Abstract:
A pixel structure includes a backup thin film transistor with a float gate electrode or a float drain electrode and a working thin film transistor. When the pixel of the working thin film transistor does not work, the backup thin film transistor replaces the working thin film transistor to drive the pixel. The method of repairing the pixel is to cut off the connection between the drain electrode of the working thin film transistor and the pixel electrode, and then to connect the gate electrode or the drain electrode of the backup thin film transistor to the gate electrode or the drain electrode of the working thin film transistor, such that the backup thin film transistor can replace the working thin film transistor to drive the pixel electrode.

Description:
1. FIELD OF THE INVENTION 
       [0001]    This invention relates to a pixel structure and repair method thereof, and, more especially, to utilize a backup thin film transistor to repair a defective pixel structure and its method. 
       2. BACKGROUND OF THE RELATED ART 
       [0002]    In general, to repair a defective pixel to a dark point or a gray point is employed in repairing defective pixel of a thin film transistor liquid crystal display (TFT LCD), but it is still a permanent defective pixel. 
         [0003]      FIG. 1  shows the conventional equivalent circuit of a pixel structure to explain the circuit of a TFT LCD. As shown in this figure, for a pixel, the gate electrode  410  electrically connects to a scan line  200 , the source electrode  420  electrically connects to a data line  100  and the drain electrode  430  electrically connects to a pixel electrode  300 . 
         [0004]      FIG. 2  is the conventional equivalent circuit of a repaired pixel, which is repaired into a dark point. The drain electrode  430  of the pixel is shortened at a repair point  510  to electrically connect to a scan line  200 . Since the pixel electrode  300  is electrically connected to the scan line  200  directly, the defective pixel shows a dark point. 
         [0005]      FIG. 3  is the equivalent circuit of a repaired pixel, which is repaired into a gray point. The drain electrode  430  of the pixel is shortened at a repair point  520  to electrically connect to a data line  100 . Since the pixel electrode  300  is electrically connected to the data line  100  directly, the defective pixel becomes a gray point. 
         [0006]    However, the dark point or the gray point, which is still a defect of a TFT LCD and influence display performance. Furthermore, a bright point is a critical defect for a TFT LCD, so it is repaired into a dark point or a gray point. However, the display performance of a TFT LCD is still not significantly improved because of the permanent existence of a defective pixel. So, how to repair a defective pixel into a normal pixel and to prevent the defect is an important technique. 
       SUMMARY OF THE INVENTION 
       [0007]    For solving the abovementioned problems, one object of this invention is to provide a pixel structure, which utilizes a backup thin film transistor (TFT) for repairing a defective pixel. Moreover, the backup TFT has a top gate electrode to hold the driven power to improve the space efficiency, so it will not affect the aperture ratio of the TFT LCD. 
         [0008]    Another object of this invention is to provide a method of repairing a defective pixel. It utilizes a backup TFT to replace the TFT of a defective pixel instead of transforming the defective point to a dark point or a gray point. 
         [0009]    For achieving the abovementioned objects, this invention provides a pixel structure disposed on a substrate. The pixel structure includes a data line, a scan line a first TFT, a second TFT, a pixel electrode and a pattern. The scan line and the data line are arranged on the substrate. The first and the second TFTs include a gate electrode, a source electrode and a drain electrode respectively. For the first TFT, the gate of the first TFT is electrically connected to the scan line, and the source electrode to the data line, and the drain electrode to the pixel electrode via a first contact hole. For the second TFT, the gate electrode of the second TFT is floating, and the source electrode is electrically connected to the data line and the drain electrode to the drain electrode of the first TFT. The pattern is designed at the layer of the source and drain electrodes of the first and the second TFTs, and its two ends, designed as the repair points, overlap the gate electrodes of the first and second TFTs in space. 
         [0010]    The method of repairing a defective pixel is to cut off the pixel electrode and the drain electrode of the first TFT, and then to electrically connect the pattern with the gate electrodes of the first and the second TFTs when the pixel damages, so the second TFT will drive the pixel instead of the first TFT. 
         [0011]    For achieving the abovementioned objects, this invention provides a pixel structure disposed on a substrate. The pixel structure includes a data line, a scan line, a first TFT, a second TFT, a pixel electrode and a pattern. The scan line and the data line are arranged on the substrate. The first and the second TFTs include a gate electrode, a source electrode and a drain electrode respectively. For the first TFT, the gate of the first TFT is electrically connected to the scan line, and the source electrode to the data line, and the drain electrode to the pixel electrode via a first contact hole. For the second TFT, the gate electrode of the second TFT is electrically connected to the scan line, and the source electrode is electrically connected to the data line and the drain electrode is floating. The pattern is designed at the layer of the gate electrodes of the first and the second TFTs, and its two ends, designed as the repair points, overlap the drain electrodes of the first and second TFTs. 
         [0012]    The method of repairing a defective pixel is to cut off the pixel electrode and the drain electrode of the first TFT, and then to electrically connect the pattern with the drain electrodes of the first and the second TFTs when the pixel damages, so the second TFT will drive the pixel instead of the first TFT. 
         [0013]    For preserving the driving power of the pixel electrode to improve the space efficiency, the second TFT is designed as a double-gate TFT. In the second TFT, it has a top gate electrode opposite to the gate electrode at the layer of the pixel electrode, and the top gate electrode is electrically connected to the gate electrode via a second contact hole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a diagram showing an equivalent circuit of a conventional pixel structure. 
           [0015]      FIG. 2  is a diagram showing an equivalent circuit of a conventional dark-repaired pixel structure. 
           [0016]      FIG. 3  is a diagram showing an equivalent circuit of a conventional gray-repaired pixel structure. 
           [0017]      FIG. 4  is a diagram showing an equivalent circuit of a pixel structure according to the first embodiment of this invention. 
           [0018]      FIG. 5  is a diagram showing the top view of a pixel structure according to the first embodiment of this invention. 
           [0019]      FIG. 6  is a diagram showing the top view of a pixel structure according to the second embodiment of this invention. 
           [0020]      FIG. 7  is a diagram showing the sectional view of a pixel structure according to the second embodiment of this invention. 
           [0021]      FIG. 8  is a diagram showing an equivalent circuit of a pixel structure according to the third embodiment of this invention. 
           [0022]      FIG. 9  is a diagram showing the top view of a pixel structure according to the third embodiment of this invention. 
           [0023]      FIG. 10  is a diagram showing the top view of a pixel structure according to the fourth embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The pixel structure of a thin film transistor liquid crystal display (TFT LCD) is disposed on a substrate, where the substrate includes a plurality of data lines and crossing scan lines to define a plurality of pixel regions. Each of those pixel regions includes a first TFT, a second TFT, a pixel electrode and a repair pattern. The first and the second TFTs include a gate electrode, a source electrode and a drain electrode respectively. Those data lines and scan lines are configured to drive the first thin film transistor and the second thin film transistor. In the first TFT, the drain electrode is electrically connected to the pixel electrode and the gate electrode to the scan line and the source electrode to the data line. In the second TFT, the drain electrode or the gate electrode is floating, but the source electrode is electrically connected to the data line. 
         [0025]    Once a pixel damages, the repair method is to cut off the pixel electrode and the drain electrode of the first TFT and then to electrically connect the gate electrodes or the drain electrodes of the first and the second TFTs. Therefore, the second TFT will drive the pixel electrode instead of the first TFT to repair the defective pixel. The laser cutting method is generally employed in cutting off the connection between the drain electrode of the first TFT and pixel electrode, and the laser welding method is employed in connecting the connection between the gate electrodes or the drain electrodes of the first and the second TFTs. 
         [0026]    For improving the space efficiency due to the area occupied by the second TFT and keeping the driving power of the pixel electrode, a top gate electrode is designed, where the top gate is designed opposite to the gate electrode of the second TFT at the layer of the pixel electrode. The top gate electrode is electrically connected to the gate electrode of the second TFT such that the current channel may be enlarged to preserve the driving power, wherein the top gate electrode may be a transparent electrode, which is configured at the same layer of and formed at the same time to the pixel electrode. 
         [0027]    For understanding this invention, the following utilizes different embodiments accompanying drawings to illustrate the spirit of this invention. 
         [0028]      FIG. 4  shows the equivalent circuit of a pixel structure according to the first embodiment of this invention. The gate electrode  410  of a first TFT is electrically connected to a scan line  200 , and the source electrode  420  to a data line  100 , and the drain electrode  430  to a pixel electrode  300 . For a second TFT, its gate electrode  810  is a float electrode, and its source electrode  820  is electrically connected to the data line  100 , and its drain electrode  830  to the pixel electrode  300 . A repair pattern  530  is designed at the layer of the source electrode  820  and the drain electrode  830  of the second TFT, and its two ends, defined repair points  531 ,  532 , overlap the gate electrodes  410 ,  810  of the first and the second TFTs. 
         [0029]    For repairing, the connection between the drain electrode  430  of the first TFT and the pixel electrode  300  is to cut off at a cutting point  600 . Next, the repair pattern  530  is electrically connected to the gate electrode  410  of the first TFT via the repair point  531  and to the gate electrode  810  of the second TFT via the repair point  532 . 
         [0030]      FIG. 5  is a schematic diagram showing the top view of the pixel structure of the first embodiment. As shown in the figure, for the second TFT, its gate electrode  810  is a float electrode, and its source electrode  820  is electrically connected to the data line  100 , and its drain electrode  830  to the pixel electrode  300  via a first contact hole  861 . For the first TFT, its gate electrode  410  is electrically connected to the scan line  200 , and its source electrode  420  to the data line  100 , and its drain electrode  430  to the pixel electrode  300  via a first contact hole (CH)  861 , and the drain electrode extends to form the drain electrode  830  of the second TFT. 
         [0031]    The cutting point  600  is defined on the drain electrode  430  that is positioned above the gap between the gate electrode  410  and the pixel electrode  300 . The repair pattern  530  is designed at the layer of the source electrodes  420 ,  820  and the drain electrodes  430 ,  830 , and its two ends, which are defined as the repair point  531  and the repair point  532 , overlap the gate electrodes  410 ,  810  of the first and the second TFTs respectively. 
         [0032]    Once a pixel damages, a laser cutting method is employed to cut the connection between the drain electrode  430  of the first TFT and the pixel electrode  300  at the cutting point  600 , and then a laser welding method is employed to connect the repair points  531 ,  532  of the repair pattern  530  with the gate electrodes  410 ,  810  of the first and the second TFTs respectively, such that the second TFT will replace the first TFT to drive the pixel electrode  300 . 
         [0033]      FIG. 6  is the top view of a schematic diagram showing the pixel structure of a second embodiment according to this invention. It differs from the first embodiment that the second TFT is a double-gate-electrode TFT. A top gate electrode  840  is designed opposite to the gate electrode  810  at the layer of the pixel electrode  300 . Furthermore, the top gate electrode  840  is electrically connected to the gate electrode  810  via a second contact hole  841 . 
         [0034]      FIG. 7  is a sectional diagram showing the structure of the second TFT in the second embodiment. As shown in figure, the second TFT includes the top gate electrode  840  and the gate electrode  810 , and these two gate electrodes are electrically connected to induce a larger current channel to keep the driving power of the pixel electrode  300 . 
         [0035]      FIG. 8  shows the equivalent circuit of the pixel structure according the third embodiment of this invention. For the first TFT, its gate electrode  410  is electrically connected to a scan line  200 , and its source electrode  420  to a data line  100 , and its drain electrode  430  to a pixel electrode  300 . For the second TFT, its gate electrode  810  is electrically connected to the scan line  200 , and its source electrode  820  to the data line  100 , and its drain electrode  830  is a float electrode, and a repair pattern  540  is designed. The repair method is to cut off the drain electrode  430  of the first TFT and the pixel electrode  300  at the cutting point  600 , and then to electrically connect the drain electrodes  430 ,  830  of the first and the second TFTs to the repair points  541 ,  542 , such that the second TFT will replace the first TFT to drive the pixel electrode  300 . 
         [0036]      FIG. 9  is a schematic diagram showing the top view of a pixel structure of the third embodiment. As shown in the figure, for the second TFT, its gate electrode  810  is electrically connected to the scan line  200 , and its source electrode  820  to the data line  100 , and its drain electrode  830  is a float electrode. For the first TFT, its gate electrode  410  is electrically connected to the scan line  200 , and its source electrode  420  to the data line  100 , and its drain electrode  430  to the pixel electrode  300  via a first contact hole  861 . 
         [0037]    The cutting point  600  is defined on the drain electrode  430  that is positioned above the gap between the gate electrode  410  and the pixel electrode  300 . The repair pattern  540  is designed at the layer of the gate electrodes  410 ,  810 , and its two ends, defined as the repair points  541 ,  542 , overlap the drain electrodes  430 ,  830  of the first and the second TFTs in space respectively. 
         [0038]      FIG. 10  is the top view of a schematic diagram showing the pixel structure of a fourth embodiment according to this invention. It differs from the third embodiment that the second TFT is a double-gate-electrode TFT. A top gate electrode  850  is designed opposite to the gate electrode  810  at the layer of the pixel electrode  300 . Furthermore, the top gate electrode  850  is electrically connected to the gate electrode  810  via a second contact hole  851 . 
         [0039]    As mentioned above, the repair method of the third and the fourth embodiments is similar with that of the first and the second embodiments. The laser cutting method is employed to cut off the drain electrode  430  of the first TFT and the pixel electrode  300  at cutting point  600 , and then the laser welding method to connect the drain electrode  430  of the first TFT and the drain electrode  830  of the second TFT to the repair pattern  540  at repair points  541 ,  542  respectively, such that the second TFT will replace the first TFT to drive the pixel electrode  300 . 
         [0040]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that other modifications and variation can be made without departing the spirit and scope of the invention as claimed.