Abstract:
TFT LCD including a first substrate and a second substrate, a scanning line on the first substrate, a signal line formed to cross the scanning line, a channel layer formed along the signal line and extended to a portion of the scanning line, source and drain electrodes formed separated on the channel layer over the scanning line, a pixel electrode connected to the drain electrode, and a liquid crystal layer formed between the first substrate and the second substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display, and more particularly, to a TFT (Thin Film Transistor) LCD (Liquid Crystal Display) having a large aperture ratio. 
     2. Background of the Related Art 
     Referring to  FIG. 1A , an AMLCD (Active Matrix Liquid Crystal Display Device) used in a portable TV or a notebook computer, or the like is provided with a plurality of scanning lines  10  and signal lines  17  crossing each other, and pixel regions defined by the scanning lines  10  and the signal lines  17 . These are TFTs at parts where the scanning lines  10  and the signal lines are crossed; and pixel electrodes (dashed lines in the drawing) in the pixel regions for displaying a desired picture when a voltage is applied to the scanning lines  10 , to switch the TFT, and to transmit a picture signal provided to the signal lines  17  to the pixel electrodes. 
     Referring to  FIG. 1B  showing a section across line A–A′, the AMLCD is provided with a gate electrode  11  on a transparent substrate  7 , and a gate insulating layer  13  on the gate electrode  11 . There is a channel layer  15  and an ohmic contact layer  16  of amorphous silicon a-Si on the gate insulating layer  13 , and source/drain electrodes  17   a  and  17   b  on the ohmic contact layer  16 . There is a protection layer  19  on an entire surface of the source/drain electrodes  17   a  and  17   b , a contact hole  20  in the protection layer  19 . The pixel electrode  21  and the drain electrode  17   b  are connected through the contact hole  20 . In the drawing, Cgs denotes a capacitance between the gate electrode and the source electrode, Cgd denotes a capacitance between the gate electrode and the drain electrode. The Ccross denotes a capacitance in overlap regions of the scanning lines and the signal lines. The Cgs, Cgd, and Ccross are parameters influencing to an accumulated capacitance (not shown), as well as ΔVp (change in offset voltage) and ΔVpxl (change in pixel voltage). In the related art LCD, if there is a misalignment between the scanning line  10  and the signal line  17 , minute variations of Cds and Cgd give influence to ΔVp and ΔVpxl, making flicker worse and causing non-uniform luminance, that deteriorates a picture quality. And, in a divided exposure for a large sized screen, the increased deviations of Cgs and Cgd caused by poor adjustment between shots worsens the foregoing problem, to impede providing a large sized LCD screen, and, since the TFT is formed on an extension line of the scanning line, to reduce an aperture ratio of the device. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a TFT LCD that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a TFT LCD in which an effective voltage to liquid crystal is stabilized, for making a stable operation. 
     Another object of the present invention is to provide TFT LCD which can maintain a fixed capacitance between a scanning line and a drain electrode even if the scanning line and the signal line are misaligned. 
     Another object of the present invention is to provide TFT LCD which shows no deterioration of a picture quality even in divided exposure for a large sized screen. 
     Further object of the present invention is to provide TFT LCD which has an improved aperture ratio. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the TFT LCD includes a first substrate and a second substrate, a scanning line on the first substrate, a signal line formed to cross the scanning line, a channel layer formed along the signal line and extended to a portion of the scanning line, source and drain electrodes formed separated on the channel layer over the scanning line, a pixel electrode connected to the drain electrode, and a liquid crystal layer formed between the first substrate and the second substrate. 
     The channel layer has a width smaller than a width of the scanning line and the signal line overlapped between the scanning line and the signal line. 
     The signal line, also serving as a source electrode of a TFT, requires no extension line of the related art signal line, and can maintain a capacitance Cgd between the scanning line and the drain electrode always constant even if there is misalignment between the scanning line and the signal line because a pattern extended from the drain electrode overlaps the scanning line fully. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
       In the drawings: 
         FIG. 1A  illustrates a plan view of a unit pixel of a related art LCD; 
         FIG. 1B  illustrates a section across line A–A′ in  FIG. 1A ; 
         FIG. 2A  illustrates a plan view of a unit pixel of an LCD in accordance with a preferred embodiment of the present invention; 
         FIG. 2B  illustrates a section across line B–B′ in  FIG. 2A ; and, 
         FIG. 3  illustrates a plan view of a unit pixel of an LCD in accordance with another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  FIG. 2A  illustrates a plan view of a unit pixel of an LCD in accordance with a preferred embodiment of the present invention, and  FIG. 2B  illustrates a section across line B–B′ in  FIG. 2A . 
     Referring to  FIG. 2B , the LCD in accordance with a preferred embodiment of the present invention includes a scanning line (a gate electrode)  111  of a metal, such as aluminum, formed on a transparent substrate  107  by sputtering. There is an insulating layer  113  of SiNx or SiOx or the like formed thereon by APCVD (Atmospheric Chemical Vapor Deposition), and a semiconductor layer  115  and an n+ layer  115  stacked in succession thereon. The semiconductor layer  115  is formed of amorphous silicon a-Si, and the insulating layer  113  is formed of SiO 2  having good bulk characteristics, and can prevent short circuit of the gate electrode  111  and formation of hillock at the gate electrode  11  without an anodized film. There are a channel layer and an ohmic contact layer formed by etching the semiconductor layer  115  and the n+ layer  116 , and source/drain electrodes  117   a  and  117   b  of aluminum or chromium formed by sputtering and patterning. The ohmic contact layer  116  is formed by dry etching by using the source/drain electrode  117   a  and  117   b  as a mask. There is a protection layer  119  of SiNx on an entire surface of the substrate  107  formed by PECVD (Plasma Enhanced Chemical Vapor Deposition), and a pixel electrode  121  thereon in a pixel region formed by sputtering and patterning ITO (Indium Tin Oxide). The pixel electrode  121  is connected to the drain electrode  117   b  electrically through a contact hole  120  in the protection layer  119 . The channel layer  115  has a width smaller than the widths of the scanning line  111  and the signal line  117   a , and is positioned between the scanning line  111  and the signal line  117   a . In this instance, since the channel layer  115  is covered with the signal line  117   a , generation of off-current is prevented, to prevent deterioration of a picture quality caused by a residual image. The signal line  117   a , serving as a source electrode of the TFT also, requires no extension pattern of the signal line as in the related art, and maintains a constant capacitance Ccross formed in an overlap region of the scanning line  111  and the signal line  117   a . And, the signal line  117   a  maintains a constant capacitance Cgs between the scanning line  111  and the signal line  117   a  even if a misalignment occurs between the scanning line  111  and the signal line  117   a . Since a pattern extended from the drain electrode  117   b  overlaps the scanning line  111  completely, even if there is a misalignment between the scanning line  111  and the signal line  117   a , the capacitance between the scanning line  111  and the drain line  117   b  is always maintained constant. Accordingly, the ΔVp and ΔVpxl involved in the effective voltage for driving the LCD are maintained constant owing to the Cgs, Cgd and Ccross, which are always constant regardless of the misalignment. In the drawing, ‘δ’ is greater than a movement caused by the misalignment of the scanning line  111  and the signal line  117   a , ‘Δ’ is greater than a movement caused by the misalignment of the scanning line  111  and the channel layer  115 , and “Δ” is greater than a movement caused by the misalignment of the signal line  117   a  and the channel layer  115 . 
       FIG. 3  illustrates a plan view of a unit pixel of an LCD in accordance with another preferred embodiment of the present invention, wherein reference symbols are the same as the foregoing embodiment, and operation of the present embodiment is also the same. This embodiment is different from the previous embodiment in that a portion for the channel layer is extended by ‘β’ to increase a width ‘w’ of the channel layer on the scanning line, which is wider than a variation of the Cgd caused by the misalignment, to assure a more stable operation of the LCD. As shown in  FIG. 3 , it is possible that the width can be increased by ‘β′’ instead of ‘β’ (dashed portion), or by both the ‘β′’ and ‘β’. 
     As explained, the TFT LCD of the present invention has the following advantages. 
     The TFT LCD of the present invention can provide an LCD which can make a stable operation by maintaining both a capacitance Ccross formed in an overlap region of the scanning line and the signal line and the capacitance Cgs between the scanning line and the source electrode always constant, and by maintaining a capacitance Cgd between the scanning line and the drain electrode always constant by overlapping a pattern extended from the drain electrode with the scanning line completely. 
     And, the TFT LCD of the present invention is favorable for fabrication of a large sized LCD as a picture quality can be made uniform even in a case of a divided exposure, can improve an aperture ratio of the device because the TFT is formed on the scanning line. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the TFT LCD of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.