Abstract:
A liquid crystal display includes a first substrate including a first electrode; a second substrate including thereon a second electrode having at least one elongate hole having a longitudinal direction and facing to the first electrode; a third electrode positioned under the at least one hole and between the second electrode and the second substrate; and a liquid crystal layer comprising a plurality of liquid crystal molecules and interposed between the first substrate and the second substrate. The third electrode has a bias voltage being two volts higher than a pixel voltage of the sub-electrode.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display having biased bending vertical alignment. 
   BACKGROUND OF THE INVENTION 
   In the conventional liquid display  1  as shown in  FIG. 1 , the first substrate  10  has a first electrode  11 , the second substrate  30  has a second electrode  31 , and a liquid crystal layer  20  is disposed between the first electrode  11  and the second electrode  31 . The protrusion  12  disposed on the first electrode  11  can divide each pixel area into a plurality of domains. So, each liquid crystal molecules  21  is not vertical but has an angle to the first substrate  10 , called multi-domain vertical alignment (MVA). As such, the view angle of a user is increased. 
   However, the manufacture of the protrusion  12  is difficult so the cost of the conventional display  1  is very high and is easy to malfunction. 
   The other conventional liquid crystal display  4  is shown in  FIGS. 2-1  and  2 - 2 .  FIG. 2-1  is a cross-sectional view of another conventional liquid crystal display. And  FIG. 2-2  is a top view of the conventional liquid crystal display as shown in  FIG. 2-1 . The first substrate  40  has a first electrode  41 . The second substrate  60  has a plurality of second electrodes  62  each of which further has sub-electrodes  62 ′ divided by the slit  62   a . A third electrode  63  is disposed under the slit  62   a  so the liquid crystal molecules  51  of the liquid crystal layer  50  is always parallel to the first electrode  41 . And the other liquid molecules  52  have an angle to the first electrode  41 . The second electrode  62  and the third electrode  63  are disposed separately by insulating film  61 . 
   However, as shown in  FIG. 3 , the liquid crystal molecules  53  far from the third electrode  63  is pointed to the third electrode  63 . But when an electric field is present across between the first and second substrates  40  and  60 , the liquid crystal molecules  51  above the slit  62   a  are tilted and flow along the longitudinal direction of the third electrode  63  first, and after a period of time, the liquid crystal molecules  51  then rotate as the liquid crystal molecules  53 . It causes the slow responding time. 
   Another problem is that the liquid crystal molecules  52  above the edge  63 ′ of the third electrode  63  will rotate suddenly because of the electric field resulting from the overlapping of the second electrode  62  and third electrode  63 . The unstable states of the liquid crystal molecules  52  not only cause the slow responding time but also cause the flicker of the liquid crystal display  4 . 
   And yet another problem is that when the conventional liquid crystal display wants to increase the transmittance, the pixel voltage must be increased relatively. Generally speaking, the transmittance is raised while the pixel voltage is (of the sub-electrode  62 ′) raised. Please refer to the  FIG. 2-1 . However, when the pixel voltage gets closing to the value of the bias voltage (of the third electrode  63 ), the transmittance gets going down because the liquid crystal molecules  55 ′ rotate reversely and block the light which passes through the liquid crystal layer  50 . In the  FIG. 2-1 , when the pixel voltage (of the sub-electrode  62 ′) gets higher, because of the reverse area  55 , the reversely rotating molecules  5 ′ is created. Then the liquid crystal molecules  55 ′ collide with the liquid crystal molecules  53 ′ in the normal area  53  and then a colliding area  57  is created. Therefore, the transmittance of the conventional liquid crystal display are decreased by the colliding area  57 . And the importance of controlling the voltage interval between the sub-electrode  62 ′ and the third electrode  63  is also described below. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a liquid crystal display having a short responding time. 
   It is another object of the present invention to provide a liquid crystal with flicker-free. It is another object of the present invention to increase transmittance of a liquid crystal display. 
   According to one aspect of the present invention, a liquid crystal display includes a first substrate including a first electrode; a second substrate including thereon a second electrode having at least one elongate hole having a longitudinal direction and facing to the first electrode and said second electrode is supplied by a pixel voltage; a third electrode positioned under the at least one hole and between the second electrode and the second substrate and said third electrode is supplied by a bias voltage; and a liquid crystal layer including a plurality of liquid crystal molecules and interposed between the first substrate and the second substrate, wherein an interval between said pixel voltage and said bias voltage is for preventing said liquid crystal molecules rotating reversely. 
   In accordance with the present invention, the third electrode has at least one notch disposed on an edge thereof and a longitudinal direction perpendicular to the longitudinal direction of the elongate hole. 
   In accordance with the present invention, the second electrode is divided into the plurality of sub-electrodes by a plurality of slits. In accordance with the present invention, the second electrode further includes a plurality of gaps respectively aligned with the slit and pointed to the third electrode. 
   In accordance with the present invention, the liquid crystal molecules are negative dielectric anisotropy material. 
   In accordance with the present invention, the second substrate further comprises a switching element connected to said second electrode. 
   In accordance with the present invention, the third electrode is connected to an independent electrode. 
   In accordance with the present invention, the third electrode is electrically connected to a gate electrode. 
   In accordance with the present invention, the first electrode is made of a transparent material. 
   In accordance with the present invention, the second electrode is made of a transparent material. 
   In accordance with the present invention, the third electrode is made of an opaque material. 
   In accordance with the present invention, the second electrode is electrically connected to a switching element. 
   In accordance with the present invention, the interval between the second electrode and the third electrode is at least two volts. In accordance with the present invention, the first electrode is supplied by a common voltage. 
   In accordance with the present invention, while the pixel voltage is higher than the common voltage, the bias voltage is at least two volts larger than the pixel voltage. 
   In accordance with the present invention, while the pixel voltage is lower than the common voltage, the bias voltage is at least two volts smaller than the pixel voltage. 
   The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional view of a conventional liquid crystal display; 
       FIG. 2-1  is a cross-sectional view of another conventional liquid crystal display; 
       FIG. 2-2  is a top view of the conventional liquid crystal display as shown in  FIG. 2-1 ; 
       FIG. 3  is a perspective view of the structure of the second electrode and the third electrode shown in  FIG. 2-1 ; 
       FIG. 4  is a cross-section view of a liquid crystal display according to a preferred embodiment of the present invention; 
       FIG. 5  is a perspective view of the structure of the second electrode and the third electrode shown in  FIG. 4 ; 
       FIG. 6  is a top view of the structure of the second electrode and the third electrode according to a first embodiment of the present invention; 
       FIG. 7  is a top view showing the gap formed on the second electrode; 
       FIG. 8  is a top view of a second substrate according to a first embodiment of the present invention; 
       FIG. 9  is a top view of a second substrate according to a second embodiment of the present invention; 
       FIG. 10  is a top view of a second substrate according to a third embodiment of the present invention; 
       FIG. 11  is a top view of a second substrate according to a fourth embodiment of the present invention; 
       FIG. 12  is a chart showing the variation of brightness between the embodiment of FIG.  10  and the conventional liquid crystal display of  FIG. 2-2 ; 
       FIG. 13  is a chart showing the variation of brightness according to the embodiment of FIG.  10  and the embodiment of  FIG. 11 ; 
       FIG. 14  shows a voltage difference between bias voltage and pixel voltage when the liquid crystal display is in a positive electric field; and 
       FIG. 15  shows a voltage difference between bias voltage and pixel voltage when the liquid crystal display is in a negative electric field. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   According to the  FIG. 4 , the cross-sectional view of the present invention is shown. The second electrode  82  has at least one hole  82   a  above the third electrode  83 . The liquid crystal layer  50  is disposed between the first electrode  71  of the first substrate  70  and the second electrode  82  of the second substrate  80 . The slit  82   b  divides the second electrode  82  into a plurality of sub-electrodes  82 ′. The edge  83 ′ of the third electrode  83  is formed with at least one notch  83 ″. The second electrode  82  and the third electrode  83  are separated by the insulating film  81 , and the second electrode  82  is connected to the switching element  91  (FIG.  8 ). 
   According to  FIG. 5 , the holes  82   a  is elongate, and under the existance of a sufficient electric field across the first and second substrates  70  and  80  (FIG.  4 ), the liquid crystal molecules  51  will tilt along the longitudinal direction of the hole  82   a  because of the fringe electric field. So the tilted direction of the liquid crystal molecules  51  is perpendicular to the longitudinal direction of the third electrode  83 . The notch  83 ″ also has a oblique electric field to push the liquid crystal molecules  52  to be pointed to the center above the third electrode  83 , as same as the tilt direction of the liquid crystal molecules  53 . 
   According to the  FIG. 6 , the second electrode  82  has a plurality of holes  82   a  above the third electrode  83  to create the fringe electric field, and to force the liquid crystal molecules  51  to tilt as shown in FIG.  5 . 
   According to the  FIG. 7 , the sub-electrode  82 ′ of the second electrode  82  also includes a plurality of gaps  82   c  respectively aligned with the slits  82   b  ( FIG. 4 ) and pointed to the third electrode  83 . The gaps  82   c  generate a fringe electric field to restrict the liquid crystal molecules in the liquid crystal layer  50  and make the molecules recovered to original arrangement easily when the liquid crystal molecule is disordered by an outer force. 
     FIG. 8  shows the top view of the substrate according to the first embodiment of the present invention in practice. The second electrode  82  is divided into several sub-electrodes  82 ′ by the slit  82   b . The third electrode  83  is disposed under the second electrode  82  and the holes  82   a . The notch  83 ″ is formed on the third electrode  83 . The third electrode  83  is connected to the gate electrode  90 , so the third electrode  83  will be activated simultaneously with the gate electrode  90 . The second electrode  82  is connected to the switching element  91 . 
     FIG. 9  shows another top view of the second substrate  80  ( FIG. 4 ) according to the second embodiment of the present invention in practice. The third electrode  83  is electrically connected to the gate electrode  90 . And the second electrode  82  further includes gaps  82   c  respectively aligned with the slits  82   b . The second electrode  82  is connected to the switching element  91 . 
     FIG. 10  shows a further top view of the second substrate  80  ( FIG. 4 ) according to the third embodiment of the present invention in practice. The third electrode  83  is disposed under the second electrode  82  and the holes  82   a . But the third electrode  83  is not connected with the gate electrode  90 . The third electrode  83  is connected to the independent electrode  100 , so the voltage of the third electrode  83  can be controlled independently and the gate signal delay time of the present invention will be decreased. The second electrode  82  is connected to the switching element  91 . 
     FIG. 11  shows further a top view of the second substrate according to the fourth embodiment of the present invention in practice. The third electrode  83  is disposed under the second electrode  82  and the holes  82   a . But the third electrode  83  is not connected with the gate electrode  90 . The second electrode  82  further includes the gaps  82   c  respectively aligned with the slits  82   b . The third electrode  83  is connected to the independent electrode  100 , so the voltage of the third electrode  83  can be controlled independently and the gate signal delay time of the present invention can be decreased. The second electrode  82  is connected to the switching element  91 . 
   In order to raise the pixel voltages and maintain the light transmittance without dropping down, the present invention further defines that when the electric field thereof is positive, that is the voltage of the second electrode is higher than the voltage of the first electrode, the third electrode has a bias voltage being two volts higher than a pixel voltage of the sub-electrode. And when the electric field thereof is negative, that is the voltage of the second electrode is lower than the voltage of the first electrode the third electrode has a bias voltage being two volts lower than the pixel voltage of the sub-electrode. 
   Please refer to the  FIG. 12 , which is a chart showing the variation of transmittance base on the embodiment of FIG.  10  and the conventional liquid crystal display of  FIG. 2-2 , which is a top view of the liquid crystal display as shown in  FIG. 2-1 . The embodiment of the  FIG. 10  has the third electrode  83  disposed under the holes  82   a  of the second electrode  82  and connected to the independent electrode  100 . The conventional liquid crystal display  4  of  FIG. 2-2  has a third electrode  63  disposed under the slit  62   a . The embodiment of the  FIG. 10  is called E-type LCD and the conventional LCD in the  FIG. 2-2  is called F-type LCD. According to the line E- 10  in  FIG. 12 , the bias voltage of the third electrode  83  of the E-type LCD is ten volts. The line F- 10  of the F-type LCD indicates that the bias voltage of the third electrode  63  is ten volts. The voltage of the first electrode ( 41  in  FIG. 2-1  and  71  in  FIG. 4 ) of both F-type LCD and E-type LCD is kept at zero volt. Therefore, referring to the  FIG. 12 , it is clear that the light transmittance is decreased when the pixel voltage of the sub-electrode  82 ′ is over five volts and when the pixel voltage of the sub-electrode  62 ′ is over 4.5 volts. When both of the third electrode  83  and  63  are supplied by fifteen volts shown by lines E- 15  and F- 15 , the transmittance of the E-type LCD is increased until the voltage of the sub-electrode  82 ′ is increased over than 8.5 volts. But, the transmittance of the F-type LCD is dropped when the pixel voltage of the sub-electrode  62 ′ is increased over than 7 volts because of the liquid crystal molecules rotating reversely. Therefore, when the third electrode  83  and  63  are both supplied by fifteen volts, and the pixel voltage is over 8.5 volts, the liquid crystal molecules of the E-type LCD rotating reversely. And when the third electrode  83  of the E-type LCD is supplied by twenty volts, the pixel voltage can be raised as around 9.5 volts that the transmittance will not drop in the embodiment. Nevertheless, according to the F-type LCD of  FIG. 2-2 , when the pixel voltage is over eight volts, the transmittance thereof will be dropped. Therefore, to maintain the voltage of the third electrode higher than that of the sub-electrode within a proper interval for preventing the liquid crystal molecules rotating reversely is very important. Further, in the  FIG. 12 , it is clear that the proper voltage interval between the third electrode and the sub-electrode of the E-type LCD is narrower than that of the F-type LCD. Therefore, comparing to the conventional LCD in  FIG. 2-2 , the LCD in  FIG. 10  of the present invention can effectively improve the problem about the reversely rotation of the liquid crystal molecules. 
   Please refer to the  FIG. 13 , which is a chart showing the variation of transmittance in the A-type LCD of the FIG.  11  and the E-type LCD of the FIG.  10 . Both embodiments of the  FIGS. 11 and 10  have the third electrode  83  disposed under the holes  82   a  of the second electrode  82  and connected to the independent electrode  100 . But the second electrode  82  of A-type LCD of the  FIG. 11  further has gap  82   c  respectively aligned with the slits  82   b . The voltage of the first electrode ( 71  as shown in  FIG. 4 ) of both A-type LCD and E-type LCD is kept at zero volt. The line E- 10  shows that if the third electrode  83  of the E-type LCD is supplied by ten volts, the transmittance will drop when the pixel voltage is over 5 volts. And the line A- 10  shows when the third electrode  83  of the A-type LCD is supplied by ten volts, the transmittance will drop due to liquid crystal molecules rotating reversely when the pixel voltage is over eight volts. The line E- 15  indicates that the third electrode  83  of the F-type LCD is supplied by fifteen volts. The line A- 15  indicates that the third electrode  83  of the A-type LCD is supplied by fifteen volts. Referring to lines E- 15  and A- 15 , when the pixel voltage of the sub-electrode  82 ′ of E-type LCD is over 8.5 volts, the transmittance begins to drop. Similary, the transmittance of A-type LCD does not drop until the pixel voltage is over 9 volts. The lines E- 20  and A- 20  indicate that the third electrodes  83  of the E-type LCD and the A-type LCD are both supplied by 20 volts. According to the line E- 20 , when the pixel voltage is over 9.5 volts, the transmittance drops. And the line A- 20  shows that although the pixel voltage increase to ten volts, the liquid crystal molecules of the A-type LCD will not rotate reversely. Certainly, to maintain the voltage of the third electrode higher than that of the sub-electrode in a proper interval is still very important. Furthermore, from  FIG. 13 , it is clear that the proper voltage interval between the third electrode and the sub-electrode of the A-type LCD is smaller than that of the E-type LCD. So, comparing to the first embodiment of the electrodes of  FIG. 10 , the second embodiment in  FIG. 11  of the present invention can fiber improve the liquid crystal molecules rotating reversely. No matter to the first embodiment of E-type LCD or A-type LCD, the bias voltage applied on the third electrode must higher than the pixel voltage of the sub-electrode in a proper interval to prevent the liquid crystal molecules rotating reversely. Therefore, according to  FIG. 14 , when the electrical field is positive, the bias voltage Vbias, which is applied to the third electrode, must higher than the pixel voltage Vpixel, which is applied to the second electrode, an interval Va, and the interval Va is at least two volts. Base on the same reason, according to  FIG. 15 , when the electrical field is negative, the bias voltage Vbias, which is applied to the third electrode, must lower than the pixel voltage Vpixel, which is applied to the second electrode, an interval Vb, and the interval Vb is at least two volts. Furthermore, the common voltage Vcom, which is applied to the first electrode, is kept at a predetermined voltage. 
   While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.