Abstract:
A liquid crystal display includes a plurality of pixel regions ( 3 ), each of which includes a substrate portion ( 39 ) and a storage capacitor ( 37 ) arranged on the substrate portion. The storage capacitor includes a first capacitor electrode ( 38 ), an insulating layer ( 36 ) formed on the first capacitor electrode, and a second capacitor electrode ( 34 ) positioned on the insulating layer. At least one through hole ( 380 ) is arranged in at least one of the first and second capacitor electrodes. With the edge effect of the through hole(s) of the capacitor electrode(s), the capacitance of the storage capacitors can be increased significantly. Therefore, each of the storage capacitors can have a high capacitance without reducing the aperture ratio of the liquid crystal display.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to storage capacitors, and particularly relates to a storage capacitor used in a liquid crystal display (LCD).  
       BACKGROUND  
       [0002]     An active matrix LCD generally includes a plurality of pixel regions defined by a plurality of gate lines and a plurality of data lines that cross each other. A plurality of thin film transistors (TFTs) are arranged at intersections of the gate lines and the data lines. Each pixel region includes a pixel electrode, which is controlled by a corresponding TFT.  
         [0003]     When a signal is applied to a TFT, a corresponding pixel electrode is activated; that is, a voltage is applied to the pixel electrode. In order to obtain a high display effect, the pixel electrode must be maintained at a constant voltage until a next signal is applied to the TFT. For maintaining the voltage of the pixel electrode, a storage capacitor is needed.  
         [0004]     Referring to  FIG. 6 , this shows a pixel region of a conventional LCD. The pixel region  2  includes a pixel electrode  20 , a plurality of data lines  23  and gate lines  28 , a TFT  200 , and a storage capacitor  27 . The data lines  23  cross the gate lines  28  to define the pixel region  2 . The TFT  200  includes a gate electrode, a source electrode, and a drain electrode respectively connected to one of the gate lines  28 , one of the data lines  23 , and the pixel electrode  20 . The TFT  200  acts as a switch for turning on and turning off the storage capacitor  27 .  
         [0005]     Referring to  FIG. 7 , this shows a cross-section of the storage capacitor  27 . The storage capacitor  27  is arranged on a glass substrate  29 , and includes a first capacitor electrode (the gate line)  28 , a first insulating layer  26  arranged on the glass substrate  29  and the first capacitor electrode  28 , and a second capacitor electrode  24  positioned on the first insulating layer  26  over the first capacitor electrode  28 . A second insulating layer  22  is formed on the second capacitor electrode  24 . The pixel electrode  20  is arranged on the second insulating layer  22 , and is electrically connected to the second capacitor electrode  24  at a contact hole  220 .  
         [0006]     In order to achieve a high display effect, the storage capacitor  27  should maintain a constant voltage applied on the pixel electrode  20 . Therefore, the storage capacitor  27  must have a certain capacitance value. Since the storage capacitor  27  is equivalent to a capacitor with two parallel planes, the following capacitance formula is applicable:  
         C   ST     =       ɛ   ·   A     d         
 
 where “C ST ” denotes the storage capacitance value; “ε” denotes the dielectric constant of the first insulating layer  26  between the first capacitor electrode  28  and the second capacitor electrode  24 ; “A” denotes the effective area of the first capacitor electrode  28  and the second capacitor electrode  24 ; and “d” denotes the thickness of the first insulating layer  26  between the first capacitor electrode  28  and the second insulating layer  24 . Therefore, the capacitance of the storage capacitor  27  is directly proportional to the effective area “A,” and inversely proportional to the thickness “d.”
 
         [0007]     For a constant thickness “d,” the only way to increase the capacitance of the storage capacitor is to increase the effective area “A.” However, if the effective area “A” is increased, an aperture ratio of the pixel region  2  is reduced. This can significantly limit the display quality of the liquid crystal display.  
         [0008]     What is needed, therefore, is a thin film transistor which can have a large capacitance while not reducing the aperture ratio of a corresponding liquid crystal display.  
       SUMMARY  
       [0009]     In a first preferred embodiment, a storage capacitor includes a first capacitor electrode, an insulating layer formed on the first capacitor electrode, and a second capacitor electrode positioned on the insulating layer. At least one of the first and second capacitor electrodes has at least one through hole.  
         [0010]     In a second preferred embodiment, a liquid crystal display includes a plurality of pixel regions, each of which includes a substrate portion and a storage capacitor arranged on the substrate portion. The storage capacitor includes a first capacitor electrode, an insulating layer formed on the first capacitor electrode, and a second capacitor electrode positioned on the insulating layer. At least one through hole is defined in at least one of the first and second capacitor electrodes.  
         [0011]     In the storage capacitor of the above-described preferred embodiments, at least one of the first and second capacitor electrodes has at least one through hole. With the edge effect of the through hole(s) in the capacitor electrode(s), the capacitance of the storage capacitor is increased significantly. That is, the capacitance of the storage capacitor can be increased without increasing areas of the capacitor electrodes. Furthermore, the through hole(s) can improve an aperture ratio of the pixel region having the storage capacitor.  
         [0012]     Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic, top view of a pixel region of an LCD according to a first preferred embodiment of the present invention;  
         [0014]      FIG. 2  is a schematic, cross-sectional view taken along line II-II of  FIG. 1 ;  
         [0015]      FIG. 3  is a schematic, top view of a pixel region of an LCD according to a second preferred embodiment of the present invention;  
         [0016]      FIG. 4  is schematic, a top view of a pixel region of an LCD according to a third preferred embodiment of the present invention;  
         [0017]      FIG. 5  is schematic, a schematic, cross-sectional view of a pixel region of an LCD according to a fourth preferred embodiment of the present invention;  
         [0018]      FIG. 6  is a schematic, top view of a pixel region of a conventional LCD; and  
         [0019]      FIG. 7  is a schematic, cross-sectional view taken along line VI-VI of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     Referring to  FIG. 1 , this shows a pixel region of an LCD according to the first preferred embodiment of the present invention. The pixel region  3  includes a pixel electrode  30 , a plurality of data lines  33  and gate lines  38 , a TFT  300 , and a storage capacitor  37 . The data lines  33  cross the gate lines  38  to define the pixel region  3 . The TFT  300  includes a gate electrode, a source electrode, and a drain electrode respectively connected to one of the gate lines  38 , one of the data lines  33 , and the pixel electrode  30 . The TFT  300  acts as a switch for turning on and off the storage capacitor  37 .  
         [0021]     Referring to  FIG. 2 , this shows a cross-section of the storage capacitor  37 . The storage capacitor  37  is arranged on a glass substrate  39 , and includes a first capacitor electrode (the gate line)  38 , a first insulating layer  36  arranged on the glass substrate  39  and the first capacitor electrode  38 , a second capacitor electrode  34  positioned on the first insulating layer  36  over the first capacitor electrode  38 , and a second insulating layer  32  formed on the second capacitor electrode  34 . The pixel electrode  30  is arranged on the second insulating layer  32  and electrically connected to the second capacitor electrode  34  at a contact hole  320 . The first capacitor electrode  38  includes a plurality of through holes  380  defined therein.  
         [0022]     Because of the through holes  380  in the first capacitor electrode  38 , and because the first capacitor electrode  38  has a thickness not substantially different from that of the insulating layer  36 , the storage capacitor  37  is not an ideal parallel plate capacitor. According to electrostatics theory, the charge density of the first capacitor electrode  38  at the edges of the through holes  380  is greater than that of other portions of the first capacitor electrode  38 . This is known as the edge effect. Therefore if the first capacitor electrode  38  has a same area as the first capacitor electrode  28  of the above-described conventional storage capacitor  27 , and if a same voltage is applied to the capacitor electrodes  38 ,  28 , the quantity of electric charge on the first capacitor electrode  38  is much more than that of the electric charge on the first capacitor electrode  28 .  
         [0023]     The capacitance formula C=q/V is applicable, where “C” denotes the capacitance of a capacitor, “q” denotes the quantity of electric charge on one electrode of the capacitor, and “V” denotes the voltage applied on the capacitor. Therefore, the capacitance of the capacitor is directly proportional to the quantity of the electric charge “q,” and inversely proportional to the voltage “V” applied on the capacitor. In the above description, the quantity of the electric charge on the first capacitor electrode  38  is much more than that of the electric charge on the conventional first capacitor electrode  28 . Accordingly, the capacitance of the storage capacitor  37  is greater than that of the conventional storage capacitor  27 .  
         [0024]     As described above, with the edge effect of the through holes  380  of the first capacitor electrode  38 , the capacitance of the storage capacitor  37  is increased greatly. That is, the capacitance of the storage capacitor  37  can be increased without increasing areas of the first and second capacitor electrodes  38 ,  34 . Furthermore, the through holes  380  can improve the aperture ratio of the pixel region having the storage capacitor  37 . Therefore, the LCD can have a corresponding improved aperture ratio.  
         [0025]     Referring to  FIG. 3 , this shows a pixel region of an LCD according to the second preferred embodiment of the present invention. The pixel region  5  is similar to the above-described pixel region  3  of the first preferred embodiment. However, unlike with the pixel region  3 , the pixel region  5  further includes a plurality of protrusions  502  arranged at one long side of a second capacitor electrode  54  of a storage capacitor  57 . The protrusions  502  are connected with each other by a main body of the second capacitor electrode  54 . The pixel region  5  includes a plurality of gaps at the long side of the second capacitor electrode  54 , the gaps interleaving the protrusions  502 .  
         [0026]     Each protrusion  502  and a first capacitor electrode  58  cooperatively define a sub-capacitor. That is, the storage capacitor  57  is formed by a plurality of sub-capacitors connected in parallel with each other. Accordingly, the capacitance of the storage capacitor  57  is greater than that of the storage capacitor  37 . Furthermore, light beams can pass through the gaps. Accordingly, an aperture ratio of the pixel region  5  is increased.  
         [0027]     Referring to  FIG. 4 , this shows a pixel region of an LCD according to the third preferred embodiment of the present invention. The pixel region  6  is similar to the above-described pixel region  5  of the second preferred embodiment. However, unlike with the pixel region  5 , the pixel region  6  further includes a plurality of through holes  640  positioned in a second capacitor electrode  64  of a storage capacitor  67 .  
         [0028]     Referring to  FIG. 5 , this shows a pixel region of an LCD according to the fourth preferred embodiment of the present invention. The pixel region  7  is similar to the above-described pixel region  3  of the first preferred embodiment. However, in the pixel region  7 , a plurality of through holes  780  positioned in a first capacitor electrode  78  of a storage capacitor  77 , and a plurality of through holes  740  positioned in a second capacitor electrode  74  of the storage capacitor  77 . Further some of the through holes  740  and the through holes  780  are correspondingly arranged.  
         [0029]     Similar to the through holes  380  of the first preferred embodiment, with the edge effect of the through holes  640  of the second capacitor electrode  64 , the capacitance of the storage capacitor  67  is increased greatly. That is, the capacitance of the storage capacitor  67  can be increased without increasing an area of the second capacitor electrode  64 . Furthermore, the through holes  640  can improve the aperture ratio of the pixel region  6  having the storage capacitor  67 . Moreover, light beams can pass through gaps between protrusions of the storage capacitor  67 . Therefore, the LCD can have a corresponding improved aperture ratio.  
         [0030]     It is to be understood that the storage capacitor of the present invention is not limited in the above-described preferred embodiments. For example, in the pixel region  3  of the first preferred embodiment, the first capacitor electrode  38  and the second capacitor electrode  34  can be made of the same material or different materials. Any of such materials may include transparent conductive material; for example, indium tin oxide (ITO), indium zinc oxide (IZO), and so on. In other examples, in the pixel region  5  of the second preferred embodiment, instead of having the protrusions  502 , the first capacitor electrode  58  of the storage capacitor  57  may have a plurality of protrusions arranged at one long side thereof. In such case, the pixel region  5  includes a plurality of gaps at the long side of the first capacitor electrode  58 , the gaps interleaving the protrusions. Further, the storage capacitor  57  may have both the protrusions  502  and the protrusions of the first capacitor electrode  58 . Still further, the protrusions  502  and/or the protrusions of the first capacitor electrode  58  may be trapezoidal, triangular, or have another suitable shape or shapes.  
         [0031]     It is to be further understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only, and changes may be made in detail to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.