrons are attracted by the high voltage applied to the anode electrode, and collide against the corresponding phosphor layers, thereby light-emitting them. 
   With common electron emission devices, when the presence and the amount of electron emission are quickly and correctly controlled in accordance with the driving signals applied to the cathode and the gate electrodes, accurate displaying in accordance with image signals can be accomplished. The amount of electron emission is observed by the emission current reaching the anode electrode. The amount of electron emission and the emission current will hereinafter be used interchangeably. 
   The above problem can be exacerbated when the electron emission device displays moving images or the images are shifted while inducing considerably large-scaled variation in the emission current. For instance, when the images are shifted from the black mode to the white mode, the emission current should be quickly recovered in accordance with the driving signals. Otherwise, the luminance is deteriorated at the initial section of the white mode. 
   Rectangular wave pulses are typically applied to the cathode and the gate electrodes. The rectangular wave pulses involve relatively high voltages, and the larger the number of pixels involved, the shorter the application period of the ON time becomes. A signal distortion may result from a delay in the driving signals due to the parasitic capacitance generated at the crossed regions of the cathode and the gate electrodes, or from the internal resistance of the cathode and the gate electrodes. The delay of the driving signals results in an emission delay, and deteriorates the display quality. 
   SUMMARY OF THE INVENTION 
   In one exemplary embodiment of the present invention, an electron emission device inhibits the emission delay to reduce or minimize the luminance deterioration in displaying moving images and enhances the display quality. Another exemplary embodiment relates to a method of driving the electron emission device. 
   In one exemplary embodiment of the present invention, an electron emission device includes cathode electrodes, gate electrodes formed over the cathode electrodes, and an insulating layer disposed between the cathode electrodes and the gate electrodes. Electron emission regions are formed on the cathode electrodes to emit electrons under the application of the electric fields generated due to the difference between the voltages applied to the cathode electrodes and the gate electrodes. A driving unit applies voltages to the cathode and the gate electrodes. An anode electrode receives a positive voltage to accelerate the electrons emitted from the electron emission regions. The first voltage Vc applied to the cathode electrodes and the second voltage Vg applied to the gate electrodes satisfy the following condition: 0.4≦Vc/Vg&lt;0.8. 
   The driving unit respectively applies the first voltage and the second voltage to the cathode electrodes and the gate electrodes such that electrons are not emitted from the electron emission regions. 
   The driving unit applies pulses with the first voltage and a third voltage lower than the first voltage to the cathode electrodes while applying the second voltage to the gate electrode. The amount of electrons emitted from the electron emission regions is determined by the period of time during which the third voltage is applied to the cathode electrodes. 
   A focusing electrode is further formed over the gate electrodes to receive 0V or a negative voltage. 
   A method of driving an electron emission device is used with first electrodes, second electrodes insulated from the first electrodes, and electron emission regions formed on any one of the first and the second electrodes to emit electrons due to a difference between the voltages applied to the first and the second electrodes. A first voltage is applied to the first electrodes. Pulses with a second voltage and a third voltage are applied to the second electrodes while applying the first voltage to the first electrodes. The first voltage and the second voltage satisfy the condition of 0.4≦Vc/Vg&lt;0.8, where Vc indicates a lower one of the first and the second voltages, and Vg indicates a higher one of the first and the second voltages. 
   When the third voltage is applied to the second electrodes, electrons are emitted from the electron emission regions. 
   In one embodiment, the electron emission regions are formed at the second electrodes, Vc is the second voltage, and Vg being the first voltage. 
   An amount of electrons emitted from the electron emission regions may also be determined by the period of time during which the third voltage is applied. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an electron emission device according to an embodiment of the present invention. 
       FIG. 2  is a partial exploded perspective view of a display panel for the electron emission device according to an embodiment of the present invention. 
       FIG. 3  is a partial sectional view of a display panel according to an embodiment of the present invention. 
       FIG. 4  illustrates driving waveforms applied to cathode and gate electrodes for an electron emission device according to an embodiment of the present invention. 
       FIG. 5  is a graph illustrating the emission delay characteristics of an electron emission device according to an Example 1. 
       FIG. 6  is a graph illustrating the emission delay characteristics of an electron emission device according to an Example 2. 
       FIG. 7  is a graph illustrating the emission delay characteristics of an electron emission device according to a Comparative Example 1. 
       FIG. 8  is a graph illustrating the emission delay characteristics of an electron emission device according to a Comparative Example 2. 
       FIG. 9  is a graph illustrating the emission delay characteristics of an electron emission device according to a Comparative Example 3. 
       FIG. 10  is a graph illustrating the emission delay characteristics of an electron emission device according to a Comparative Example 4. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , an electron emission device includes a display panel  110 , a scan driving unit  140  for driving scan electrodes S 1 –Sn, a data driving unit  130  for driving data electrodes D 1 –Dm, and a control unit  150  for controlling the scan and the data driving units  140  and  130 . 
   The display panel  110  has n scan electrodes S 1 –Sn, and m data electrodes D 1 –Dm, and electron emission regions are provided at the respective crossed regions of the scan and the data electrodes S 1 –Sn and D 1 –Dm. 
   The control unit  150  receives image signals R, G and B, vertical synchronization signals V-SYNC and horizontal synchronization signals H-SYNC, and generates scan and data driving signals. The control unit  150  then applies those signals to the scan and the data driving units  140  and  130 . The scan driving unit  140  applies scan driving voltages to the scan electrodes S 1 –Sn based on the input scan driving signals, and the data driving unit  130  applies data driving voltages to the data electrodes D 1 –Dm based on the input data driving signals. 
   As shown in  FIGS. 2 and 3 , the display panel  110  includes first and second substrates  2  and  4  arranged parallel to each other and spaced apart from each other at a predetermined distance. An electron emission structure is provided at the first substrate  2 , and a light emission structure is provided at the second substrate  4  to emit visible rays and display the desired images. 
   Cathode electrodes  6  are stripe-patterned on the first substrate  2  in a first direction, and a first insulating layer  8  is formed on and covers substantially the entire area of the first substrate  2 . The first insulating layer  8  also covers the cathode electrodes  6 . Gate electrodes  10  are stripe-patterned on the first insulating layer  8  perpendicular to the first direction. 
   The crossed regions of the cathode and the gate electrodes  6  and  10  form unit pixels, and one or more electron emission regions  12  are formed on the cathode electrodes  6  per the respective unit pixels. Openings  14  are formed at the first insulating layer  8  and the gate electrodes  10  corresponding to the respective electron emission regions  12  while exposing the electron emission regions  12  on the cathode electrodes  6  patterned on the first substrate  2 . 
   The electron emission regions  12  are formed with a material emitting electrons when an electric field is applied under a vacuum atmosphere, such as a carbonaceous material or a nanometer-sized material. The electron emission regions  12  may be formed with, for example, carbon nanotube, graphite, graphite nanofiber, diamond, diamond-like carbon, C 60 , silicon nanowire, or a combination thereof, by way of, for example, screen-printing, direct growth, chemical vapor deposition, or sputtering. 
   As seen in  FIGS. 2 and 3 , electron emission regions  12  are formed with a circular shape, and are linearly arranged at the respective unit pixels along the length of the cathode electrodes  6 . However, the shape, number per unit pixel, and arrangement of the electron emission regions  12  are not limited to the illustrated, but may be altered in various manners. 
   A second insulating layer  16  and a focusing electrode  18  are formed on the gate electrodes  10  and the first insulating layer  8 . Openings  20  are formed at the second insulating layer  16  and the focusing electrode  18  to allow passage of the electron beams. For instance, the openings  20  may be provided in a one-to-one correspondence at the respective unit pixels. In this case, the focusing electrode collectively focuses the electrons emitted at the one unit pixel. 
   The focusing electrode  18  may cover substantially the entire area of the first substrate  2 . The focusing electrode  18  may be formed as a metallic film coated on the second insulating layer  16 , or as a metallic plate with opening portions attached to the second insulating layer  16 . 
   Red, green and blue phosphor layers  22 R,  22 G and  22 B are formed on a surface of the second substrate  4  facing the first substrate  2  while being spaced apart from each other by a particular distance. Black layers  24  are disposed between the respective phosphor layers  22  to enhance the screen contrast. 
   In this embodiment, an anode electrode  26  is formed on the phosphor layers  22  and the black layers  24  with an aluminum-based metallic material. The anode electrode  26  receives a voltage required to accelerate the electron beams, and reflects the visible rays radiated from the phosphor layers  22 , thereby increasing the screen luminance. 
   Alternatively, the anode electrode  26  may be formed with a transparent conductive material, such as indium tin oxide (ITO). In this case, the anode electrode is placed on a surface of the phosphor layers  22  and the black layers  24  facing the second substrate  4 , and may be patterned with a plurality of portions. 
   Spacers  28  are arranged between the first and the second substrates  2  and  4 , and the peripheries of the substrates  2  and  4  are sealed to each other using a sealing member, such as a glass frit. The inner space between the substrates  2  and  4  is exhausted and kept in a vacuum state, thereby forming the display panel  110 . The spacers  28  are located at the non-light emission area of the black layers  24 . 
   With the above structure, the cathode electrodes  6  are connected to the data driving unit  130  to receive data driving voltages therefrom, and the gate electrodes  10  are connected to the scan driving unit  140  to receive scan driving voltages therefrom. The focusing electrode  16  receives 0V or a negative (−) direct current (DC) voltage of several to several tens of volts, and the anode electrode  26  receives a positive (+) direct current (DC) voltage of several hundred to several thousand volts. 
     FIG. 4  illustrates the driving waveforms applicable to the electron emission device according to one embodiment of the present invention. 
   As shown in  FIG. 4 , an ON voltage V S  of the scan signal is applied to the scan electrode Sn within the period T 1 , and an ON voltage V 1 , of the data signal is applied to the data electrode Dm. Then, electrons are emitted from the electron emission regions due to the difference V S −V 1  of the voltages applied to the scan electrode Sn and the data electrode Dm. The emitted electrons collide against the phosphor layers to thereby light-emit them. 
   Thereafter, the ON voltage V S  of the scan signal is held at the scan electrode Sn within the period T 2 , and an OFF voltage VD of the data signal is applied to the data electrode Dm. Then, the difference V S −V D  of the voltages applied to the scan electrode Sn and the data electrode Dm is reduced, and hence, electrons are not emitted from the electron emission regions. The pulse width within the periods T 1  and T 2  may be varied to thereby express the desired gray scales. 
   An OFF voltage V 1 , of the scan signal is applied to the scan electrode Sn within the period T 3 , and an OFF voltage V 1 , of the data signal is applied to the data electrode Dm. Therefore, electrons are not emitted from the electron emission regions. The OFF voltage V 1  of the scan signal may be established to be the same as the ON voltage V 1 , of the data signal, such as 0V. 
   The ON/OFF of the emission current and the amount of emission current are controlled by the combination of the scan and the data pulses. Referring to  FIGS. 2–4 . the electrons emitted from the electron emission regions  12  are focused at the center of the bundle of electron beams by the focusing voltage while passing the focusing electrode  16 , and attracted by the anode voltage. The emitted electrons are accelerated toward the corresponding phosphor layers  22 , and collide against the phosphor layers  22 . In this process, the focusing electrode  16  and the second insulating layer  14  weaken the influence of the anode electric field with respect to the electron emission regions  12 . 
   The emission current can be controlled by forming electric fields due to the voltage difference between the gate and the cathode electrodes  10  and  6  without interference by external factors (such as an anode electric field) under the application of a predetermined voltage to the anode electrode. In this case, the difference V D  of the OFF voltage of the data signal from the ON voltage V S  of the scan signal, that is, the turn-off voltage, is a factor in determining the driving characteristic of the electron emission device. 
   In one embodiment, when the ON voltage V S  of the scan signal applied to the gate electrode is indicated by Vg, and the OFF voltage V D  of the data signal applied to the cathode electrode is indicated by Vc, the values of Vg and Vc satisfy the following condition (Formula 1): 0.4≦Vc/Vg&lt;0.8. 
     FIGS. 5 to 10  are graphs illustrating the emission delay characteristic as a function of the variation in Vc/Vg. The horizontal axis of the graph indicates time, and the vertical axis thereof indicates emission current. The emission delay experiments were conducted by measuring the emission current per one minute while realizing a white mode for ten (10) minutes, then a black mode for one (1) minute, and then shifting back into a white mode for the remainder of the experiment. 
     FIG. 5  illustrates the experimental results obtained according to an Example 1, where Vc/Vg was 0.4. The value of Vg was established to be 50V, and the value of Vc was established to be 20V. As shown, the emission current quickly recovered at the initial shift time point of the second white mode so that the emission delay was not observed. 
     FIG. 6  illustrates the experimental results obtained in an Example 2, where Vc/Vg was 0.73. The value of Vg was established to be 110V, and the value of Vc to be 80V. As shown in the drawing, the emission current was quickly recovered at the initial shift time point of the second white mode so that an emission delay was not observed. 
     FIG. 7  illustrates the experimental results obtained in a Comparative Example 1, where Vc/Vg was 0.8. The value of Vg was established to be 100V, and the value of Vc was established to be 80V. As shown, the emission reduction ratio was approximately 9.3% at the initial shift time point of the second white mode, and the emission delay time was approximately one (1) minute. 
   The emission reduction ratio is the ratio of the emission current measured at the initial time point of the second white mode to the emission current measured at the final time point of the first white mode. The emission delay time is the time required for the emission current measured at the final time point of the first white mode to be recovered within ±5%. The screen luminance is deteriorated during this emission delay time. 
     FIG. 8  illustrates the experimental results obtained according to a Comparative Example 2, where Vc/Vg was 0.83. The value of Vg was established to be 120V, and the value of Vc was established to be 100V. As shown, the emission reduction ratio was approximately 24.4% at the initial shift time point of the second white mode, and the emission delay time was about one (1) minute. 
     FIG. 9  illustrates the experimental results obtained according to a Comparative Example 3, where Vc/Vg was 0.89. The value of Vg was established to be 90V, and the value of Vc was established to be 80V. As shown, the emission reduction ratio was approximately 31% at the initial shift time point of the second white mode, and the emission delay time was approximately four (4) minutes. 
     FIG. 10  illustrates the experimental results obtained according to a Comparative Example 4, where Vc/Vg was 1. The values of Vg and Vc were both established to be 70V. As shown, the emission reduction ratio was approximately 75.6% at the initial shift time point of the second white mode, and the emission current was not recovered even nine (9) minutes later. 
   It is estimated that the above-described results are obtained because as the ratio of Vc to Vg increases, the driving signals become distorted and delayed due to the external factors such as a resistance of the cathode and the gate electrodes or a parasitic capacitance between the two electrodes. When the value of Vc/Vg is less than 0.4, the difference between the turn-on voltage (V S −V 1  shown in  FIG. 4 ) and the turn-off voltage (V S −V D  shown in  FIG. 4 ) is so slight that the basic data-off operation might not occur. 
   Establishing the value of Vc/Vg to satisfy the condition of the Formula 1 can thus result in excellent moving image quality as emission delay is limited, even during an image shift inducing a considerably large emission current variation. 
   Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concept herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention, as defined in the appended claims and their equivalents.