Abstract:
The active matrix display device has a thin film transistor and a pixel electrode, which is provided with a pixel voltage through the thin film transistor, for each of pixels. A supplemental pixel electrode, which is connected to the pixel electrode of one of the pixels adjacent to each other, and which extends to the region between the two pixels adjacent to each other, is also disposed. The supplemental pixel electrode enables the region between the pixels to be used as a part of the display region. The liquid crystal of this region is also driven by the voltage same as the pixel electrode. The configuration of the peripheral circuit of the pixel portion is simplified, reducing the framing area of the panel.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an active matrix display device, specifically to an active matrix display device with an improved display quality.  
           [0003]    2. Description of the Related Art  
           [0004]    A flat panel display device including a reflection type active matrix liquid crystal display device (referred to as LCD hereinafter) can be thin, small and light, and it operates with low-power consumption. The LCD has been used as a display part in various devices such as mobile information device. The LCD, whose pixel has a switching element and a thin film transistor, is known as an active matrix type. The panel of the active matrix display device is highly reliable for maintaining displayed contents of the pixels, which provides the active matrix display device with high display quality.  
           [0005]    [0005]FIG. 7 shows an equivalent circuit of a pixel in the active matrix LCD. Each pixel has a thin film transistor (TFT)  11  connected to a gate line and a data line. When the TFT is turned on by a selection signal outputted to the gate line, the data corresponding to the display content is supplied to a liquid crystal capacitance  12  (Clc) from the data line through the TFT. It is necessary to accurately keep the display data from the time when the TFT is first selected for writing to the time when the TFT is selected again in the next sequence. Therefore, a storage capacitance  13  (Csc) is connected to the TFT in series with the liquid crystal capacitance Clc.  
           [0006]    [0006]FIG. 8 is a plan view showing the configuration of the pixel portion on a TFT forming substrate (a first substrate  100  in FIG. 9) of the conventional LCD. FIG. 9 is a cross-sectional view of the LCD configuration along with the X-X cross sectional line in FIG. 8. The LCD has a first substrate and a second substrate with the liquid crystal between them. In the active matrix LCD, the TFTs  11  and pixel electrodes  74  are arranged in a matrix configuration on the first substrate  100 . A common electrode  56 , to which a common voltage V com is supplied, and a color filter  54  are disposed on the second substrate  500 , which is disposed facing to the first substrate  100 . The voltage applied between the pixel electrode  74  and the common electrode  56 , which are facing each other with the liquid crystal between them, drives the liquid crystal capacitance Clc.  
           [0007]    The TFT disposed for each of the pixels on the first substrate  100  side is a top-gate type TFT, whose gate electrode is located above an active layer  64 , as seen from FIG. 9. The active layer  64  of the TFT is patterned on the substrate  100  as shown in FIG. 8. A gate insulating layer  66  is disposed covering over the active layer  64 , and the gate line, which also functions as a gate electrode, is disposed on the gate insulating layer  66 . The part of the active layer  64  facing against the gate electrode is a channel region. A drain region  64   d  and a source region  64   s  with an impurity doped are formed at the corresponding sides of the channel region.  
           [0008]    The drain region  64   d  of the active layer  64  is connected to the data line, which functions also as a drain electrode  70 , through a contact hole formed in an interlayer insulating layer  68  covering the gate electrode.  
           [0009]    Also, a flattening insulating layer  72  is disposed covering the data line and the drain line  70 . The source region  64   s  of the active layer  64  is connected to pixel electrode  74  made of ITO (Indium Tin Oxide) on the flattening insulating layer  72  through a contact hole.  
           [0010]    The source region  64   s  of the active layer  64  functions also as a first electrode  80  of the storage capacitance Csc disposed for each of the pixels and extends further, as shown in FIG. 8, from the contact region of the pixel electrode  74 . A second electrode  84  of the storage capacitance element Csc is formed simultaneously with and in the same layer as the gate electrode as seen from FIG. 9, but it is formed in a region away from the gate electrode, keeping a certain distance between them. The gate insulating layer  66  also works as a dielectric between the first electrode  80  and the second electrode  84 . The second electrode  84  of the storage capacitance element Csc, are not independently disposed for each of the pixel as seen from FIG. 8. But it is disposed in the pixel region along the row direction of the matrix in the same manner as the gate line  60 . A predetermined storage capacitance voltage Vsc is applied to the second electrode  84 .  
           [0011]    The storage capacitance element Csc disposed for each of the pixels maintains the electric charge corresponding to the display contents, which should be applied to the liquid crystal Clc, when the TFT is not selected. Therefore, the voltage change of the pixel electrode  74  can be maintained, enabling the display contents to be kept unchanged during one sequence.  
           [0012]    The gate line  60  and the second electrode  84  (a storage capacitance line) for forming the storage capacitance element Csc are disposed in parallel. The location of these two lines with respect to the location of the pixel electrode  74  is shown in FIG. 10. The gate line  60  and the second electrode  84  (a storage capacitance line) are disposed in the layer under the pixel electrodes  74 , adjacent to each other. The portions of the lines between the pixels shown as shaded areas in the figure are not covered by either of the pixel electrodes  74 .  
           [0013]    The liquid crystal corresponding to the portions of the lines shown as shaded areas occasionally shows white light. This is often observed when the adjacent pixels appear as black elements. When white light is observed, the display quality is deteriorated.  
           [0014]    The gate line  60  and the second-electrode  84  (the storage capacitance line) are usually made of reflecting material such as aluminum, molybdenum and chrome, which reflect light. Therefore, because the pixel electrodes  74 , which control the reflection of the light, do not exist at the upper layer of the lines (shaded in the figure) in the conventional device, the alignment of the liquid crystal corresponding to this portion can not be controlled. This leads to a frequent white light observation. Although this problem can be solved by disposing a black matrix (BM), the aperture ratio would be reduced.  
           [0015]    Therefore, this invention is directed to prevent the abovementioned problem and to offer the active matrix display device of high quality display.  
         SUMMARY OF THE INVENTION  
         [0016]    The active matrix display device of this invention has a thin film transistor and a pixel electrode, which is provided with a pixel voltage through the thin film transistor, for each of pixels. A supplemental pixel electrode, which is connected to one of two pixel electrodes adjacent to each other, and which extends to the region between the two pixels adjacent to each other, is also disposed.  
           [0017]    With the supplemental pixel electrode of above configuration, the region between the pixels can be utilized as a part of display region. In this configuration, the same voltage as that of the pixel electrode can drive the liquid crystal in this region. Therefore, even when a black matrix is not used to provider a larger aperture ratio, this region will not show white light when the pixel displays black, leading to high display quality.  
           [0018]    It is also possible to place a floating electrode, which configures a capacity coupling at the both pixel electrodes of the pixels adjacent to each other through an insulating layer. In this configuration, the voltage of the floating electrode changes in accordance with the voltage change of the pixel electrode. Since the liquid crystal in the adjacent region in the liquid crystal display device can be driven by the similar voltage to the voltage of the pixel region, this region will not show white light when the pixel displays black, solving the abovementioned problem. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a plan view of a display pixel in an active matrix LCD of a first embodiment of this invention.  
         [0020]    [0020]FIG. 2 is a cross-sectional view of the LCD of FIG. 1 cut along the X-X cross-sectional line shown in FIG. 1.  
         [0021]    [0021]FIG. 3 is a cross-sectional view of a display pixel in an active matrix LCD of a second embodiment of this invention.  
         [0022]    [0022]FIG. 4 is a plan view of a display pixel in an active matrix LCD of a third embodiment of this invention.  
         [0023]    [0023]FIG. 5 is a cross-sectional view of the LCD of FIG. 4 cut along the X-X cross-sectional line shown in FIG. 4.  
         [0024]    [0024]FIG. 6 is a cross-sectional view of a display pixel in an active matrix LCD of a forth embodiment of this invention.  
         [0025]    [0025]FIG. 7 shows an equivalent circuit of a pixel of a conventional active matrix LCD.  
         [0026]    [0026]FIG. 8 is a plan view of the pixel portion on a TFT forming substrate of the conventional LCD.  
         [0027]    [0027]FIG. 9 is a cross-sectional view of the LCD of FIG. 8 cut along the X-X cross-sectional line shown in FIG. 8.  
         [0028]    [0028]FIG. 10 schematically shows the location of the pixel electrode  74  with respect to the location of the second electrode  84  of the LCD device of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The embodiment of this invention will be explained in detail by referring to FIGS.  1 - 6 . A LCD is used as an example to describe the invention.  
         [0030]    [0030]FIG. 1 is a plan view of a display pixel in an active matrix LCD of a first embodiment of this invention. FIG. 2 is a cross-sectional view of the LCD cut along the X-X cross-sectional line shown in FIG. 1. The same reference numerals will be given to the same components as in FIGS. 9 and 10, and the explanation on those components will be omitted.  
         [0031]    The LCD has a first substrate  100  and a second substrate  500  made of transparent insulating material such as glass, and a liquid crystal  200  placed between the two substrates.  
         [0032]    An equivalent circuit of each of pixels is shown in FIG. 7. Pixel electrodes  24  are disposed in a matrix configuration on the first substrate  100  as seen from FIG. 1. A top gate type TFT  1  is placed for each of the pixel electrodes.  
         [0033]    An active layer  14  of the TFT  1  in each of the pixel is in a U-shape and intersects, at two points, a gate line  20 , which extends straight in a row direction. At the portion where the active layer and the gate line intersect each other, the active layer  14  makes a channel region  14   c,  and the gate line  20  becomes the gate. A gate insulating layer  66  is formed between the gate and the channel region  14   c.  A drain  14   d  of the active layer  14  is connected to a data line  22  extending in a column direction through a contact hole made in an interlayer insulating layer  68  and the gate insulating layer  66 .  
         [0034]    And a source  14   s  of the active layer  14  is connected to a supplemental pixel electrode  40  through a contact hole C 1  made in the interlayer insulating layer  68  and the gate insulating layer  66 . The supplemental pixel electrode  40  is disposed in the same layer (for example, an aluminum layer) as the data line  22  on the interlayer insulating layer  68  and extends over the region between the pixels adjacent to each other in the row direction. That is, the supplemental pixel electrode  40  extends to cover the area between the pixel electrodes  24 , of the pixel adjacent to each other. Additionally, the supplemental pixel electrode  40  is connected to the pixel electrode  24  (a reflecting electrode) on the upper layer through a contact hole C 2  made in a flattening insulating layer  72 .  
         [0035]    Also, a storage capacitance line  84  is configured in the same layer (for example, molybdenum film, chrome film) as the gate line  20 , and extends in a row direction. The storage capacitance line  84  is overlapped with a part of the active layer  14  through the gate insulating layer  66 . This overlapped part forms a storage capacitance element.  
         [0036]    According to this embodiment, the supplemental pixel electrode  40  enables the area between the pixels to be utilized as a part of the display region. Since the same voltage applied to the pixel electrode  24  through the top gate type TFT  1  also drives the liquid crystal in the area between the pixels, this area will not show white light, thus improving the display quality.  
         [0037]    [0037]FIG. 3 is a cross-sectional view of a display pixel in an active matrix LCD of a second embodiment of this invention. This figure shows a cross section of the device corresponding to the cross section of FIG. 2. The planar configuration of the device of this embodiment is substantially the same as that of FIG. 2 except the cut in the insulating layer  72  as described below.  
         [0038]    In the first embodiment mentioned above, there is the thick flattening insulating layer  72  on the supplemental pixel electrode  40  extending over the area between the pixel electrodes  24 , adjacent to each other. And the liquid crystal  200  is placed on the flattening insulating layer  72 . However, when the insulating layer is placed between the electrode and the liquid crystal, an electric charge is accumulated in this part, resulting in burning of the liquid crystal.  
         [0039]    Therefore, the thick flattening insulating layer  72  on the supplemental pixel electrode  40  is removed, as seen from FIG. 3, in this embodiment. For example, etching can be performed to expose the supplemental pixel electrode  40  by using the pixel electrodes  24 , as masks. This prevents the burning of the liquid crystal and improves the display quality.  
         [0040]    [0040]FIG. 4 is a plan view of a display pixel in an active matrix LCD of a third embodiment of this invention. FIG. 5 is a cross-sectional view of the LCD cut along the X-X cross-sectional line shown in FIG. 4. Again, the same reference numerals are used to indicate the same components. The explanation on these components will be omitted.  
         [0041]    In this embodiment, the portion of the supplemental pixel electrode  40  extending over the area between the adjacent pixels of FIG. 1 is cut off from the remaining portion of the supplemental electrode  40 . Thus, the cut-off portion becomes a floating electrode  41  of this embodiment and forms a capacity coupling with each of the pixel electrodes  24 ,  24  of the adjacent pixels through the flattening insulating layer  72 .  
         [0042]    The voltage of the supplemental pixel electrode  40  is the same voltage as the pixel electrode  24  in the first embodiment, as the supplemental pixel electrode  40  is connected to the pixel electrode  24  of one of the pixels. However, since the floating electrode  41  is electrically floating, the voltage is determined by the voltage of the pixel electrode  24 , with which the floating electrode  40  forms the capacity coupling. That is, the voltage is determined based on the capacitance of the pixel electrode  24  and the floating electrode  41 , and the capacity coupling of these two electrodes.  
         [0043]    Therefore, when both of the adjacent pixels appear black, the voltage corresponding to that of these two pixel electrodes  24 , (the voltage corresponding to black display) is applied to the floating electrode  41 , resulting in substantially similar black representation at the area between the adjacent pixels. On the other hand, when one of the adjacent pixels appears black and the other pixel appears white, a voltage between the black display and the white display is applied to the floating electrode  41 , resulting in a gray representation. In this manner, the voltage of the floating electrodes  41  changes along with the voltage change of the pixel electrodes  24 . The alignment of the liquid crystal  200  on the floating electrode  41  is also determined by the voltage change of the pixel electrodes  24 , eliminating the conventional problems of the conventional device.  
         [0044]    The floating electrode  41  of this embodiment is configured by cutting off a portion of the supplemental pixel electrode  40  of the first embodiment. The floating electrode  41  can also be configured by using a different layer.  
         [0045]    In FIG. 5, the floating electrode  41  equally overlaps the pixel electrodes  24 , located at the both sides. However, it is possible to make the induced voltage of the floating electrode  41  be controlled substantially by the voltage of one of the pixel electrodes by disposing the floating electrode  41  substantially toward that pixel electrode to overlap more with that pixel electrode than the other electrode. This configuration is suitable for displaying letters and drawing, which requires clear-cut boundary.  
         [0046]    [0046]FIG. 6 is a cross-sectional view of a display pixel in an active matrix LCD of a forth embodiment of this invention. This figure shows a cross section of the device corresponding to the cross section of FIG. 4. The planar configuration of the device of this embodiment is substantially the same as that of FIG. 4 except the cut in the insulating layer  72  as described below.  
         [0047]    The thick flattening insulating layer  72  on the floating electrode  41  of the third embodiment is removed in this embodiment. The etching is performed to expose the floating electrode  41  by using, for example, the pixel electrodes  24 , as masks. This prevents the burning of the liquid crystal  200  and improves the display quality, for the same reason as the second embodiment.  
         [0048]    The supplemental pixel electrode, which is connected to the pixel electrode of one of the adjacent pixels and which extends to the area between the adjacent pixels, is disposed in the active matrix LCD of this invention, enabling the area between the pixels to be used as a part of the display area. In this configuration, since the voltage same as the pixels electrode also drives the area between the pixels, this area does not show white light when the pixel displays black, leading to the improved display quality.  
         [0049]    Also, the floating electrode, which forms a capacity coupling at the both pixel electrodes of the pixels adjacent to each other through the insulating layer, is disposed. The voltage of the floating electrode changes in accordance with the voltage change of the pixel electrode. Since the voltage similar to that of the pixel region also drives the liquid crystal in the area between the pixels of the LCD, the region will not show white light when the pixel displays black.