Abstract:
A pixel structure includes a scan line, a data line, a first thin film transistor (TFT), a second TFT, a first pixel electrode, a second pixel electrode and a third pixel electrode. The first TFT and the second TFT respectively possessing a first drain electrode and a second drain electrode are electrically connected to the scan line and the data line. The first pixel electrode is electrically connected to the first drain electrode. The second pixel electrode is placed on and coupled to parts of the first drain electrode, and the third pixel electrode is placed on and coupled to parts of the second drain electrode. As a result, the pixel structure is capable of reducing display quality variations arisen from different viewing angles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Taiwan application serial no. 96111243, filed Mar. 30, 2007. All disclosure of the Taiwan application is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a pixel structure of a liquid crystal display (LCD) panel, and more particularly, to a pixel structure of a multi-domain vertical alignment (MVA) LCD panel. 
         [0004]    2. Description of Related Art 
         [0005]    Currently, the LCDs have been mostly developed towards high luminance, high contrast ratio, large display size and wide viewing angle. In order to increase the view angle of the LCDs, several wide-viewing-angle techniques have been proposed. The most popular LCDs with the wide-viewing-angle feature include, for example, an MVA LCD, an in-plane switching (IPS) LCD, and a fringe field switching (FFS) LCD. 
         [0006]      FIG. 1  is a top view of a pixel structure applied to a MVA display according to the conventional art. Referring to  FIG. 1 , a pixel structure  100  is disposed on a thin film transistor (TFT) array substrate and includes a scan line  110 , a data line  120 , a TFT  130 , a pixel electrode  140  and an alignment member  150 . The TFT  130  includes a gate electrode  132 , a semiconductor layer  134 , a source electrode  136   a,  a drain electrode  136   b  and a contact hole  138 . The gate electrode  132  and the scan line  110  are electrically connected to each other, and the semiconductor layer  134  is disposed over the gate electrode  132 . The source electrode  136   a  and the drain electrode  136   b  are disposed on the semiconductor layer  134 , and the source electrode  136   a  is electrically connected to the data line  120 . 
         [0007]    The pixel electrode  140  is electrically connected to the drain electrode  136   b  via the contact hole  138 . In addition, in order to enable liquid crystal molecules to generate an MVA, the alignment member  150  is disposed on the pixel electrode  140 , and a plurality of alignment members (not shown) is disposed on a corresponding color filter substrate (not shown). Therefore, with the alignment member  150  and the plurality of the alignment members (not shown), the liquid crystal molecules disposed between the TFT array substrate and the color filter substrate may have various tilt directions, and the wide-viewing-angle effect can then be achieved. 
         [0008]    Said MVA LCD is able to increase a range of the viewing angle. However, the light transmission rate of the MVA LCD may vary corresponding to a gray-level gamma curve when the viewing angle is increased from 0 degree to 90 degrees. In brief, image color and image luminance both provided by the MVA LCD may be distorted to a greater extent due to different viewing angles. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing, the present invention is directed to provide a pixel structure for reducing display quality variations arisen from different viewing angles. 
         [0010]    The present invention provides a pixel structure including a substrate, a scan line, a data line, a first TFT, a first pixel electrode, a second pixel electrode, a second TFT and a third pixel electrode. Here, the scan line, the data line, the first TFT, the first pixel electrode, the second pixel electrode, the second TFT and the third pixel electrode are all disposed on the substrate. The first TFT is electrically connected to the scan line and the data line and has a first drain electrode. The first pixel electrode is electrically connected to the first drain electrode. The second pixel electrode is disposed over and is coupled to the first drain electrode. The second TFT is electrically connected to the scan line and the data line and has a second drain electrode. The third pixel electrode is disposed over and is coupled to the second drain electrode. 
         [0011]    According to an embodiment of the present invention, the pixel structure further comprises a first common line, a second common line and a plurality of alignment members. The first common line is disposed on the substrate, wherein the first pixel electrode and the second pixel electrode overlap parts of the first common line, respectively. The second common line is disposed on the substrate, wherein the third pixel electrode overlaps parts of the second common line. The alignment members are disposed on the first pixel electrode, the second pixel electrode and the third pixel electrode. 
         [0012]    According to an embodiment of the present invention, the alignment members include protrusions or slits. 
         [0013]    According to an embodiment of the present invention, the pixel structure further includes a fourth pixel electrode disposed on the substrate and electrically connected to the second drain electrode. The fourth pixel electrode overlaps parts of the second common line, and the alignment members are further disposed on the fourth pixel electrode. 
         [0014]    According to an embodiment of the present invention, the first pixel electrode is disposed between the second pixel electrode and the scan line. 
         [0015]    According to an embodiment of the present invention, the fourth pixel electrode is disposed between the third pixel electrode and the scan line. 
         [0016]    According to an embodiment of the present invention, the first TFT and the second TFT share a common source electrode. 
         [0017]    According to an embodiment of the present invention, the first pixel electrode and the second pixel electrode are positioned at one side of the scan line, and the third pixel electrode is positioned at another. 
         [0018]    According to an embodiment of the present invention, the first pixel electrode and the second pixel electrode are positioned at one side of the scan line, and the third pixel electrode and the fourth pixel electrode are positioned at another. 
         [0019]    The present invention further provides a driving method of a pixel structure. The driving method is adapted to drive the aforesaid pixel structure and includes the following steps. First, the first TFT and the second TFT are turned on through the scan line. Thereafter, a data voltage is inputted to the first pixel electrode through the data line. Here, the second pixel electrode generates an induced voltage through the first drain electrode, and the third pixel electrode generates another induced voltage through the second drain electrode. 
         [0020]    According to an embodiment of the present invention, the first common line and the second common line have different voltages. 
         [0021]    According to an embodiment of the present invention, the first common line and the second common line have anti-phase voltages. 
         [0022]    The present invention farther provides a driving method of a pixel structure. The driving method is adapted to drive the aforesaid pixel structure and includes the following steps. First, the first TFT and the second TFT are turned on through the scan line. Thereafter, a data voltage is inputted to the first pixel electrode and the fourth pixel electrode through the data line. Here, the second pixel electrode generates an induced voltage through the first drain electrode, the third pixel electrode generates another induced voltage through the second drain electrode, and the first common line and the second common line have different voltages. 
         [0023]    According to an embodiment of the present invention, the first common line and the second common line have anti-phase voltages. 
         [0024]    Based on the above, since the pixel structure of the present invention enables each of the pixel electrodes to reach different voltage levels according to said driving method, liquid crystal molecules disposed over each of the pixel electrodes then have different tilt angles, reducing the light transmission rate of an MVA LCD corresponding to a gray-level gamma curve to a certain degree based on variations in the viewing angles. 
         [0025]    In order to the make the aforementioned and other objects, features and advantages of the present invention comprehensible, several embodiments accompanied with figures are described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a top schematic view of a pixel structure applied to an MVA LCD according to the conventional art. 
           [0027]      FIG. 2  is a top view of a pixel structure according to a first embodiment. 
           [0028]      FIG. 3  is an equivalent circuit diagram of the pixel structure illustrated in  FIG. 2 . 
           [0029]      FIG. 4  is a driving waveform of each of the pixel electrodes in the pixel structure illustrated in  FIG. 2  after a driving method described in the first embodiment is performed. 
           [0030]      FIG. 5  is a top view of a pixel structure according to a second embodiment. 
           [0031]      FIG. 6  is an equivalent circuit diagram of the pixel structure illustrated in  FIG. 5 . 
           [0032]      FIG. 7  is a driving waveform of each of the pixel electrodes in the pixel structure illustrated in  FIG. 5  after the driving method described in the second embodiment is performed. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0033]      FIG. 2  is a top view of a pixel structure  200  according to a first embodiment. Referring to  FIG. 2 , the pixel structure  200  includes a substrate  210 , a scan line  220 , a data line  230 , a first TFT  240 , a first pixel electrode  260 , a second pixel electrode  262 , a second TFT  250  and a third pixel electrode  264 . Here, the scan line  220 , the data line  230 , the first TFT  240 , the first pixel electrode  260 , the second pixel electrode  262 , the second TFT  250  and the third pixel electrode  264  are all disposed on the substrate  210 . 
         [0034]    Particularly, the first TFT  240  is electrically connected to the scan line  220  and the data line  230  and has a first drain electrode  240   a  electrically connected to the first pixel electrode  260 . In more details, the first drain electrode  240   a  is electrically connected to the first pixel electrode  260  via a first contact hole  290 . The second pixel electrode  262  is floatingly disposed over parts of the first drain electrode  240   a  and is coupled to an extending portion of the first drain electrode  240   a.  More specifically, the first drain electrode  240   a  extends towards the second pixel electrode  262  in a direction parallel to the data line  230 . After the first drain electrode  240   a  extends below the second pixel electrode  262 , the extending portion is then coupled to the second pixel electrode  262  floatingly disposed over the extending portion. The second TFT  250  is electrically connected to the scan line  220  and the data line  250  and has a second drain electrode  250   a.  The third pixel electrode  264  is floatingly disposed over and is coupled to the second drain electrode  250   a.  On the other hand, in the present embodiment, the pixel structure  200  further includes a first common line  270  and a second common line  272 . Here, the first pixel electrode  260  and the second pixel electrode  262  overlap parts of the first common line  270 , respectively, while the third pixel electrode  264  overlaps parts of the second common line  272 . However, the first common line  270  and the second common line  272  are not limited in the present invention. Moreover, as the pixel structure  200  has an MVA design, the pixel structure  200  further includes a plurality of alignment members  280 . As shown in  FIG. 2 , the alignment members  280  are disposed on the first pixel electrode  260 , the second pixel electrode  262  and the third pixel electrode  264 . Nevertheless, as the pixel structure  200  has a twisted nematic (TN) design, the plurality of the alignment members  280  may not be included in the pixel structure  200 . The plurality of the alignment members  280  is not limited in the present invention. In the present embodiment, the alignment members  280  are protrusions, while the alignment members  280  may be slits in another embodiment. 
         [0035]    In the pixel structure  200 , the first TFT  240  and the second TFT  250  share a common source electrode  246 . However, in other embodiments, the first TFT  240  and the second TFT  250  may respectively have an individual source electrode. In other words, the modes and the types of the TFTs are not limited to those disclosed in  FIG. 2  of the present invention. For example, in the present embodiment, the TFTs have straight channels and are directly disposed on the scan line. However, the TFTs may have U-shaped channels and may be disposed on the protrusions extended from the scan line. 
         [0036]    Besides, the first pixel electrode  260  is disposed between the second pixel electrode  262  and the scan line  220 . The first pixel electrode  260  and the second pixel electrode  262  are positioned at one side of the scan line  220 , while the third pixel electrode  264  is positioned at another. Nevertheless, the disposition of said three pixel electrodes is merely exemplified but not limited in the present invention. 
         [0037]      FIG. 3  is an equivalent circuit diagram of the pixel structure  200 . The pixel structure  200  includes the scan line  220 , the first common line  270 , the second common line  272 , a data line  230  and another data line  232  adjacent to the pixel structure  200 , a first TFT  240  and a second TFT  250 . Referring to  FIGS. 2 and 3  together, C lc1  represents a first liquid crystal capacitance generated by the first pixel electrode  260  and a common electrode (not shown) on an opposite substrate. C sta  denotes a total storage capacitance including the storage capacitance produced by the first pixel electrode  260  and the common line  270  and the storage capacitance produced by the second pixel electrode  262  and the common line  270 . C lc2  refers to a second liquid crystal capacitance generated by the second pixel electrode  262  and the common electrode (not shown) on the opposite substrate. Moreover, the second pixel electrode  262  of the pixel structure  200  is floatingly disposed over parts of the first drain electrode  240   a,  and thus the second pixel electrode  262  and the underlying extending portion of the first drain electrode  240   a  are coupled to the first drain electrode  240   a.  Thereby, a second coupled capacitance C cp2  is generated between the second pixel electrode  262  and the first drain electrode  240   a.    
         [0038]    Referring to  FIG. 3  again, C lc3  represents a third liquid crystal capacitance generated by the third pixel electrode  264  and the common electrode (not shown) on the opposite substrate, and C s3  denotes the storage capacitance generated by the third pixel electrode  264  and the common line  272 . Furthermore, the third pixel electrode  264  of the pixel structure  200  is coupled to the second drain electrode  250   a,  and thus a third coupled capacitance C cp3  is generated between the third pixel electrode  264  and the second drain electrode  250   a.    
         [0039]    A driving method of the pixel structure  200  will be described hereinafter. Referring to  FIGS. 2 and 3  together, the driving method of the pixel structure  200  includes the following steps. First, the first TFT  240  and the second TFT  250  are turned on through the scan line  220 . Thereafter, a data voltage Va is inputted to the first pixel electrode  260  through the data line  230 . Here, the second pixel electrode  262  generates an induced voltage Vb 2  through the first drain electrode  240   a,  and the third pixel electrode  264  generates another induced voltage Vb 3  through the second drain electrode  250   a.    
         [0040]    To be more specific, the coupled capacitances C cp2  and C cp3  and signals of the first common line  270  and the second common line  272  are adopted in the present invention, such that the three pixel electrodes reach different voltage levels.  FIG. 4  is a driving waveform of each of the pixel electrodes in the pixel structure  200  after the driving method described above is performed. The first pixel electrode  260  has a driving waveform Va 1 , the second pixel electrode  262  has a driving waveform Vb 2 , and the third pixel electrode  264  has a driving waveform Vb 3 . According to the present embodiment, the first common line  270  and the second common line  272  have anti-phase voltages, but the inputted voltages of the first common line  270  and the second common line  272  are not limited in the present embodiment. In other embodiments, the voltages inputted by the first common line  270  and the second common line  272  may also have a difference. The dissimilarities of the signal waveforms Va 1 , Vb 2 , and Vb 3  are clearly indicated in  FIG. 4 . That is to say, the pixel structure  200  of the present invention enables the three pixel electrodes in the pixel structure  200  to reach different voltage levels after said driving method is carried out, such that liquid crystal molecules disposed over the three pixel electrodes have different tilt angles, reducing the light transmission rate of an MVA LCD corresponding to a gray-level gamma curve to a certain degree according to variations in the viewing angles. 
       Second Embodiment 
       [0041]      FIG. 5  is a top view of a pixel structure  300  according to another embodiment of the present invention. With reference to  FIG. 5 , the pixel structure  300  in the present embodiment and the pixel structure  200  in the first embodiment are similar, and the difference therebetween mainly lies in that a fourth pixel electrode  266  is further disposed between the third pixel electrode  264  and the scan line  220  in the pixel structure  300  of the present embodiment. In the present embodiment, the fourth pixel electrode  266  is electrically connected to the second drain electrode  250   a  and overlaps parts of the second common line  272 . In details, the first drain electrode  240   a  is electrically connected to the first pixel electrode  260  via a first contact hole  290 , while the second drain electrode  250   a  is electrically connected to the fourth pixel electrode  266  via a second contact hole  292 . More specifically, in the pixel structure  300  of the present embodiment, the first pixel electrode  260  and the second pixel electrode  262  are positioned at one side of the scan line  220 , while the third pixel electrode  264  and the fourth pixel electrode  266  are positioned at another. 
         [0042]      FIG. 6  is an equivalent circuit diagram of the pixel structure  300 . Referring to  FIGS. 6 and 7  together, the equivalent circuit diagram of the pixel structure  300  in the present embodiment and that of the pixel structure  200  in the first embodiment are similar, and the difference therebetween mainly lies in that the present embodiment further includes a fourth liquid crystal capacitance C lc4  generated by the fourth pixel electrode  266  and the common electrode (not shown) on the opposite substrate and a storage capacitance C s4  generated by the fourth pixel electrode  266  and the common line  272 . In  FIG. 6 , C sta2  represents the total storage capacitance of the third storage capacitance C s3  and the fourth storage capacitance C s4 . 
         [0043]    A driving method of the pixel structure  300  will be described hereinafter. Referring to  FIGS. 5 and 6  together, the driving method of the pixel structure  300  includes the following steps. First, the first TFT  240  and the second TFT  250  are turned on through the scan line  220 . Thereafter, a data voltage Va is inputted to the first pixel electrode  260  and the fourth pixel electrode  266  through the data line  230 . Here, the second pixel electrode  262  generates the induced voltage Vb 2  through the first drain electrode  240   a,  and the third pixel electrode generates the induced voltage Vb 3  through the second drain electrode  250   a.    
         [0044]    More particularly, the coupled capacitances C cp2  and C cp3  and the signals of the first common line  270  and the second common line  272  are adopted in the present invention, such that the four pixel electrodes reach different voltage levels.  FIG. 7  is a driving waveform of each of the pixel electrodes in the pixel structure  300  after the driving method described above is performed. The first pixel electrode  260  has the driving waveform Va 1 , the second pixel electrode  262  has the driving waveform Vb 2 , the third pixel electrode  264  has the driving waveform Vb 3 , and the fourth pixel electrode  266  has a driving waveform Va 4 . According to the present embodiment, the first common line  270  and the second common line  272  have anti-phase voltages, but the inputted voltages of the first common line  270  and the second common line  272  are not limited in the present embodiment. In other embodiments, the voltages inputted by the first common line  270  and the second common line  272  may also have a difference. The dissimilarities of the signal waveforms Va 1 , Vb 2 , Vb 3  and Va 4  are clearly indicated in  FIG. 7 . 
         [0045]    Based on the above, the pixel structures  200  and  300  according to said two embodiments of the present invention enable each of the pixel electrodes in the pixel structure  200  or in the pixel structure  300  to reach different voltage levels after the afore-mentioned driving methods are performed, such that the liquid crystal molecules disposed over each of the pixel electrodes have the different tilt angles, reducing the light transmission rate of the MVA LCD corresponding to the gray-level gammua curve to a certain degree according to the variations in the viewing angles. 
         [0046]    Although the present invention has been disclosed above by the embodiments, they are not intended to limit the present invention. Anybody skilled in the art can make some modifications and alteration without departing from the spirit and scope of the present invention. Therefore, the protecting range of the present invention falls in the appended claims.