Abstract:
An active matrix liquid crystal display device includes a thin film transistor (TFT) array substrate and a counter electrode substrate with a liquid crystal layer held therebetween. On a surface of the counter electrode substrate adjacent to the liquid crystal layer, a plurality of counter electrodes are formed perpendicular to gate lines on the TFT array substrate. Each of the counter electrodes faces at least one column of pixel electrodes connected to drain electrodes of the TFTs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to active matrix liquid crystal display devices, and more particularly to a configuration in which counter electrodes face pixel electrodes with a liquid crystal layer therebetween.  
           [0003]    2. Description of the Related Art  
           [0004]    Typically, a liquid crystal display device includes a TFT array substrate  54 , as shown in FIG. 5, having thin film transistors and pixel electrodes, and a counter electrode substrate  56 , as shown in FIG. 6, having counter electrodes facing the pixel electrodes, with a liquid crystal layer (not shown) being held therebetween.  
           [0005]    The TFT array substrate  54  includes a plurality of gate lines  51 , a plurality of data lines  52 , a plurality of thin film transistors  53  formed in the vicinity of intersections of the gate lines  51  and the data lines  52 , and pixel electrodes  57  individually connected to the plurality of thin film transistors  53 .  
           [0006]    On the other hand, the counter electrode substrate  56  facing the TFT array substrate  54  simply includes a single counter electrode  55  common to all the pixel electrodes  57 .  
           [0007]    A signal voltage from the data lines  52  is applied to the pixel electrodes  57  formed on the TFT array substrate  54  via the thin film transistors  53 . A power supply  59  is connected to the counter electrode  55  formed on the counter electrode substrate  56  through a plurality of connector terminals  58 . Although two connector terminals  58  are shown in FIG. 5, at least one connector terminal  58  is required, and the connector terminal  58  may be disposed at any location. This allows a liquid crystal layer (not shown) to be driven by using a voltage difference between the pixel electrodes  57  and the common electrode  55 .  
           [0008]    In the above-mentioned liquid crystal display device, a voltage applied to the counter electrode  55  is selected to be Vo so that positive voltages and negative voltages are applied to the liquid crystal layer in a symmetric manner, as illustrated in FIG. 7, in order to avoid a flicker or display failure which results from the sticking phenomenon (image retention) when the liquid crystal display device is driven.  
           [0009]    Recently, the demand for such a liquid crystal display device with high definition display has been increasing. As a result, the number of intersections between the gate lines  51  and the data lines  52 , and the number of thin film transistors  53  connected to the gate lines  51  are drastically being increased. The gate lines  51  exhibit parasitic capacitance at locations such as at the intersections with the data lines  52  and the gate electrodes of the thin film transistors  53  in the vicinity of the intersections.  
           [0010]    Therefore, as the desire for higher definition display increases, such capacitance in the gate lines  51  is increased, thus increasing signal delay in the gate lines  51 .  
           [0011]    When a signal delay occurs in the gate lines  51 , the signal waveform of the gate electrodes becomes rounded, and the thin film transistors  53  suffer from the leakage of charge at the timing when they are turned off.  
           [0012]    The leakage of charge at the thin film transistors  53  is greater at portions further from gate signal sources. Hence, the further the thin film transistors are from the gate signal sources, the greater the leakage becomes.  
           [0013]    Accordingly, the amount of variation in voltage applied to the pixel electrodes  57 , which depends upon the leakage of charge at the thin film transistors  53 , is also increased at portions of the gate lines  51  that are further from the gate signal sources. Portions  51   a ,  51   b ,  51   c ,  51   d , and  51   e  of each of the gate lines  51  shown in FIG. 5 extend further from the gate signal source, in the order stated.  
           [0014]    When the amount of variation in voltage applied to the pixel electrodes  57  differs depending upon a distance in the gate lines  51  from the gate signal sources, voltages applied to the liquid crystal layer, as indicated by B 1  to B 5  in FIG. 8, are increased at portions of the gate lines  51  that are further from the gate signal sources in a manner such that |B 1   a | to |B 1   b |), . . . , (|B 5   a | to |B 5   b |). Thus, the applied voltage has less symmetrical polarity.  
           [0015]    Less symmetrical polarity may result in problems of flicker or display failure which results from the sticking phenomenon.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, it is an object of the present invention to provide an active matrix liquid crystal display device in which no flicker or sticking of images occurs on a display screen when a signal delay in gate lines causes voltages applied to pixels at portions of the gate lines that are closer to and further from signal sources to differ.  
           [0017]    To this end, the present invention provides an active matrix liquid crystal display device including a pair of substrates facing each other with a liquid crystal layer held therebetween. On a surface of one of the substrates adjacent to the liquid crystal layer, there are formed a plurality of gate lines and a plurality of data lines intersecting to form a matrix; thin film transistors in the vicinity of intersections of the gate lines and the data lines, the thin film transistors having gate electrodes connected to the gate lines and source electrodes connected to the data lines; and pixel electrodes connected to the drain electrodes of the thin film transistors. On a surface of the other substrate adjacent to the liquid crystal layer, there are formed a plurality of counter electrodes in the direction perpendicular to the gate lines on the one substrate. Each of the counter electrodes faces at least one column of the pixel electrodes.  
           [0018]    Therefore, a voltage is applied at different magnitudes to the plurality of counter electrodes depending upon a distance in the gate lines from signal sources. This prevents a flicker or sticking of images from occurring on a display screen when the amount of variation in voltages applied to the pixel electrodes at portions of the gate lines that are closer to and further from the signal sources differs.  
           [0019]    Preferably, the plurality of counter electrodes are connected to power supplies for supplying different voltages, so that the voltages applied to the counter electrodes may be independently set.  
           [0020]    Preferably, the plurality of counter electrodes are respectively connected to a plurality of output terminals of a voltage controller connected to a signal power supply to generate different magnitudes of voltage, so that the number of power supplies required may be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Some illustrative embodiments of the present invention will be described with reference to the drawings, in which:  
         [0022]    [0022]FIG. 1 is an exploded view showing a main portion of an active matrix liquid crystal display device according to a first embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is an exploded view showing another main portion of the active matrix liquid crystal display device shown in FIG. 1;  
         [0024]    [0024]FIG. 3 is a graph showing the operation of the active matrix liquid crystal display device shown in FIG. 1;  
         [0025]    [0025]FIG. 4 is an exploded view showing a main portion of an active matrix liquid crystal display device according to a second embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is an exploded view showing a main portion of a conventional liquid crystal display device;  
         [0027]    [0027]FIG. 6 is an exploded view showing another main portion of the liquid crystal display device shown in FIG. 5;  
         [0028]    [0028]FIG. 7 is a graph showing a operation of the liquid crystal display device shown in FIG. 5; and  
         [0029]    [0029]FIG. 8 is a graph showing another operation of the liquid crystal display device shown in FIG. 5. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIGS. 1 and 2 are exploded views showing main portions of an active matrix liquid crystal display device according to a first embodiment of the present invention.  
         [0031]    Referring to FIGS. 1 and 2, the liquid crystal display device includes a TFT array substrate  4  and a counter electrode substrate  6  facing each other with a liquid crystal layer (not shown) held therebetween. The TFT array substrate  4  includes a plurality of gate lines  1 , a plurality of data lines  21 ,  22 ,  23 ,  24 ,  25 ,  26 ,  27 ,  28 ,  29 , and  30 , a plurality of thin film transistors  3  formed in the vicinity of intersections between the gate lines  1  and the data lines  21  to  30 , and pixel electrodes  7  individually connected to the thin film transistors  3 . The counter electrode substrate  6  contains counter electrodes  11 ,  12 ,  13 ,  14 , and  15  which face columns of the pixel electrodes  7  defined by the data lines  21  and  22 ,  23  and  24 ,  25  and  26 ,  27  and  28 , and  29  and  30 , respectively.  
         [0032]    A signal voltage is applied to the pixel electrodes  7  formed on the TFT array substrate  4  from the associated data lines  21  to  30  via the thin film transistors  3 . The plurality of counter electrodes  11 ,  12 ,  13 ,  14 , and  15  formed on the counter electrode substrate  6  are supplied different voltages from power supplies  16 ,  17 ,  18 ,  19 , and  20  via connector terminals  11   a ,  12   a ,  13   a ,  14   a , and  15   a , respectively.  
         [0033]    As shown in FIG. 3, the voltages applied to the counter electrodes  11 ,  12 ,  13 ,  14 , and  15  are selected to be V 1 , V 2 , V 3 , V 4 , and V 5  for columns of the pixel electrodes  7  depending upon a distance in the gate lines  1  from signal sources so that positive voltages and negative voltages are applied to the liquid crystal layer in a symmetric manner such that (|A 1   a |=|A 1   b |), . . . , (|A 5   a |=|A 5   b |).  
         [0034]    Therefore, even though a signal delay in the gate lines  1  causes voltages applied to the pixel electrodes  7  at portions of the gate lines  1  that are closer to and further from the signal sources to differ, positive voltages and negative voltages are applied to the liquid crystal layer in a symmetric manner. This avoids a flicker or display failure which results from the sticking phenomenon when the present display device is driven.  
         [0035]    While five divided counter electrodes, i.e., the counter electrodes  11 ,  12 ,  13 ,  14 , and  15  are employed in the first embodiment shown in FIGS. 1 and 2, the number of divided counter electrodes is not limited to this number. The larger the number of divided counter electrodes, the more symmetric the positive and negative voltages applied to the liquid crystal layer.  
         [0036]    This feature would be more remarkably exhibited in high definition display of the SVGA class or higher.  
         [0037]    A liquid crystal display device according to a second embodiment of the present invention is described with reference to FIG. 4. The liquid crystal display device according to the second embodiment is different from that of the first embodiment in that the counter electrodes  11 ,  12 ,  13 ,  14 , and  15  are supplied with different magnitudes of voltage through output terminals  50   a ,  50   b ,  50   c ,  50   d , and  50   e  of a voltage drop section  50  coupled to a single power supply  49  to generate different fractional voltages.  
         [0038]    In the liquid crystal display device according to the second embodiment, the same reference numerals are assigned to the same elements as those of the liquid crystal display device of the first embodiment shown in FIG. 4, and a description thereof is thus omitted.  
         [0039]    The plurality of counter electrodes  11 ,  12 ,  13 ,  14 , and  15  formed on the counter electrode substrate  6  of the liquid crystal display device shown in FIG. 4 are electrically connected to the output terminals  50   a ,  50   b ,  50   c ,  50   d , and  50   e  of the voltage drop section  50  via the connector terminals  11   a ,  12   a ,  13   a ,  14   a , and  15   a , respectively.  
         [0040]    The voltage drop section  50  having one end connected to the power supply  49  and the other end connected to the ground includes resistors R 1 , R 2 , R 3 , R 4 , and R 5  connected in series. Although the other end of the voltage drop section  50  is connected to the ground in FIG. 4, it may be connected to any other terminal with a voltage.  
         [0041]    The voltages applied to the plurality of counter electrodes  11 ,  12 ,  13 ,  14 , and  15  from the power supply  49  are set to have different magnitudes in the voltage drop section  50 . In FIG. 4, the plurality of resistors R 1 , R 2 , R 3 , R 4 , and R 5  are used to generate a voltage drop having different magnitudes. The resistance values of the resistors R 1 , R 2 , R 3 , R 4 , and R 5  are selected for V 1 , V 2 , V 3 , V 4 , and V 5  and for the associated columns of pixel electrodes  7  depending upon a distance in the gate lines  1  from signal sources so that positive voltages and negative voltages are applied to the liquid crystal layer in a symmetric manner, as in FIG. 3.  
         [0042]    Therefore, the desired voltage is applied to the plurality of counter electrodes  11 ,  12 ,  13 ,  14 , and  15  in order to avoid a flicker or display failure which results from the sticking phenomenon when the present liquid crystal display device is driven, allowing the liquid crystal display device to be driven using voltage differences between the pixel electrodes  7  and the plurality of counter electrodes  11 ,  12 ,  13 ,  14 , and  15 .  
         [0043]    Although the present invention has been described through illustrations of its preferred forms, it is to be understood that the described embodiments are only illustrative and various changes and modifications may be imparted thereto without departing from the scope of the present invention which is limited solely by the appended claims.