Abstract:
The present invention provides a liquid crystal display with a plurality of pixel units. Each pixel unit includes two sub-pixels. Each sub-pixel includes a thin film transistor, a liquid crystal capacitor and a storage capacitor. One of the storage capacitors is a changeable capacitor. By the changeable capacitor, two different data voltages are generated in respective sub-pixels during adjacent frames. The different data voltages are symmetrical with respect to a common voltage to improve image quality.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-part of application Ser. No. 11/119,773 filed May 3, 2005 hereby incorporated by reference as it fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a pixel structure, and more particularly to a pixel unit with improved viewing angles of a liquid crystal display. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays have been widely applied in electrical products, such as computer monitors and TV monitors, for a long time. To provide a wider viewing range, Fujitsu commercialized a multi-domain vertically aligned liquid crystal display (MVA-LCD) in 1997). MVA has almost perfect viewing angle characteristics. However, a notable weak point is that the skin color of Asian people (light orange or pink) appears whitish from an oblique viewing direction. 
     The solid line in  FIG. 1  shows the transmittance-voltage (T-V) characteristics of the MVA in the normal direction. The vertical axis is the transmittance rate. The horizontal axis is the applied voltage. When the applied voltage increases, the transmittance rate curve  101  in the normal direction also increases. The transmittance changes monotonically as the applied voltage increases. However, in the oblique direction, the transmittance rate curve  102  winds and the various gray scales become the same. Especially in the region  100 , the transmittance changes decrease as the applied voltage increases. This is the main reason that the skin color of Asian people appears or whitish from an oblique viewing direction. 
     A method is provided to improve this foregoing problem. This method combines two different T-V characteristics. The dashed line  201  in  FIG. 2  shows the original T-V characteristics in the oblique viewing direction. The dashed line  202  in  FIG. 2  shows other T-V characteristics with a higher threshold voltage. By optimizing the threshold voltage and the maximum transmittance of these two lines, monotonic characteristics can be achieved, as shown by the solid line  203  in  FIG. 2 . According to the typical method, each pixel is divided into two areas. One area has the original threshold voltage and the other area has a higher one. 
     There is a residual image problem in the typical method. According to the typical method, each pixel unit includes a plurality of sub-pixels. Each sub-pixel may generate different voltage changes after the voltage applied to the pixel unit is removed. The different voltage change may generate different data voltage in two adjacent frames when corresponding to a common electrode, which may affect the image quantity. 
     Therefore, it is also an objective to improve the image quality. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a liquid crystal display comprises a first substrate and a plurality of data lines and a plurality of scan lines arranged in the first substrate, wherein the scan lines cross the data lines to define a plurality of pixel units, each pixel unit including a first sub-pixel and a second sub-pixel, wherein each pixel unit comprises a first transistor located in the first sub-pixel, the first transistor has a gate electrode coupled to a first scan line, a drain electrode coupled to a first data line and a source electrode coupled to a first storage capacitor and a second transistor located in the second sub-pixel, the second transistor has a gate electrode coupled to the first scan line, a drain electrode coupled to the first data line and a source electrode coupled to a second storage capacitor, wherein at least one of the first storage capacitors and the second storage capacitor is a changeable capacitor. 
     According to another embodiment of the present invention, a liquid crystal display driving method is provided. The liquid crystal display has a plurality of pixel units, each pixel unit includes a first sub-pixel with a first transistor and a second sub-pixel with a second transistor, wherein the gate electrodes of the first transistor and the second transistor couple to a first scan line, and the drain electrodes of the first transistor and the second transistor couple to a first data line. The method comprises providing a high level electric potential to the first scan line for writing a data signal transferred in the first data line to a pixel electrode in the first sub-pixel and a pixel electrode in the second sub-pixel and to provide a low level electric potential to the first scan line to isolate the first transistor and the second transistor from the first data line. According to this method, in a first frame of two adjacent frames, when the first scan line is transferred from the high level electric potential to the low level electric potential, a first voltage change happens in the pixel electrode of the first sub-pixel and a second voltage change happens in the pixel electrode of the second sub-pixel, and in a second frame, when the first scan line is transferred from the high level electric potential to the low level electric potential, a third voltage change happens in the pixel electrode of the first sub-pixel and a fourth voltage change happens in the pixel electrode of the second sub-pixel, wherein at least the first voltage change is not equal to the third voltage change. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates a transmittance-voltage (T-V) characteristic of the MVA in the normal and oblique directions; 
         FIG. 2  illustrates the combination T-V characteristics in the oblique direction; 
         FIG. 3  illustrates a schematic diagram of a pixel unit according to the first embodiment of the present invention; 
         FIG. 4A  illustrates a schematic structure diagram of a metal-insulator-semiconductor capacitor; 
         FIG. 4B  illustrates a capacitance-voltage characteristic of a metal-insulator-semiconductor capacitor; 
         FIG. 5A  illustrates a cross-sectional view of a sub-pixel in accordance with the first embodiment of the present invention; 
         FIG. 5B  illustrates a cross-sectional view of a sub-pixel in accordance with the first embodiment of the present invention; 
         FIG. 6  illustrates a waveform for operating the pixel region in accordance with the first embodiment of the present invention; 
         FIG. 7  illustrates a schematic diagram of a pixel unit according to the second embodiment of the present invention; 
         FIG. 8A  illustrates a cross-sectional view of a sub-pixel in accordance with the second embodiment of the present invention; 
         FIG. 8B  illustrates a cross-sectional view of a sub-pixel in accordance with the second embodiment of the present invention;and 
         FIG. 9  illustrates a waveform for operating the pixel region in accordance with the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Without limiting the spirit and scope of the present invention, the pixel unit structure proposed in the present invention is illustrated with a plurality of embodiments. One with ordinary skill in the art, upon acknowledging the embodiments, can apply the pixel unit structure and the operation method of the present invention to various liquid crystal displays. Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     First Embodiment 
       FIG. 3  is a schematic diagram of a pixel unit according to the first embodiment of the present invention. The pixel unit  300  includes two sub-pixels  302  and  304 . 
     The sub-pixel  302  includes a thin film transistor  3021 . According to the thin film transistor  3021 , the gate electrode is connected to the scanning line  306 , the drain electrode is connected to the data line  308  and the source electrode is connected to the pixel electrode  3022 . The storage capacitor  3023  is composed of the pixel electrode  3022  and the bias electrode V bias . The liquid crystal capacitor  3024  is composed of the pixel electrode  3022  and the common electrode V com . A diffusion capacitor  3025  exists between the gate and the source electrode of the thin film transistor  3021 . 
     The sub-pixel  304  includes a thin film transistor  3041 . According to the thin film transistor  3041 , the gate electrode is connected to the scanning line  306 , the drain electrode is connected to the data line  308  and the source electrode is connected to the pixel electrode  3042 . The storage capacitor  3043  is composed of the pixel electrode  3042  and the bias electrode V bias . The liquid crystal capacitor  3044  is composed of the pixel electrode  3042  and the common electrode V com . A diffusion capacitor  3045  exists between the gate and the source electrode of the thin film transistor  3041 . 
     In this embodiment, a metal-insulator-semiconductor-metal structure, MIS structure, is used to form the storage capacitor  3023 . A metal-insulator-metal structure is used to form the storage capacitor  3043 . 
       FIG. 4A  is a schematic diagram of the storage capacitor  3023  with a metal-insulator-semiconductor-metal structure. An insulator layer  403  and a semiconductor layer  404  are located between the first metal layer  401  and the second metal layer  402 . The metal-insulator-semiconductor (MIS) structure forms a capacitor. The capacitance of a MIS capacitor is changeable and related to the value of the voltage difference (V M1 −V M2 ) between the first metal layer  401  and the second metal layer  402 .  FIG. 4B  illustrates a relation diagram between the capacitance and the voltage difference value (V M1 −V M2 ). When the voltage (V M1 ) applied to the first metal layer  401  is larger than the voltage (V M2 ) applied to the second metal layer  402 , that is the voltage difference value (V M1 −V M2 ) is larger than zero, the capacitance increases when the voltage difference value increases. When the voltage (V M1 ) applied to the first metal layer  401  is less than the voltage (V M2 ) applied to the second metal layer  402 , that is the voltage difference value (V M1 −V M2 ) is less than zero, the capacitance decreases when the voltage difference value increases. 
     The curve in the  FIG. 4B  is not symmetrical around the origin. Therefore, a bias voltage V bias  is applied to the first metal layer  401  or the second metal layer  402  to shift the original point to make the curve symmetrical around the shifted origin. In this case, when the voltage difference value (V M1 −V M2 ) is larger than the positive threshold voltage value (V thod+ ) or less than the negative threshold voltage value (V thod− ), the capacitance tends to a specific positive value or a specific negative value. In this embodiment, the capacitor may generate the capacitance C 3023, on  when a voltage difference value that is larger than the positive threshold voltage value (V thod+ ) is applied to this capacitor. The capacitor may generate the capacitance C 3023, off  when the voltage difference value that is less than the negative threshold voltage value (V thod− ) is applied to this capacitor. Moreover, the capacitor with a metal-insulator-semiconductor (MIS) structure as illustrated in  FIG. 4A  is called a changeable capacitor or voltage control capacitor, VCCAP. 
     Many pixel structure types may be used to form the pixel unit  300 .  FIG. 5A  and  FIG. 5B  illustrates one of the pixel structure types.  FIG. 5A  is a schematic diagram of the thin film transistor  3021  and the metal-insulator-semiconductor (MIS) storage capacitor  3023  in the sub-pixel  302 .  FIG. 5B  is a schematic diagram of the thin film transistor  3041  and the metal-insulator-metal (MIM) storage capacitor  3043  in the sub-pixel  304 . In  FIG. 5A , in sub-pixel  302 , the common electrode V com  is formed over a glass substrate  510 . The thin film transistor  3021  and the metal-insulator-semiconductor (MIS) storage capacitor  3023  are formed over a glass substrate  500 . A metal layer  502  is formed over the glass substrate  500  to serve as the gate metal of the thin film transistor  3021  and the first metal layer  401  (shown in the  FIG. 4A ) of the storage capacitor  3023 . An insulator layer  503  is formed over the glass substrate  500  to cover the metal layer  502  to serve as the gate insulator of the thin film transistor  3021  and the insulator layer  403  (shown in the  FIG. 4A ) of the storage capacitor  3023 . An amorphous silicon layer  504  and an n+ amorphous silicon layer  505  are sequentially formed over the gate insulator of the thin film transistor  3021  and the insulator layer  403  of the storage capacitor  3023 . The amorphous silicon layer  504  and the n+ amorphous silicon layer  505  formed over the gate insulator are used as an active region of the thin film transistor  3021 . The amorphous silicon layer  504  and the n+ amorphous silicon layer  505  formed over the insulator layer  403  (shown in the  FIG. 4A ) are used as the semiconductor layer  404  (shown in the  FIG. 4A ) of the storage capacitor  3023 . A metal layer  506  is formed over the n+ amorphous silicon layer  505 . The metal layer  506 , the amorphous silicon layer  504  and the n+ amorphous silicon layer  505  form the source and the drain electrode structure. A metal layer  506  formed over the semiconductor layer  404  (shown in the  FIG. 4A ) is used as the second metal layer  402  (shown in the  FIG. 4A ) of the storage capacitor  3023 . A passivation layer  507  is formed over the glass substrate  500  to cover the source and the drain electrode structure of the thin film transistor  3021  and the second metal layer  402  (shown in the  FIG. 4A ) of the storage capacitor  3023 . A plurality of through holes  509 ,  511  and  512  are formed in the passivation layer  507 . The through hole  509  is used to expose the source electrode of the thin film transistor  3021 . The through holes  511  and  512  are used to expose the first metal layer of the storage capacitor  3023 . An indium tin oxide, ITO, layer  508  is formed over the passivation layer  507  to connect with the source electrode of the thin film transistor  3021  and the first metal layer of the storage capacitor  3023  to serve as the pixel electrode of the sub-pixel  302 . The diffusion capacitor  3025  (shown in the  FIG. 3 ) is composed of the gate metal layer  502  and the source electrode structure of the thin film transistor  3021 . The liquid crystal capacitor  3024  (shown in the  FIG. 3 ) is composed of the ITO layer  508  and the common electrode V com  formed over the glass substrate  510 . 
     In  FIG. 5B , in sub-pixel  304 , the common electrode V com  is formed over the glass substrate  510 . The thin film transistor  3041  and the metal-insulator-metal (MIM) storage capacitor  3043  are formed over the glass substrate  500 . A metal layer  502  is formed over the glass substrate  500  to serve as the gate metal of the thin film transistor  3041  and the first electrode of the storage capacitor  3043 . An insulator layer  503  is formed over the glass substrate  500  to cover the metal layer  502  to serve as the gate insulator of the thin film transistor  3041  and the insulator layer of the storage capacitor  3043 . An amorphous silicon layer  504  and an n+ amorphous silicon layer  505  sequentially formed over the gate insulator are used as an active region of the thin film transistor  3041 . A metal layer  506  is formed over the n+ amorphous silicon layer  505 . The metal layer  506 , the amorphous silicon layer  504  and the n+ amorphous silicon layer  505  form the source and the drain electrode structure of the thin film transistor  3041 . The metal layer  506  is also used as the second electrode of the storage capacitor  3043 . A passivation layer  507  is formed over the glass substrate  500  to cover the source and the drain electrode structure of the thin film transistor  3041  and the second electrode of the storage capacitor  3043 . A through hole  513  is formed in the passivation layer  507  to expose the second electrode of the storage capacitor  3043 . An indium tin oxide, ITO, layer  508  is formed over the passivation layer  507  to connect with the second electrode of the storage capacitor  3043 . The diffusion capacitor  3045  (shown in the  FIG. 3 ) is composed of the gate metal layer  502  and the source electrode structure of the thin film transistor  3041 . The liquid crystal capacitor  3044  (shown in the  FIG. 3 ) is composed of the ITO layer  508  and the common electrode V com  formed over the glass substrate  510 . 
       FIG. 6  shows a waveform diagram for driving this pixel unit  300  according to the first embodiment of the present invention. Referring to  FIGS. 6 and 3 , in this embodiment, during the time segment T 1  of the odd frame, the scan line  306  is selected and is charged to a high-level state, V gh , to turn on the thin film transistors  3021  and  3041 . At this time, data, V P , with positive polarity transferred in the data line  308  is transferred to the storage capacitor  3023  and  3043  and the liquid crystal capacitor  3024  and  3044  through the thin film transistors  3021  and  3041 . When the time segment T 1  is almost complete, the electric potential of the scan line  306  is pulled down to a low-level state, V gL , to turn off the thin film transistors  3021  and  3041 . At this time, the voltage of the liquid crystal capacitors  3024  and  3044  are maintained by the corresponding storage capacitors  3023  and  3043 . 
     However, the instant the thin film transistors  3021  and  3041  are turned off, the voltage value of the data, V P , may fall by ΔV. The ΔV is related to the diffusion capacitor between the gate and source electrodes of thin film transistor, liquid crystal capacitor and the storage capacitor. According to the first embodiment, the pixel unit  300  includes sub-pixel  302  and sub-pixel  304 . Therefore, the pixel unit includes two ΔV values, ΔV 1  and ΔV 2 , to make the two sub-pixels have different voltage values, V P1  and V P2 . The ΔV 1  value related to the diffusion capacitor  3025  of thin film transistor  3021 , liquid crystal capacitor  3204  and the storage capacitor  3023  is shown as follows:
 
Δ V   1 =( V   gh   −V   gL )× C   3025 /( C   3025   +C   3024   +C   3023 )
 
     The ΔV 2  value related to the diffusion capacitor  3045  of thin film transistor  3041 , liquid crystal capacitor  3044  and the storage capacitor  3043  is shown as follows:
 
Δ V   2 =( V   gh   −V   gL )× C   3045 /( C   3045   +C   3044   +C   3043 )
 
     According to this embodiment, the storage capacitor  3023  is a changeable metal-insulator-semiconductor capacitor. Therefore, during the odd frame for writing positive polarity data, the voltage value of the data, V P , is larger than the bias voltage value, V bias . That is the voltage applied to the first metal layer  401  is larger than the voltage applied to the second metal later  402  as shown in the  FIG. 4A . In this case, the voltage difference value (V M1 −V M2 ) is larger than not only than zero but also the positive threshold voltage value (V thod+ ). According to this embodiment, the storage capacitor  3023  may generate the capacitance C 3023, on  as shown in the  FIG. 4B . Therefore, during the odd frame for writing positive polarity data, the ΔV 2  is shown as follows:
 
Δ V   1 (on)=( V   gh   −V   gL )× C   3025 /( C   3025   +C   3024   +C   3023,on )
 
     During the even frame for writing negative polarity data time segment T 2 , the scan line  306  is selected and is charged to a high-level state, V gh , to turn on the thin film transistors  3021  and  3041 . At this time, data, −V P , with negative polarity transferred in the data line  308  is transferred to the storage capacitor  3023  and  3043  and the liquid crystal capacitors  3024  and  3044  through the thin film transistors  3021  and  3041 . When the time segment T 2  is almost over, the electric potential of the scan line  306  is pulled down to a low-level state, V gL , to turn off the thin film transistors  3021  and  3041 . At this time, the voltage of the liquid crystal capacitors  3024  and  3044  are maintained by the corresponding storage capacitors  3023  and  3043 . 
     However, the instant the thin film transistors  3021  and  3041  are turned off, the voltage value of the data, −V P , may fall by ΔV. The ΔV is related to the diffusion capacitor between the gate and source electrodes of the thin film transistor, liquid crystal capacitor and the storage capacitor. 
     According to this embodiment, the storage capacitor  3023  is a changeable metal-insulator-semiconductor capacitor. Therefore, during the even frame for writing negative polarity data, the voltage value of the data, −V P , is less than the bias voltage value, V bias . That is the voltage applied to the first metal layer  401  is less than the voltage applied to the second metal later  402  as shown in the  FIG. 4A . In this case, the voltage difference value (V M1 −V M2 ) is less than not only zero but also the negative threshold voltage value (V thod− ). According to this embodiment, the storage capacitor  3023  may generate the capacitance C 3023, off  as shown in the  FIG. 4B . Therefore, during the even frame for writing negative polarity data, the ΔV 1  is shown as follows:
 
Δ V   1 (off)=( V   gh   −V   gL )× C   3025 /( C   3025   +C   3024   +C   3023,off )
 
     In the sub-pixel  302 , the ΔV 2  value related to the diffusion capacitor  3045  of thin film transistor  3041 , the liquid crystal capacitor  3044  and the storage capacitor  3043  is shown as follows:
 
Δ V   2 =( V   gh   −V   gL )× C   3045 /( C   3045   +C   3044   +C   3043 )
 
     The storage capacitor  3023  is a changeable metal-insulator-semiconductor capacitor. Therefore, for the sub-pixel  302 , the voltage change when the negative polarity data is written is different from the voltage change when the positive polarity data is written. According to this embodiment, Because the capacitance C 3023, on  is larger than the capacitance C 3023, off , the voltage change ΔV 1 (ON) when positive polarity data is written is less than the voltage change ΔV 1 (Off) when negative polarity data is written. The storage capacitor  3043  is a metal-insulator-metal capacitor. Therefore, for the sub-pixel  304 , the voltage change is always ΔV 2  no matter if the voltage change ΔV 2  occurs when negative polarity data is written or when positive polarity data is written. 
     According to this embodiment, the capacitance of the diffusion capacitor  3025  is equal to the capacitance of the diffusion capacitor  3045 . The capacitance of the liquid crystal capacitor  3024  is equal to the capacitance of the liquid crystal capacitor  3044 . The storage capacitor  3023  is a changeable metal-insulator-semiconductor capacitor. Therefore, when positive polarity data is written, the capacitance C 3023, on  of the storage capacitor  3023  is larger than the capacitance of the storage capacitor  3043 . On the other hand, when negative polarity data is written, the capacitance C 3023, off , of the storage capacitor  3023  is less than the capacitance of the storage capacitor  3043 . Therefore, the relationship of the voltage change values is ΔV 1 (Off)&gt;ΔV 2 &gt;ΔV 1 (On). In the foregoing embodiment, the diffusion capacitor  3025  is set to be equal to the capacitance of the diffusion capacitor  3045  and the capacitance of the liquid crystal capacitor  3024  is set to be equal to the capacitance of the liquid crystal capacitor  3044 . However, the foregoing capacitance set does not limit the present invention. 
     Please refer to the  FIG. 6  again. The storage capacitor  3023  is a changeable metal-insulator-semiconductor capacitor. Therefore, for the sub-pixel  302 , the voltage change from writing negative polarity data and the voltage change from writing positive polarity data is different in the instant that the thin film transistors  3021  and  3041  in the sub-pixel  302  are turned off. The storage capacitor  3043  is a non-changeable metal-insulator-metal capacitor. Therefore, for the sub-pixel  304 , the voltage change from writing negative polarity data and the voltage change from writing positive polarity data is same in the instant that the thin film transistors  3021  and  3041  in the sub-pixel  302  are turned off. Therefore, in this embodiment, adjusting the capacitance of the storage capacitor  3023  makes the data voltage of the even frame symmetrical to the data voltage of the odd frame in the common electrode V com  after the thin film transistors  3021  and  3041  in the sub-pixel  302  are turned off. In other words, for the sub-pixel  302 , the data voltage V 1,o  in the odd frame is equal to the data voltage V 1,e  in the even frame. For the sub-pixel  304 , the data voltage V 2,o  in the odd frame is equal to the data voltage V 2,e  in the even frame. 
     The optical characteristics of the sub-pixel  302  can be evaluated by the root mean square voltage of V 1,0  and V 1,e . The optical characteristic of the sub-pixel  304  can be evaluated by the root mean square voltage of V 2,0  and V 2,e . 
     The root mean square voltage value of the sub-pixel  302  is shown as follows: 
     
       
         
           
             
               RMS 
               ⁢ 
               
                   
               
               ⁢ 
               of 
               ⁢ 
               
                   
               
               ⁢ 
               sub 
               ⁢ 
               
                   
               
               ⁢ 
               pixel 
               ⁢ 
               
                   
               
               ⁢ 
               302 
             
             = 
             
               
                 
                   
                     V 
                     
                       1 
                       , 
                       0 
                     
                     2 
                   
                   + 
                   
                     V 
                     
                       1 
                       , 
                       e 
                     
                     2 
                   
                 
                 2 
               
             
           
         
       
     
     The root mean square voltage value of the sub-pixel  304  is shown as follows: 
     
       
         
           
             
               RMS 
               ⁢ 
               
                   
               
               ⁢ 
               of 
               ⁢ 
               
                   
               
               ⁢ 
               sub 
               ⁢ 
               
                   
               
               ⁢ 
               pixel 
               ⁢ 
               
                   
               
               ⁢ 
               304 
             
             = 
             
               
                 
                   
                     V 
                     
                       2 
                       , 
                       0 
                     
                     2 
                   
                   + 
                   
                     V 
                     
                       2 
                       , 
                       e 
                     
                     2 
                   
                 
                 2 
               
             
           
         
       
     
     According to the first embodiment, each pixel unit includes two sub-pixels. Therefore, the optical characteristics of the whole pixel region are a combination of the optical characteristics of the two sub-pixels. One of the two sub-pixels has a changeable storage capacitor. Therefore, by adjusting the capacitance of the storage capacitor  3023 , the data voltage of the even frame will be symmetrical to the data voltage of the odd frame in the common electrode V com . Such a method may optimize the optical characteristic of this whole pixel. 
     It is noticed that the changeable storage capacitor is located in the sub-pixel  302  in the first embodiment. However, in other embodiments, the changeable storage capacitor is located in the sub-pixel  304 . Moreover, in other embodiments, a plurality of changeable storage capacitors is located in the sub-pixel  302  and the sub-pixel  304  respectively. On the other hand, the value of the bias voltage V bias  is not be limited in this embodiment. However, in other embodiments, the bias voltage V bias  is connected to the common electrode V com . 
     Second Embodiment 
       FIG. 7  is a schematic diagram of a pixel unit according to the second embodiment of the present invention. The pixel unit  700  includes two sub-pixels  702  and  704 . 
     The sub-pixel  702  includes a thin film transistor  7021 . According to the thin film transistor  7021 , the gate electrode is connected to the scanning line  706 , the drain electrode is connected to the data line  708  and the source electrode is connected to the pixel electrode  7022 . The storage capacitor  7023  is composed of the pixel electrode  7022  and the bias electrode V bias . The liquid crystal capacitor  7024  is composed of the pixel electrode  7022  and the common electrode V com . A diffusion capacitor  7025  exists between the gate and the source electrode of the thin film transistor  7021 . 
     The sub-pixel  704  includes a thin film transistor  7041 . According to the thin film transistor  7041 , the gate electrode is connected to the scan line  706 , the drain electrode is connected to the data line  708  and the source electrode is connected to the pixel electrode  7042 . The storage capacitor  7043  is composed of the pixel electrode  7042  and the bias electrode V bias . The liquid crystal capacitor  7044  is composed of the pixel electrode  7042  and the common electrode V com . A diffusion capacitor  7045  exists between the gate and the source electrode of the thin film transistor  7041 . 
     In this embodiment, a metal-insulator-semiconductor-metal structure, MIS structure, is used to form the storage capacitor  7023 . A metal-insulator-metal structure is used to form the storage capacitor  7043 . 
     The storage capacitor  7023  with a metal-insulator-semiconductor-metal structure is shown in the  FIG. 4A . The relation diagram between the capacitance and the voltage difference value (V M1 −V M2 ) is shown in the  FIG. 4B . As described in the foregoing first embodiment, when the voltage difference value (V M1 −V M2 ) is larger than a positive threshold voltage value (V thod+ ) or less than a negative threshold voltage value (V thod− ), the capacitance tends to a specific positive value or a specific negative value. In this embodiment, the capacitor may generate the capacitance C 7023, on  when the voltage difference value that is larger than the positive threshold voltage value (V thod+ ) is applied to this capacitor. The capacitor may generate the capacitance C 7023, off  when the voltage difference value that is less than the negative threshold voltage value (V thod− ) is applied to this capacitor. 
     The main difference between the first embodiment and the second embodiment is the connection structure between the storage capacitor and the thin film transistor. In the first embodiment, the first metal layer of the storage capacitor  3023  is connected to the source electrode structure of the thin film transistor  3021  through a through hole and the second metal layer of the storage capacitor  3023  is connected to the bias voltage V bias . However, in the second embodiment, the first metal layer of the storage capacitor  7023  is connected to the bias voltage V bias  and the second metal layer of the storage capacitor  7023  is connected to the source electrode structure of the thin film transistor  7021 . 
     Many pixel structure types may be used to form the pixel unit  700 .  FIG. 8A  and  FIG. 8B  illustrates one of the pixel structure types.  FIG. 8A  is a schematic diagram of the thin film transistor  7021  and the metal-insulator-semiconductor (MIS) storage capacitor  7023  in the sub-pixel  702 .  FIG. 8B  is a schematic diagram of the thin film transistor  7041  and the metal-insulator-metal (MIM) storage capacitor  7043  in the sub-pixel  704 . In  FIG. 8A , in sub-pixel  702 , the common electrode V com  is formed over a glass substrate  810 . The thin film transistor  7021  and the metal-insulator-semiconductor (MIS) storage capacitor  7023  are formed over a glass substrate  800 . A metal layer  802  is formed over the glass substrate  800  to serve as the gate metal of the thin film transistor  7021  and the first metal layer  401  (shown in the  FIG. 4A ) of the storage capacitor  7023 . An insulator layer  803  is formed over the glass substrate  800  to cover the metal layer  802  to serve as the gate insulator of the thin film transistor  7021  and the insulator layer  403  (shown in the  FIG. 4A ) of the storage capacitor  7023 . An amorphous silicon layer  804  and n+ amorphous silicon layer  805  are sequentially formed over the gate insulator of the thin film transistor  7021  and the insulator layer  403  of the storage capacitor  7023 . The amorphous silicon layer  804  and the n+amorphous silicon layer  805  are used as a semiconductor layer of the thin film transistor  7021  and used as the semiconductor layer  404  (shown in the  FIG. 4A ) of the storage capacitor  7023 . A metal layer  806  is formed over the n+ amorphous silicon layer  805 . The metal layer  806 , the amorphous silicon layer  804  and the n+ amorphous silicon layer  805  form the source and the drain electrode structure. A metal layer  806  is used as the second metal layer  402  (shown in the  FIG. 4A ) of the storage capacitor  7023 . It is noticed that in this embodiment, the source electrode of the thin film transistor  7021  is connected to the second metal layer of the storage capacitor  7023 . The drain electrode of the thin film transistor  7021  is connected to the data line. A passivation layer  807  is formed over the glass substrate  800  to cover the source and the drain electrode structure of the thin film transistor  7021  and the second metal layer  402  (shown in the  FIG. 4A ) of the storage capacitor  7023 . A through hole  809  is formed in the passivation layer  807  to expose the second metal layer of the storage capacitor  7023 . An indium tin oxide, ITO, layer  808  is formed over the passivation layer  807  to connect with the second metal layer  402  of the storage capacitor  7023  to serve as the pixel electrode  7022  of the sub-pixel  702 . The diffusion capacitor  7025  (shown in the  FIG. 7 ) is composed of the gate metal layer  802  and the source electrode structure of the thin film transistor  7021 . The liquid crystal capacitor  7024  (shown in the  FIG. 7 ) is composed of the ITO layer  808  and the common electrode V com  formed over the glass substrate  810 . 
     In  FIG. 8B , in sub-pixel  704 , the common electrode V com  is formed over the glass substrate  810 . The thin film transistor  7041  and the metal-insulator-metal (MIM) storage capacitor  7043  are formed over the glass substrate  800 . A metal layer  802  is formed over the glass substrate  800  to serve as the gate metal of the thin film transistor  7041  and the first electrode of the storage capacitor  7043 . An insulator layer  803  is formed over the glass substrate  800  to cover the metal layer  802  to serve as the gate insulator of the thin film transistor  7041  and the insulator layer of the storage capacitor  7043 . An amorphous silicon layer  804  and an n+ amorphous silicon layer  805  sequentially formed over the gate insulator are used as semiconductor layers of the thin film transistor  7041 . A metal layer  806  is formed over the source electrode and the drain electrode of the thin film transistor  7041  and the insulator layer of the storage capacitor  7043 . The metal layer  806 , the amorphous silicon layer  804  and the n+ amorphous silicon layer  805  form the source and the drain electrode structure of the thin film transistor  7041 . The metal layer  806  is also used as the second electrode of the storage capacitor  7043 . In this embodiment, the source electrode of the thin film transistor  7041  is connected to the second electrode of the storage capacitor  7043 . The drain electrode of the thin film transistor  7041  is connected to the data line. A passivation layer  807  is formed over the glass substrate  800  to cover the source and the drain electrode structure of the thin film transistor  7041  and the second electrode of the storage capacitor  7043 . A through hole  811  is formed in the passivation layer  807  to expose the second electrode of the storage capacitor  7043 . An indium tin oxide, ITO, layer  808  is formed over the passivation layer  807  to connect with the second electrode of the storage capacitor  7043 . The diffusion capacitor  7045  (shown in the  FIG. 7 ) is composed of the gate metal layer  802  and the source electrode structure of the thin film transistor  7041 . The liquid crystal capacitor  7044  (shown in the  FIG. 7 ) is composed of the ITO layer  808  and the common electrode V com  formed over the glass substrate  810 . 
       FIG. 9  shows a waveform diagram for driving this pixel unit  700  according to the second embodiment of the present invention. Referring to  FIG. 7  and  FIG. 9 , in this embodiment, during the time segment T 1  of the odd frame, the scan line  706  is selected and is charged to a high-level state, V gh , to turn on the thin film transistors  7021  and  7041 . At this time, data, V P , with positive polarity transferred in the data line  708  is transferred to the storage capacitor  7023  and  7043  and the liquid crystal capacitor  7024  and  7044  through the thin film transistors  7021  and  7041 . When the time segment T 1  is almost over, the electric potential of the scan line  706  is pulled down to a low-level state, V gL , to turn off the thin film transistors  7021  and  7041 . At this time, the voltage of the liquid crystal capacitors  7024  and  7044  are maintained by the corresponding storage capacitors  7023  and  7043 . 
     However, the instant the thin film transistors  7021  and  7041  are turned off, the voltage value of the data, V P , may fall by ΔV. The ΔV is related to the diffusion capacitor, liquid crystal capacitor and the storage capacitor. 
     According to the second embodiment, the storage capacitor  7023  is a changeable metal-insulator-semiconductor capacitor as shown in  FIG. 4A . Therefore, during the odd frame for writing positive polarity data, the voltage value of the data, V P , is larger than the bias voltage value, V bias . That is the voltage applied to the second metal later  402  is larger than the voltage applied to the first metal layer  401  as shown in the  FIG. 4A . In this case, the voltage difference value (V M1 −V M2 ) is less than not only zero but also a negative threshold voltage value (V thod− ). According to this embodiment, the storage capacitor  7023  may generate the capacitance C 7023, off  as shown in the  FIG. 4B . Therefore, during the odd frame for writing positive polarity data, the ΔV 1  is shown as follows:
 
Δ V   1 (off)=( V   gh   −V   gL )× C   7025 /( C   7025   +C   7024   +C   7023,off )
 
     The ΔV 2  value related to the diffusion capacitor  7045  of thin film transistor  7041 , liquid crystal capacitor  7044  and the storage capacitor  7043  is shown as follows:
 
Δ V   2=(   V   gh   −V   gL )× C   7045 /( C   7045   +C   7044   +C   7043 )
 
     During the even frame for writing negative polarity data time segment T 2 , the scan line  706  is selected and is charged to a high-level state, V gh , to turn on the thin film transistors  7021  and  7041 . At this time, data, −V P , with negative polarity transferred in the data line  708  is transferred to the storage capacitor  7023  and  7043  and the liquid crystal capacitor  7024  and  7044  through the thin film transistors  7021  and  7041 . When the time segment T 2  being almost over, the electric potential of the scan line  706  is pulled down to a low-level state, V gL , to turn off the thin film transistors  7021  and  7041 . At this time, the voltage of the liquid crystal capacitors  7024  and  7044  are maintained by the corresponding storage capacitors  7023  and  7043 . 
     However, the instant the thin film transistors  7021  and  7041  are turned off, the voltage value of the data, −V P , may fall by ΔV. The ΔV is related to the diffusion capacitor, the liquid crystal capacitor and the storage capacitor. 
     According to this embodiment, the storage capacitor  7023  is a changeable metal-insulator-semiconductor capacitor as shown in the  FIG. 4A . Therefore, during the even frame for writing negative polarity data, the voltage value of the data, −V P , is less than the bias voltage value, V bias . That is the voltage applied to the first metal layer  401  is larger than the voltage applied to the second metal later  402  as shown in the  FIG. 4A . In this case, the voltage difference value (V M1   31  V M2 ) is larger than not only zero but also the positive threshold voltage value (V thod+ ). According to this embodiment, the storage capacitor  7023  may generate the capacitance C 7023, on  as shown in the  FIG. 4B . Therefore, during the even frame for writing negative polarity data, the ΔV 1  is shown as follows:
 
Δ V   1 (on)=( V   gh   −V   gL )× C   7025 /( C   7025   +C   7024   +C   7023,on )
 
     In the sub-pixel  704 , the ΔV 2  value related to the diffusion capacitor  7045  of thin film transistor  7041 , liquid crystal capacitor  7044  and the storage capacitor  7043  is shown as follows:
 
Δ V   2 =( V   gh   −V   gL )× C   7045 /( C   7045   +C   7044   +C   7043 )
 
     The storage capacitor  7023  is a changeable metal-insulator-semiconductor capacitor. Therefore, for the sub-pixel  702 , the voltage change when negative polarity data is written is different from the voltage change when positive polarity data is written. Because the capacitance C 7023, on  is larger than the capacitance C 7023, off , the voltage changeΔV 1 (ON) when negative polarity data is written is smaller than the voltage change ΔV 1 (Off) when positive polarity data is written. The storage capacitor  7043  is a metal-insulator-metal capacitor. Therefore, for the sub-pixel  704 , the voltage change is always ΔV 2  no matter when negative polarity data is written or when positive polarity data is written. 
     According to this embodiment, the capacitance of the diffusion capacitor  7025  is equal to the capacitance of the diffusion capacitor  7045 . The capacitance of the liquid crystal capacitor  7024  is equal to the capacitance of the liquid crystal capacitor  7044 . The storage capacitor  7023  is a changeable metal-insulator-semiconductor capacitor. Therefore, when positive polarity data is written, the capacitance C 7023, off  of the storage capacitor  7023  is less than the capacitance of the storage capacitor  7043 . On the other hand, when negative polarity data is written, the capacitance C 7023, on , of the storage capacitor  7023  is larger than the capacitance of the storage capacitor  7043 . Therefore, the relationship of the voltage change value is ΔV 1 (Off)&gt;ΔV 2 &gt;ΔV 1 (On). 
     Please refer to the  FIG. 9  again. The storage capacitor  7023  is a changeable metal-insulator-semiconductor capacitor. Therefore, for the sub-pixel  702 , the voltage change for writing negative polarity data and the voltage change for writing positive polarity data is different in the instant that the thin film transistors  7021  and  7041  are turned off. The storage capacitor  7043  is a non-changeable metal-insulator-metal capacitor. Therefore, for the sub-pixel  704 , the voltage change for writing negative polarity data and the voltage change for writing positive polarity data is the same in the instant that the thin film transistors  7021  and  7041  are turned off. Therefore, in this embodiment, adjusting the capacitance of the storage capacitor  7023  makes the data voltage of the even frame symmetrical to the data voltage of the odd frame in the common electrode V com  after the thin film transistors  7021  and  7041  are turned off. In other words, for the sub-pixel  702 , the data voltage V 1,o  in the odd frame is equal to the data voltage V 1,e  in the even frame. For the sub-pixel  704 , the data voltage V 2,o  in the odd frame is equal to the data voltage V 2,e  in the even frame. 
     Also, the optical characteristic of the sub-pixel  702  can be evaluated by the root mean square voltage of V 1,0  and V 1,e , and the optical characteristic of the sub-pixel  704  can be evaluated by the root mean square voltage of V 2,0  and V 2,e , like in the foregoing first embodiment 
     According to the second embodiment, each pixel unit includes two sub-pixels. Therefore, the optical characteristic of the whole pixel region is the combination of the optical characteristics of the two sub-pixels. One of the two sub-pixels has a changeable storage capacitor. Therefore, by adjusting the capacitance of the storage capacitor  7023 , the data voltage of the even frame will be symmetrical to the data voltage of the odd frame in the common electrode V com . Such a method may optimize the optical characteristics of this whole pixel. 
     Similarly, according to this embodiment, the changeable storage capacitor is located in the sub-pixel  702  in the second embodiment. However, in other embodiments, the changeable storage capacitor is located in the sub-pixel  704 . Moreover, in other embodiments, a plurality of changeable storage capacitors is located in the sub-pixel  702  and the sub-pixel  704  respectively. On the other hand, the value of the bias voltage V bias  is not limited in this embodiment. However, in other embodiments, the bias voltage V bias  is connected to the common electrode V com . 
     Accordingly, each pixel unit includes two sub-pixels. Each sub-pixel includes a storage capacitor, a liquid crystal capacitor and a thin film transistor. One of the storage capacitors is a changeable capacitor or voltage control capacitor. By adjusting the capacitance of the changeable storage capacitor, the data voltage of the even frame is symmetrical to the data voltage of the odd frame in the common electrode V com  after the thin film transistors are turned off to improve the optical characteristic of the pixel unit. 
     As is understood by a person skilled in the art, the foregoing descriptions of the preferred embodiment of the present invention are an illustration of the present invention rather than a limitation thereof. Various modifications and similar arrangements are included within the spirit and scope of the appended claims. The scope of the claims should be accorded to the broadest interpretation so as to encompass all such modifications and similar structures. While a preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.