Patent Publication Number: US-7916235-B2

Title: Pixel structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of an application Ser. No. 11/840,995, filed on Aug. 19, 2007, now pending, which claims the priority benefit of Taiwan application serial no. 96120514, filed on Jun. 7, 2007. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a pixel structure, and more particularly, to a pixel structure capable of improving display quality. 
     2. Description of Related Art 
     The rapid development of multi-media comes mostly as a result of the recent advance in the production of semiconductor devices and display apparatus. Liquid crystal display, with its high display quality, good spatial utilization, low power consumption and radiation-free operation, has gradually become the mainstream display product in the market. To provide better display quality to the liquid crystal display, all kinds of wide viewing angle liquid crystal displays have been developed. The most common ones include, for example, the in-plane switching (IPS) liquid crystal display, the fringe field switching liquid crystal display, the multi-domain vertical alignment (MVA) liquid crystal display and so on. 
       FIG. 1  is a schematic cross-sectional view of a conventional pixel structure. As shown in  FIG. 1 , the conventional pixel structure  100  includes a substrate  112 , a thin film transistor T, a capacitor-coupling electrode  118   c , a first pixel electrode  119   a , a second pixel electrode  119   b  and an alignment film PI. The thin film transistor T mainly includes a gate  114 , a gate insulating layer  116 , a semiconductor layer  117 , a source  118   a  and a drain  118   b . In  FIG. 1 , the thin film transistor T is a bottom gate structure, and the thin film transistor T is covered by the passivation layer  120 . More specifically, the thin film transistor T is disposed on the substrate  112 , and the capacitor-coupling electrode  118   c  is electrically connected to the drain  118   b  of the thin film transistor T. In addition, the first pixel electrode  119   a  is electrically connected to the drain  118   b  of the thin film transistor T, and the capacitor-coupling electrode  118   c  is located between the second pixel electrode  119   b  and the substrate  112 . 
     In an ideal condition, the first pixel electrode  119   a  is electrically insulated from the second pixel electrode  119   b , and the second pixel electrode  119   b  is coupled to the capacitor-coupling electrode  118   c  underneath. In other words, after turning on the active device T, the first pixel electrode  119   a  and the second pixel electrode  119   b  can have different voltages so that the liquid crystals (not shown) corresponding to the first pixel electrode  119   a  and the second pixel electrode  119   b  can have different inclining states. It should be noted that residual charges on the second pixel electrode  119   b  and the alignment film PI are difficult to remove because the second pixel electrode  119   b  is in a floating state. Consequently, the performance of the second pixel electrode  119   b  will be affected by the residual charges so that the problem of having a residual image in the next display picture needs to be solved. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a pixel structure capable of preventing the display of a residual image in a picture. 
     The invention also provides another pixel structure with good display quality. The invention also provides a pixel structure suitable for disposing on a substrate. The pixel structure of the invention includes a thin film transistor, a first pixel electrode, a second pixel electrode, a scan line and a data line. The thin film transistor is disposed on the substrate. The thin film transistor of the invention includes a gate, a source, a first drain and a second drain. Additionally, a main thin film transistor is formed by the gate, the source and the first drain. A sub-thin film transistor (sub-TFT) is formed by the gate, the first drain and the second drain. When the main thin film transistor and the sub-TFT are turned on, the conducting current of the sub-TFT is substantially smaller than the conducting current of the main thin film transistor, and their conducting currents form a specific ratio. The first pixel electrode of the invention is electrically connected to the first drain, and a portion of the first drain extends between the second pixel electrode and the substrate such that a capacitor-coupling electrode is formed. Moreover, the second pixel electrode is electrically connected to the second drain of the sub-TFT. The scan line is disposed on the substrate and electrically connected to the gate, and the data line is electrically connected to the source. 
     In an embodiment of the invention, the specific ratio is between 0.05˜0.3, for example. 
     In an embodiment of the invention, the first pixel electrode has a plurality of slits. 
     In an embodiment of the invention, the second pixel electrode has a plurality of slits. 
     In an embodiment of the invention, the pixel structure further includes a common line distribution pattern disposed on the substrate and electrically connected to a common voltage. 
     In an embodiment of the invention, at least a portion of the common line distribution pattern extends along the edge of the first pixel electrode and the second pixel electrode. 
     In the invention, a sub-thin film transistor having a very small conducting current is electrically connected to a second pixel electrode corresponding to the capacitor-coupling electrode. Therefore, the second pixel electrode may utilize the sub-thin film transistor of the invention to remove unwanted residual charges so as to prevent the performance of the second pixel electrode from being adversely affect and effectively suppress the problem of having residual image in a display picture. 
     In order to make the aforementioned and other features and advantages of the invention comprehensible, embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic cross-sectional view of a conventional pixel structure. 
         FIG. 2A  is a pixel structure according to a first embodiment of the invention. 
         FIG. 2B  is a circuit diagram of the pixel structure according to the first embodiment of the invention. 
         FIG. 3A  is a pixel structure according to a second embodiment of the invention. 
         FIG. 3B  is a circuit diagram of the pixel structure according to the second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     First Embodiment 
       FIG. 2A  is a pixel structure according to a first embodiment of the invention.  FIG. 2B  is a circuit diagram of the pixel structure according to the first embodiment of the invention. As shown in  FIGS. 2A and 2B , the pixel structure  200  of the invention includes a thin film transistor  220 , a first pixel electrode  230   a , a second pixel electrode  230   b , a scan line  240  and a data line  250 . The thin film transistor  220  is disposed on a substrate  210  and electrically connected to the scan line  240  and the data line  250 . In practice, a switching signal can be transmitted through the scan line  240  to turn on the thin film transistor  220 . After turning on the thin film transistor  220 , a display signal can be transmitted to the first pixel electrode  230   a  and the second pixel electrode  230   b  through the data line  250 . 
     The thin film transistor  220  of the invention includes a gate  222 , a semiconductor layer  223 , a source  224 , a first drain  226   a , a second drain  226   b  and a third drain  226   c . It should be mentioned that the thin film transistor  220  shown in  FIG. 2A  is a bottom gate structure. Obviously, the thin film transistor  220  can also be a top gate structure. Here, the bottom gate structure is chosen only as an example and is not intended to limit the invention. 
     More specifically, the scan line  240  is electrically connected to the gate  222 , and the data line  250  is electrically connected to the source  224 . The gate  222  can be part of the extension of the scan line  240 , and the source  224  can be part of the extension of the data line  250 . As shown in  FIG. 2B , it should be mentioned that a thin film transistor  220   a  is formed by the gate  222 , the source  224  and the first drain  226   a  of the invention. A second thin film transistor  220   b  is formed by the gate  222 , the source  224  and the second drain  226   b . A sub-thin film transistor (sub-TFT)  220   c  is formed by the gate  222 , the second drain  226   b  and the third drain  226   c . In particular, when the sub-TFT  220   c  and the second thin film transistor  220   b  are turned on, the conducting current of the sub-TFT  220   c  is substantially smaller than the conducting current of the second thin film transistor  220   b , and there is a specific ratio between the conducting currents. For example, the conducting current of the sub-TFT  220   c  can be 0.05 to 0.3 times the conducting current of the second thin film transistor  220   b.    
     It should be noted from  FIGS. 2A and 2B  that the first pixel electrode  230   a  of the invention could be electrically connected to the first drain  226   a  of the first thin film transistor  220   a  through a contact opening C 1 . In addition, the second drain  226   b  of the second thin film transistor extends to a portion between the second pixel electrode  230   b  and the substrate  210  such that a capacitor-coupling electrode  230   c  is formed. In particular, the second pixel electrode  230   b  can be electrically connected to the third drain  226   c  of the sub-TFT through a contact opening C 2 . In general, the first pixel electrode  230   a  and the second pixel electrode  230   b  may have a plurality of slits S so that liquid crystals (not shown) can be divided into a plurality of domains. 
     Furthermore, the pixel structure  200  of the invention may also includes a common line pattern  242  disposed on the substrate  210 . At least a portion of the common line pattern  242  of the invention extends along the edge of the first pixel electrode  230   a  and the second pixel electrode  230   b . Obviously, the common line pattern  242  shown in  FIG. 2A  can have other shapes and can be selected on demand. Here, the shape of the common line pattern  242  is used only as an example and is not intended to limit the invention. In practice, the common line pattern  242  only has to be electrically connected to a common voltage. 
     More specifically, after turning on the first thin film transistor  220   a , the second thin film transistor  220   b  and the sub-TFT  220   c , a display signal can be transmitted to the first pixel electrode  230   a  through the first thin film transistor  220   a . Here, it should be mentioned that the first pixel electrode  230   a  could form a first storage capacitor Cst 1  (as shown in  FIG. 2B ) with the common line pattern  242  underneath and form a first liquid crystal capacitor Clc 1  with a common electrode above an upper color filter (not shown). 
     Additionally, the second pixel electrode  230   b  can couple with the capacitor-coupling electrode  230   c  underneath to form a coupling capacitor Cc (as shown in  FIG. 2B ), and form a second storage capacitor Cst 2  with the common line pattern  242 . On the other hand, a second liquid crystal capacitor Clc 2  can be formed by the second pixel electrode  230   b  and the common electrode of the upper color filter (not shown). In other words, the first pixel electrode  230   a  and the second pixel electrode  230   b  can have different voltages. As a result, the liquid crystals (not shown) corresponding to the first pixel electrode  230   a  and the second pixel electrode  230   b  can have different inclining states so as to provide a wide viewing angle in the display. 
     It should be note that the performance of the second pixel electrode  230   b  would not be affected because the conducting current of the sub-TFT  220   c  is very small. Here, it should be mentioned that the residual charges on the second pixel electrode  230   b  of a previous display picture could be removed through the sub-TFT  220   c . As a result, the problem of having residual charges on the second pixel electrode  119   b  (as shown in  FIG. 1 ) in the conventional design can be resolved. Therefore, the pixel structure  200  of the invention can effectively suppress residual image and improve display quality. Moreover, it should be said the magnitude of the conducting current of the sub-TFT  220   c  can be suitably adjusted according to the actual requirements as long as it does not affect the normal performance of the second pixel electrode  230   b.    
     Second Embodiment 
       FIG. 3A  is a pixel structure according to a second embodiment of the invention.  FIG. 3B  is a circuit diagram of the pixel structure according to the second embodiment of the invention. As shown in  FIGS. 3A and 3B , the pixel structure  300  of the invention includes a thin film transistor  320 , a first pixel electrode  330   a , a second pixel electrode  330   b , a scan line  340  and a data line  350 . The thin film transistor  320  is disposed on a substrate  310  and electrically connected to the scan line  340  and the data line  350 . In practice, a switching signal can be transmitted to turn on the thin film transistor  320  through the scan line  340 . After turning on the thin film transistor  320 , a display signal can be transmitted to the first pixel electrode  330   a  and the second pixel electrode  330   b  through the data line  350 . 
     More specifically, the thin film transistor  320  of the invention includes a gate  322 , a semiconductor layer  323 , a source  324 , a first drain  326   a  and a second drain  326   b . It should be mentioned that the thin film transistor  320  shown in  FIG. 3A  is a bottom gate structure. Obviously, the thin film transistor  320  can also be a top gate structure. Here, the bottom gate structure is chosen only as an example and is not intended to limit the invention. 
     In practice, the scan line  340  is electrically connected to the gate  322 , and the data line  350  is electrically connected to the source  324 . The gate  322  can be part of the extension of the scan line  340 , and the source  324  can be part of the extension of the data line  350 . Here, it should be mentioned that the gate  322 , the source  324  and the first drain  326   a  of the invention form a main thin film transistor  320   a  as shown in  FIG. 3B . As shown in  FIG. 3B , a sub-thin film transistor (sub-TFT)  320   b  is formed by the gate  322 , the first drain  326   a  and the second drain  326   b . In particular, when the main thin film transistor  320  and the sub-TFT  320   b  are turned on, the conducting current of the sub-TFT  320   b  is substantially smaller than the conducting current of the main thin film transistor  320   a , and there is a a specific ratio between the conducting currents. For example, the conducting current of the sub-TFT  320   b  can be 0.05 to 0.3 times the conducting current of the main thin film transistor  320   a.    
     As shown in  FIGS. 3A and 3B , the first pixel electrode  330   a  of the invention can be electrically connected to the first drain  326   a  through a contact opening C 3 . Moreover, part of the first drain  326   a  extends into a portion between the second pixel electrode  330   b  and the substrate  310  such that a capacitor-coupling electrode  330   c  is formed. In particular, the second pixel electrode  330   b  can be electrically connected to the second drain  326   b  of the sub-TFT  320   b  through a contact opening C 4 . In general, the first pixel electrode  330   a  and the second pixel electrode  330   b  can have a plurality of slits S so that liquid crystals (not shown) can be divided into a plurality of domains. 
     Furthermore, the pixel structure  300  of the invention may also include a common line pattern  342  disposed on the substrate  310 . At least a portion of the common line pattern  342  of the invention extends along the edge of the first pixel electrode  330   a  and the second pixel electrode  330   b . Obviously, the common line pattern  342  shown in  FIG. 3A  can have other shapes and can be selected on demand. Here, the shape of the common line pattern  342  is used only as an example and is not intended to limit the invention. In practice, the common line pattern  342  only has to be electrically connected to a common voltage. 
     More specifically, after turning on the main thin film transistor  320   a  and the sub-TFT  320   b , a display signal can be transmitted to the first pixel electrode  330   a  through the main thin film transistor  320   a . Here, it should be mentioned that a first storage capacitor Cst 1  (as shown in  FIG. 3B ) could be formed by the first pixel electrode  330   a  and the common line pattern  342  underneath. A first liquid crystal capacitor Clc 1  is formed by the first pixel electrode  330   a  and a common electrode of an upper color filter (not shown). 
     Additionally, a coupling capacitor Cc is formed by the second pixel electrode  330   b  and the capacitor-coupling electrode  330   c . The capacitor-coupling electrode  330   c  underneath the second pixel electrode  330   b  is coupled to the second pixel electrode  330   b . A second storage capacitor Cst 2  is formed by the second pixel electrode  330   b  and the common line pattern  342 . On the other hand, a second liquid crystal capacitor Clc 2  is formed by the second pixel electrode  330   b  and the common electrode above the upper color filter (not shown). In other words, the first pixel electrode  330   a  and the second pixel electrode  330   b  can have different voltages. As a result, the liquid crystals (not shown) corresponding to the first pixel electrode  330   a  and the second pixel electrode  330   b  can have different inclining states so as to provide a wide viewing angle in the display. 
     It should be note that the performance of the second pixel electrode  330   b  would not be affected because the conducting current of the sub-TFT  320   b  is very small. Here, it should be mentioned that the residual charges on the second pixel electrode  330   b  of a previous display picture could be removed through the sub-TFT  320   b . As a result, the problem of having residual charges on the second pixel electrode  119   b  (as shown in  FIG. 1 ) in the conventional design can be resolved. Therefore, the pixel structure  300  of the invention can effectively suppress residual image and improve display quality. Moreover, it should be said the magnitude of the conducting current of the sub-TFT  320   b  can be suitably adjusted according to the actual requirements as long as it does not affect the normal performance of the second pixel electrode  330   b.    
     In summary, a sub-thin film transistor having a very small conducting current is electrically connected to a second pixel electrode corresponding to the capacitor-coupling electrode. Therefore, the second pixel electrode may utilize the sub-thin film transistor of the invention to remove unwanted residual charges so as to prevent the performance of the second pixel electrode from being adversely affect and effectively suppress the problem of having residual image in a display picture. Ultimately, the display quality is improved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.