Patent Publication Number: US-2010127267-A1

Title: Alternative thin film transistors for liquid crystal displays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/456,794 filed on Jun. 6, 2003. This application is also related to co-owned U.S. Pat. No. 6,903,754, issued on Jun. 7, 2005, U.S. Pat. No. 7,187,353, issued on Mar. 6, 2007, U.S. Pat. No. 7,268,758, issued on Sept. 11, 2007, U.S. Pat. No. 7,397,655, issued on Jul. 8, 2008, the contents of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Thin-film transistor (TFT) misalignment and parasitic capacitance can degrade the quality and performance of electronic devices such as liquid crystal displays (LCDs). One known attempt to correct for TFT misalignments and any associated increase in parasitic capacitance is found in U.S. Pat. No. 5,191,451 to Katayama et at (“the ‘451 patent”).  FIG. 1A  depicts the “double TFT” arrangement  100  of the ‘451 patent. Source line  104  connects to the TFT via source electrode  106 . Two gate electrodes  108  are connected to gate line  102 . Two drain electrodes  110  connect to the pixel and are formed such that the two gate electrodes  108  affect conduction from the source electrode to the drain electrodes when activated. It is noted that there are two crossover regions  112  that are connected to TFT may produce additional parasitic capacitance between the gate and the source. As discussed in the ‘451 patent, any vertical misalignment of the TFT placement is somewhat corrected by this double TFT arrangement as is discussed therein. 
     Another manner of reducing the ill effects of TFT misalignment is shown in U.S. Pat. No. 5,097,297 to Nakazawa (“the ‘297 patent”).  FIG. 4  depicts a TFT  400  made in the manner taught in the ‘297 patent. As may be seen in  FIG. 2 , gate line  402  delivers the gate signal to gate electrode  408 . Source line  404  sends image data to source electrodes  406 . When the gate electrode is activated, the image data is transferred to the pixel via the drain electrode  410 . It is noted that this TFT embodiment contains only one gate crossover  412  which aids in reducing parasitic capacitance. 
     Furthermore, prior LCDs use the same orientation to align transistor in the pixel area of the display. However, for alternative pixel arrangements, transistors may need to be located in unconventional locations of a pixel area, while addressing misalignment and parasitic capacitance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in, and constitute a part of this specification illustrate exemplary implementations and embodiments of the invention and, together with the description, serve to explain principles of the invention. 
         FIG. 1  shows a prior art TFT having a double source/drain structure. 
         FIGS. 2 and 3  show alternative TFTs having a double source/drain structure. 
         FIG. 4  shows a prior art TFT with a double gate structure. 
         FIG. 5  show TFT structures in a reverse orientation and normal orientation, respectively. 
         FIG. 6  show TFT structures in a reverse orientation and normal orientation with an added gate crossover in the normal orientation to balance any parasitic capacitance found in the reverse orientation. 
         FIGS. 7  show TFT structures in a reverse orientation and normal orientation with one fewer gate crossover in the reverse orientation to match any parasitic capacitance in the normal orientation. 
         FIG. 8  shows one novel pixel element design having a corner removed from the pixel to balance parasitic capacitances. 
         FIG. 9  shows yet another novel pixel element design having multiple corners removed to balance parasitic capacitances. 
         FIG. 10  shows yet another novel pixel structure in which at least one extra line is added to shield the pixel element from parasitic effects. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to implementations and embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The following implementations and embodiments disclose alternative thin film transistors for liquid crystal displays are disclosed. The alternative transistors can be used for panels of displays such as liquid crystal displays (LCDs), especially those having alternative pixel arrangements. These transistors can be oriented on a panel of an LCD using different, non-traditional configurations, while addressing misalignment and parasitic capacitance. 
       FIGS. 2 and 3  provide different alternative embodiments to the prior art double TFT structure shown in  FIG. 1 . These structures can provide reduced source to gate capacitance, which can cause crosstalk in certain images. However, the gate to drain crossover can lessen the damage to image quality. One advantage of the embodiment of  FIG. 3  is that there is only one crossover  132  that may reduce parasitic capacitance. 
     Another set of TFT redesigns are shown in  FIGS. 5 through 10  to handle the unevenness of parasitic capacitance that might be introduced by the above described TFT remapping. As TFTs are remapped on the panel, it is possible for some TFTs on the panel to be implemented in different corners or quadrants of a pixel area. For example, some TFTs may be constructed in the upper left hand corner of the pixel area, some in the upper right hand corner of the pixel area and so on. If all such TFTs were constructed the same way, then it would be likely that the source-drain orientation would be reversed for left hand corner and right hand corner implementation. Such non-uniformity of construction might introduce uneven parasitic capacitance in the case of a given TFT misalignment. 
       FIG. 5  is one embodiment of a TFT built with a reverse orientation  502  as compared with a TFT built with a typical orientation  1904 . For exemplary purposes, TFT  504  is constructed within the upper left hand corner of its associated pixel in the usual manner—i.e. without any crossovers to avoid any introduced parasitic capacitance. It is noted that the source (S) and drain (D) electrodes are placed in a left-to-right fashion. TFT  502  is shown constructed in the upper right hand corner of a pixel area in a reverse orientation—i.e. a crossover  514  from source line  1906  is constructed so that the source electrode  1910  and drain electrode  512  are also in left-to-right fashion. Thus, if there is a TFT misalignment in the horizontal direction, then TFTs  502  and  504  will receive the same amount of added parasitic capacitance—thus, keeping the panel&#39;s defects uniform. It will be appreciated that although TFT  502  and TFT  504  are depicted side-by-side and connected to the same column, this is primarily for explanatory purposes. It is unlikely that two adjoining subpixels would share the same column/data line—thus, TFT  504  and its associated pixel is provided to show the distinction between a normal TFT orientation and TFT  502  in a reverse orientation. 
       FIG. 6  shows another embodiment of TFTs  602  and  604 . As can be seen, a new crossover  606  is added to TFT  604  so as to balance the added parasitic capacitance via crossover  604 .  FIG. 7  is yet another embodiment of TFTs  702  and  704 . As may be seen here, the gate electrode crossover  606  in  FIG. 6  has been removed in favor of a gate line crossover  706  which may have a lesser impact on individual pixel elements. 
       FIGS. 8 and 9  are embodiments of pixel elements with corners  810  and  910  removed to match the one corner removed containing the TFT structure. These pixel elements as designed here may balance the parasitic capacitances than a normal pixel structure. 
       FIG. 10  is another embodiment of a pixel structure that employs at least one extra metal line  1010  that may help to shield the pixel element from the parasitic capacitances between the gate lines and the pixel element. Additionally, if a dot inversion scheme is employed, then the opposing polarities on both lines  1010  will also help to balance any parasitic capacitance between the source lines and the pixel elements. 
     Regarding the alternative TFT structures and pixel elements disclosed herein, standard LCD fabrication techniques can be implemented to form such structures. Moreover, the column, gate, and electrode lines can be formed of transparent material such as transparent conductive oxide so as not to degrade the optical qualities of the LCD.