Patent Publication Number: US-6982775-B2

Title: Liquid crystal display having reduced flicker

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having reduced flicker. 
   2. Description of the Prior Art 
   A thin film transistor display, such as a thin film transistor liquid crystal display (TFT-LCD), utilizes many thin film transistors, in conjunction with other elements such as capacitors and bonding pads, arranged in a matrix as switches for driving liquid crystal molecules to produce brilliant images. The advantages of the TFT-LCD over a conventional CRT monitor include better portability, lower power consumption, and lower radiation. Therefore, the TFT-LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), electronic toys, etc. 
   Referring to  FIG. 1  and  FIG. 2 .  FIG. 1  is a schematic diagram of a prior art TFT-LCD  10 .  FIG. 2  is an equivalent circuit diagram of the TFT-LCD  10 . The TFT-LCD  10  comprises a scanning line control circuit  12 , a signal line control circuit  14 , and a pixel array  16  having a plurality of pixels connected to scanning lines. For example, a pixel A, a pixel B, and a pixel C are connected to a common scanning line. As shown in  FIG. 2 , a pixel  20  comprises a liquid crystal cell (LC), connected to a common counter electrode (CE), and a thin film transistor (TFT), which comprises a gate electrode connected to a scanning line G 0 , a drain electrode connected to a signal line D 0 , and a source electrode connected to a pixel electrode of the liquid crystal cell. Additionally, the pixel  20  contains a storage capacitor (SC) connected between the liquid crystal cell and a scanning line G 1 . The storage capacitor is used to reduce the voltage variation of the liquid crystal cell due to current leakage and thus help the liquid crystal cell to store electric charges. 
   As shown in  FIG. 2 , the light passing through the pixels varies with the voltage applied to the liquid crystal cell. By changing the voltage to the liquid crystal cell, the amount of light passing through each pixel can be changed and thus the TFT-LCD can display predetermined images. The voltage applied to the liquid crystal cell is the difference between the voltage of the common counter electrode and the voltage of the pixel electrode. When the thin film transistor is turned off, the pixel electrode is on a floating status. If any fluctuations occur in the voltages of electric elements around the pixel electrode, the fluctuations will cause the voltage of the pixel electrode to deviate from its desirable voltage. The deviation of the voltage of the pixel electrode is referred to feed-through voltage (V FD ), which is represented by:
 
 V   FD   =[C   GS /( C   LC   +C   SC   +C   GS )]*Δ V   G   (1)
 
   where C LC  is the capacitance of the liquid crystal cell (LC), C SC  is the capacitance of the storage capacitor (SC), C GS  is the capacitance between the source electrode and the gate electrode of the thin film transistor, and ΔV G  is the amplitude of a pulse voltage applied to the gate electrode. 
   In general, adjusting the voltage of the common counter electrode can compensate for the feed-through voltage. However, because the resistance and the capacitance of the scanning line round the falling edge of a pulse voltage applied to the gate electrode, a feed-through voltage of a pixel decreases as the distance between the scanning line control circuit and the pixel increases. For example, as shown in  FIG. 1 , feed-through voltage of the pixel A is larger than that of the pixel B, whose feed-through voltage is larger than that of the pixel C (that is, (V FD ) A &gt;(V FD ) B &gt;(V FD ) C  where (V FD ) A ,(V FD ) B , and (V FD ) C  represent feed-through voltages of the pixels A, B, C, respectively). Accordingly, it is difficult to compensate feed-through voltages for all pixels by adjusting the voltage of the common counter electrode. Therefore, it is hard to provide a TFT-LCD without flicker. 
   The method disclosed in U.S. Pat. No. 6,028,650 attempts to solve the above-mentioned problem. Referring to  FIG. 3 .  FIG. 3  is a top view of a pixel array  30  of the TFT-LCD  10 . The pixel array  30  comprises scanning lines  32  and  32   a  connected to a scanning line control circuit (DR 1 ), signal lines  34   a ,  34   b ,  34   c , and pixels A, B, C, which correspond to pixels A, B, C shown in  FIG. 1 . Pixels A, B, C comprise thin film transistors Q A , Q B , Q C  respectively, and their corresponding liquid crystal cells. The gate electrodes of thin film transistors Q A , Q B , Q C  are connected to the scanning line  32 . The drain electrodes of thin film transistors Q A , Q B , Q C  are connected to signal lines  34   a ,  34   b ,  34   c  respectively. The source electrodes of thin film transistors Q A , Q B , Q C  are respectively connected to pixel electrodes  38   a ,  38   b ,  38   c  of the liquid crystal cells. 
   To form the pixel array  30 , first a patterned conductive layer, serving as scanning lines  32  and  32   a , is formed on a substrate (not shown). Next, an insulating layer and a semi-conductive layer are sequentially added. Then, a second patterned conductive layer, serving as signal lines  34   a ,  34   b ,  34   c , is deposited on the semi-conductive layer. Finally, a transparent conductive layer is deposited to form pixel electrodes  38   a ,  38   b , and  38   c  of pixels A, B, C. An overlapping region  40   a  of the scanning line  32   a  and the pixel electrode  38   a  is a storage capacitor of the pixel A. Similarly, overlapping regions  40   b ,  40   c  are storage capacitors of pixels B, C. Capacitances of the storage capacitors of pixels A, B, C are represented by (C SC ) A , (C SC ) B , (C SC ) C . The area of the overlapping region  40   a  is larger than that of the overlapping region  40   b , whose area is larger than that of the overlapping region  40   c . As a result, (C SC ) A  is larger than (C SC ) B , which is larger than (C SC ) C . Thus, feed-through voltages of pixels A, B, C, represented by (V FD ) A , (V FD ) B , and (V FD ) C , are approximately equal (that is, (V FD ) A ≈(V FD ) B ≈(V FD ) C ). 
   In brief, the above-mentioned method adjusts the capacitances of storage capacitors to compensate feed-through voltages of all the pixels. As a storage capacitor gets farther from the scanning line control circuit, its capacitance becomes smaller. As a result, it is hard for such storage capacitor with low capacitance to help the liquid crystal cells hold electric charges. Besides, as a storage capacitor gets closer to the scanning line control circuit, its capacitance becomes larger and thus, the width of the scanning line should be made wider so as to form the storage capacitor. However, the aperture ratio of the LCD apparatus will decrease as the width of the scanning line increases. 
   SUMMARY OF INVENTION 
   It is therefore a objective of the claimed invention to provide a liquid crystal display (LCD) having reduced flicker to solve the above-mentioned problem. 
   According to the claimed invention, a liquid crystal display (LCD) having reduced flicker includes a plurality of signal lines, a plurality of scanning lines, and a plurality of pixels. Each pixel includes a liquid crystal cell having a pixel electrode, a storage capacitor, and a switching transistor. The switching transistor includes a gate electrode connected to one of the scanning lines, a drain electrode connected to one of the signal lines, and a source electrode connected to the pixel electrode. An overlapping region is between the gate electrode and the source electrode. The area of the overlapping region increases by increasing the distance between an input end of the scanning line corresponding to the overlapping region and the pixel electrode corresponding to the overlapping region. 
   It is an advantage that the claimed invention adjusts the capacitance between the gate electrode and the source electrode of the thin film transistor by varying the areas of the overlapping regions so that feed-through voltages of all pixels are approximately equal. There are no changes occurring to the storage capacitors and the width of the scanning lines. Thus, the storage capacitors can help the liquid crystal cells hold the electric charges effectively. The aperture ratio of the LCD apparatus can be improved as well. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated with figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art TFT-LCD. 
       FIG. 2  is a circuit diagram of the TFT-LCD shown in  FIG. 1 . 
       FIG. 3  is a top view of a pixel array of the TFT-LCD in  FIG. 1 . 
       FIG. 4  is a top view of a pixel array of a TFT-LCD according to the present invention. 
       FIG. 5  is a top view of the pixel array of another TFT-LCD according to the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 4 ,  FIG. 4  is a top view of a pixel array of a TFT-LCD according to the present invention. As shown in  FIG. 4 , a pixel array  50  comprises a scanning line  52  electrically connected to a scanning line control circuit (DR 1 ), signal lines  54   a ,  54   b ,  54   c , and pixels A, B, C, which respectively correspond to pixels A, B, C shown in  FIG. 1 . Pixels A, B, C comprise thin film transistors T A , T B , T C  respectively, and their corresponding liquid crystal cells. The gate electrodes of thin film transistors T A , T B , T C  are connected to the scanning line  52 . The drain electrodes of thin film transistors T A , T B , T C  are connected to signal lines  54   a ,  54   b ,  54   c  respectively. The source electrodes of thin film transistors T A , T B , T C  are respectively connected to pixel electrodes  58   a ,  58   b ,  58   c  of the liquid crystal cells. Region  60   a  (drawn as slash) is an overlapping region of the scanning line  52  and the source electrode  56   a . Region  60   b  (drawn as slash) is an overlapping region of the scanning line  52  and the source electrode  56   b . Region  60   c  (drawn as slash) is an overlapping region of the scanning line  52  and the source electrode  56   c . In addition, the gate electrodes of thin film transistors T A , T B , T C  comprise blocks  57   a ,  57   b ,  57   c  which are located within overlapping regions  60   a ,  60   b ,  60   c . The area of the block  57   a  is smaller than that of the block  57   b , whose area is smaller than that of the block  57   c . Thus, the area of the overlapping region  60   a  is smaller than that of the overlapping region  60   b , whose area is smaller than that of the overlapping region  60   c . A pair of protective structures  62   a  is provided, preventing the block  57   a  from being separated from the gate electrode. The protective structures  62   a  are located on both sides of the block  57   a  or within the overlapping region  60   a . Similarly, protective structures  62   b ,  62   c  are provided for preventing the blocks  57   b ,  57   c  from being separated from the gate electrodes. 
   To form the pixel array  50 , first a patterned conductive layer, serving as the scanning line  52 , is formed on a substrate (not shown). Then, an insulating layer and a semi-conductive layer are sequentially deposited on the scanning line  52  and the substrate. A second patterned conductive layer, serving as signal lines  54   a ,  54   b ,  54   c , is deposited on the semi-conductive layer. Finally, a transparent conductive layer is deposited to form pixel electrodes  58   a ,  58   b ,  58   c  of pixels A, B, C. 
   Please refer to equation (1). In general, both C SC  and C LC  are much larger than C SC  (i.e. C SC , C LC &gt;&gt;C GS ). Therefore, equation (1) can be rewritten as follows:
 
 V   FD   =[C   GS /( C   LC   +C   SC )]*Δ V   G   (2)
 
   Please refer to equation (2). Regarding pixels A, B, C shown in  FIG. 4 , if (C GS ) A =(C GS ) B =(C GS ) C ,(C SC ) A =(C SC ) B =(C SC ) C , and (C LC ) A =(C LC ) B =(C LC ) C , the feed-through voltages of pixels A, B, C is (V FD ) A &gt;(V FD ) B &gt;(V FD ) C . However, if (C GS ) A &lt;C GS ) B &lt;(C GS ) C ,(C SC ) A =(C SC ) B =(C SC ) C , and (C LC ) A =(C LC ) B =(C LC ) C , then (V FD ) A ≈(V FD ) B ≈(V FD ) C . That is, feed-through voltages of pixels A, B, C, are approximately equal as long as the condition (C GS ) A &lt;C GS ) B &lt;(C GS ) C  is achieved. Accordingly, the present invention is adding blocks  57   a ,  57   b ,  57   c  beside the gate electrodes. The area of the overlapping region  60   a  is smaller than that of the overlapping region  60   b , whose area is smaller than that of the overlapping region  60   c . In this manner, (C GS ) A  is smaller than (C GS ) B , which is smaller than (C GS ) C . Thus, feed-through voltages of pixels A, B, C, are approximately equal (that is, (V FD ) A ≈(V FD ) B ≈(V FD ) C ). 
   In the first embodiment of the present invention, there are 1024 pixels in the pixel array  50 , which is divided into a plurality of regions. The blocks added beside the gate electrodes in a common region have approximately equal areas. The area of the block in a first region is greater than the area of the block in a second region adjacent to the first region by a predetermined value. For example, as shown in  FIG. 4 , as the region I is next to the region II, an area of the block  57   b  is greater than an area of the block  57   a  by the predetermined value. Similarly, as the region II is next to the region III, an area of the block  57   c  is greater than an area of the block  57   b  by the predetermined value. Additionally, the shapes of blocks  57   a ,  57   b ,  57   c  are not necessarily rectangular. They can be any shape as long as the area of the overlapping region  60   a  is smaller than that of the overlapping region  60   b , whose area is smaller than that of the overlapping region  60   c.    
   Please refer to  FIG. 5 .  FIG. 5  is a top view of a pixel array of another embodiment of TFT-LCD according to the present invention. As shown in  FIG. 5 , the source electrodes of thin film transistors T A , T B , T C  comprise blocks  59   a ,  59   b ,  59   c , which are located within the overlapping regions  60   a ,  60   b ,  60   c  (drawn as slash). The area of the block  59   a  is smaller than that of the block  59   b , whose area is smaller than that of the block  59   c . Thus, the area of the overlapping region  60   a  is smaller than that of the overlapping region  60   b , whose area is smaller than that of the overlapping region  60   c . In this manner, (C GS ) A  is smaller than (C GS ) B , which is smaller than (C GS ) C . Thus, feed-through voltages of pixels A, B, C, are approximately equal (that is, (V FD ) A ≈(V FD ) B ≈(V FD ) C ). It should be again noted that the blocks  59   a ,  59   b ,  59   c  can be any shape as long as an area of the overlapping region  60   a  is smaller than that of the overlapping region  60   b , whose area is smaller than that of the overlapping region  60   c.    
   Furthermore, in both embodiments, the pixel array  50  can be divided into 1024 regions where each region comprises only one pixel. In this manner, feed-through voltages of all pixels are precisely equal. 
   In brief, the present invention adjusts the capacitance C GS  between the gate electrode and the source electrode of the thin film transistor so that feed-through voltages of all pixels are approximately equal. To adjust the capacitance C GS , blocks with variable areas are added to the gate electrodes or to the source electrodes. An area of an overlapping region of the gate electrode and the source electrode is increased by increasing the distance between an input end of the scanning line corresponding to the overlapping region and the pixel corresponding to the overlapping region. Thus, the capacitance C GS  can be effectively adjusted. 
   In comparison with prior art, the present invention adjusts the capacitance C GS  by varying areas of the overlapping regions of the gate electrode and the source electrode so that feed-through voltages of all pixels are approximately equal. Therefore, a liquid crystal display having reduced flicker is provided. There are no changes occurring on the storage capacitors and the width of the scanning lines. Thus, the storage capacitors can help the liquid crystal cells hold an electric charge effectively. The aperture ratio of the LCD apparatus can be improved as well. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.