Abstract:
A transflective liquid crystal display comprising an active device array substrate, a facing substrate, a liquid crystal layer and a reflector. The liquid crystal molecules in the transparent area are driven by a potential between the transparent pixel electrode and the common electrode. The liquid crystal molecules in the reflective area are driven by a potential between the transparent pixel electrode and the active device array substrate or the auxiliary electrode on the facing substrate. Under the condition of a single cell gap, the electric field applied to the transparent area and the reflective area can control the change in effective phase so as to optimize the performance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the priority benefit of Taiwan application Serial no. 91132742, filed Nov. 07, 2002. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to a transflective liquid crystal display (LCD). More particularly, the present invention relates to a single cell gap transflective liquid crystal display.  
           [0004]    2. Description of Related Art  
           [0005]    Due to progress in the development of semiconductor devices or man-machine display interface, multimedia communication is almost a routine in everyday life. High-quality and economical displaying devices such as cathode ray tube (CRT) are in the market for some time. However, as a desktop terminal/display, CRT is rather bulky and occupies a lot of space, and from the standpoint of energy conservation, CRT consumes too much electrical energy. Hence, CRT can no longer meet our demands for a light, compact and energy efficient display. Since the recently developed thin film transistor liquid crystal display (TFT-LCD) has superior image quality, slim, low power consumption and radiation free characteristics, TFT-LCD is now a major sell in the market.  
           [0006]    Most liquid crystal displays (LCD) can be categorized into the transparent type, the reflective type and the transflective type. The classification is based on the different light source utilization and array arrangements. The transparent LCD uses back light as a source of illumination and the pixel electrodes on the array are transparent to facilitate the penetration of back light. The reflective LCD uses front light or external light as a source of illumination and the pixel electrons on the array are made from metal or other substances having good reflective properties so that the front or external light can be reflected. The transflective LCD uses both back light and external light as a source of illumination at the same time. Each pixel can be divided into a transparent area and a reflective area. The transparent area has a transparent electrode that facilitates the passage of back light and the reflective area has a reflective electrode capable of reflecting light from external light sources.  
           [0007]    Using a normally black transflective LCD as an example, both the transparent area and the reflective area are in a dark state before the application of a voltage. When the transparent area and the reflective area change from a dark state to the brightest state, phase in the transparent area must differ by Â±Î&gt;&gt;/2 and phase in the reflective area must differ by Â±Î&gt;&gt;/4. However, in a single cell gap LCD, the required phase differences are hard to secure at the same time. Thus, optimal utilization of light in both the transparent area and the reflective area is difficult to attain in practice. Due to intrinsic display limitations of a single cell gap transflective LCD, transflective LCD having dual cell gaps are developed. By designing the transparent area and the reflective area with different cell gaps, light from whatever sources is fully utilized.  
           [0008]    [0008]FIG. 1A is a schematic cross-sectional view of a conventional dual cell gap transflective liquid crystal display. As shown in FIG. 1A, the dual cell cap transflective LCD  100  mainly comprises of a thin film transistor (TFT) array substrate  102 , a facing substrate  104  and a liquid crystal layer  106 . The cell gap in the transmission region (T) of the transflective LCD is controlled to a distance d while the cell gap in the reflective area (R) of the transflective LCD is controlled to a distance d/2. Hence, the liquid crystal layer  106  within the transparent area (T) has a thickness d and the liquid crystal layer  106  within the reflective area (R) has a thickness d/2. In addition, the cell gap or the thickness d of the liquid crystal layer  106  must also meet the phase change relationship (Î□.d) =Â±Î&gt;&gt;/2. Therefore, through a thickness variation (d to d/2) of the liquid crystal layer  106 , there is a phase change of Â±Î&gt;&gt;/2 and Â±Î&gt;&gt;/4 inside the respective cell gaps.  
           [0009]    [0009]FIG. 1B is a schematic layout diagram of a conventional dual cell gap transflective LCD. As shown in FIG. 1B, the active device array substrate  102  has a plurality of scanning lines  200  and a plurality of data lines  202  thereon. Each pair of neighboring scanning lines  200  and each pair of neighboring data lines  202  constitute a pixel region  212 . Each pixel region  212  has an active device  204 , a transparent electrode  206  and a reflective electrode  208 . The transparent electrode  206  is positioned over a portion of the pixel region  212  to form a transparent area (T). The reflective electrode  208  is positioned over a portion of the pixel region  212  outside the transparent area (T) to form a reflective area (R).  
           [0010]    In general, the transparent electrode  206  and the reflective electrode  208  in the same pixel region  212  are electrically connected together. Hence, the transparent electrode  206  and the reflective electrode  208  within the same pixel region  212  are controlled by one active device  204 . Furthermore, the active device  204  is, for example, a thin film transistor (TFT) or a diode that may switch state when driven voltages applied to the scanning line  200  and the data line  202 .  
           [0011]    Although the dual cell gap transflective LCD is able to optimize illumination, the substrate plates are difficult to fabricate.  
         SUMMARY OF INVENTION  
         [0012]    Accordingly, one object of the present invention is to provide a transflective liquid crystal display (LCD) having a single cell cap structure that uses an applied electric field to control phase changes within the reflective area and the transparent area of the LCD. Hence, the reflective area and the transparent area in the transflective LCD are optimally illuminated.  
           [0013]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a transflective liquid crystal display. The transflective LCD mainly comprises of a thin film transistor (TFT) array, a facing substrate and a liquid crystal layer. The active device array substrate has a plurality of scanning lines and a plurality of data lines thereon. Each pair of neighboring scanning lines and each pair of neighboring data lines together constitute a pixel region. Each pixel region has an active device, a transparent electrode and a reflector. The active device is driven by voltage applied to the scan line and the data line. The transparent electrode is positioned in a portion of the pixel region to form a transparent area and the reflector is positioned in the pixel region but outside the transparent area to form a reflective area. The transparent electrode and the active device are electrically connected. The facing substrate has a plurality of common electrodes and a plurality of auxiliary electrodes. The common electrode is positioned above the transparent electrode and the auxiliary electrode is positioned above the reflector. The liquid crystal layer is positioned between the active device array substrate and the facing substrate. The transparent area and the reflective area have an identical thickness. Furthermore, a first alignment film is positioned between the liquid crystal layer and the active device array substrate and a second alignment film is positioned between liquid crystal layer and the facing substrate. In addition, a top polarizing plate is positioned just outside the active device array substrate and a bottom polarizing plate is positioned just outside the facing substrate.  
           [0014]    This invention also provides a second type of transflective liquid crystal display that comprises of an active device array substrate, a facing substrate, a liquid crystal layer and a reflector. The active device array substrate has a plurality of scan lines and a plurality of data lines thereon. Each pair of neighboring scan lines and each pair of neighboring data lines constitutes a pixel region. Each pixel region has an active device and a transparent electrode. The active device is driven by voltage applied to the scan line and the data line. The transparent electrode is positioned over the pixel region to form a transparent area. The transparent electrode and the active device are electrically connected together. The facing substrate has a plurality of common electrodes and a plurality of auxiliary electrodes thereon. The common electrode is positioned over the transparent electrode and the auxiliary electrode is positioned outside the transparent area. The reflector is positioned outside the active device array substrate to form a reflective area. The liquid crystal layer is positioned between the active device array substrate and the facing substrate. The transparent area and the reflective area have an identical thickness. Furthermore, a first alignment film is positioned between the liquid crystal layer and the active device array substrate and a second alignment film is positioned between liquid crystal layer and the facing substrate. In addition, a top polarizing plate is positioned just outside the active device array substrate and a bottom polarizing plate is positioned just outside the facing substrate.  
           [0015]    This invention also provides a third type of transflective liquid crystal display comprising of an active device array substrate, a facing substrate and a liquid crystal layer. The active device array substrate has a plurality of scanning lines and a plurality of data lines thereon. Each pair of neighboring scan lines and each pair of neighboring data lines together constitutes a pixel region. Each pixel region has an active device, a reflective electrode and an auxiliary electrode. The active device is driven by voltage applied to the scanning line and the data line. The reflective electrode is positioned over a portion of the pixel region to form a reflective area. The pixel region outside the reflective area is a transparent area. The reflective electrode and the active device are electrically connected together. The facing substrate has a plurality of common electrodes thereon. The common electrodes are positioned over the reflective electrodes. The liquid crystal layer is positioned between the active device array substrate and the facing substrate. The transparent area and the reflective area have an identical thickness. Furthermore, a first alignment film is positioned between the liquid crystal layer and the active device array substrate and a second alignment film is positioned between liquid crystal layer and the facing substrate. In addition, a top polarizing plate is positioned just outside the active device array substrate and a bottom polarizing plate is positioned just outside the facing substrate.  
           [0016]    In this invention, the active device array substrate is a thin film transistor (TFT) array substrate or a diode array substrate, for example. In addition, the LCD may also serve as a color display by adding a color filter on the facing substrate in a position that corresponds to the pixel region to form a color filtering plate.  
           [0017]    This invention also permits the installation of a first delay plate between the top polarizing plate and the active device array substrate and a second delay plate between the bottom polarizing plate and the active device array substrate.  
           [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claim. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0020]    [0020]FIG. 1A is a schematic cross-sectional view of a conventional dual cell gap transflective liquid crystal display.  
         [0021]    [0021]FIG. 1B is a schematic layout diagram of a conventional dual cell gap transflective LCD.  
         [0022]    [0022]FIGS. 2A and 2B are schematic cross-sectional views of a transflective liquid crystal display according to a first embodiment of this invention.  
         [0023]    [0023]FIG. 3 is a diagram showing the layout of a transflective LCD according the first embodiment of this invention.  
         [0024]    [0024]FIGS. 4A and 4B are schematic cross-sectional views of a transflective liquid crystal display according to a second embodiment of this invention.  
         [0025]    [0025]FIG. 5 is a diagram showing the layout of a transflective LCD according the second embodiment of this invention.  
         [0026]    [0026]FIGS. 6A and 6B are schematic cross-sectional views of a transflective liquid crystal display according to a third embodiment of this invention.  
         [0027]    [0027]FIG. 7 is a diagram showing the layout of a transflective LCD according the third embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0028]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0029]    [0029]FIGS. 2A and 2B are schematic cross-sectional views of a transflective liquid crystal display according to a first embodiment of this invention. As shown in FIG. 2A, the transflective liquid crystal display (LCD) mainly comprises of an active device array substrate  400 , a complementary panel  300  and a liquid crystal layer  500 . The Active device array substrate  400  has a plurality of pixel regions thereon. Each pixel region has an active device (not shown), a transparent electrode  402  and a reflector  404 . The active device (not shown) and the transparent electrode  402  are positioned over a portion of the pixel region to form a transparent area (T). The reflector  404  and the pixel region  212  outside the transparent area (T) form a reflective area (R). Note that the transparent electrode  402  and the reflector  404  have no electrical connection.  
         [0030]    The facing substrate  300  has a plurality of common electrode  302  and a plurality of auxiliary electrodes  304  thereon. The common electrode  302  is positioned over the transparent electrode  402  and the auxiliary electrode  304  is positioned over the reflector  404 . In addition, the liquid crystal layer  500  is positioned between the active device array substrate  400  and the facing substrate  300 .  
         [0031]    Aside from the active device array substrate  400 , the facing substrate  300  and the liquid crystal layer  500 , optical films such as a first delay plate  306  and a top polarizing plate  308  may be attached to the outer surface of the facing substrate  300 . In addition, optical films such as a second delay plate  406  and a bottom polarizing plate  408  are attached to the outer surface of the active device array substrate  400 . The first delay plate  306  is capable of delaying incoming light by Î&gt;&gt;/4 phase. Similarly, the second delay plate  406  is capable of delaying incoming light by Î&gt;&gt;/4 phase.  
         [0032]    The liquid crystal layer  500  used in this embodiment is, for example, negative liquid crystals so that slow axis of the liquid crystal molecules is parallel to the electric field provided. Without the application of a voltage, the liquid crystal molecules are aligned in a manner as shown in FIG. 2A. Hence, overall effective phase difference of the liquid crystal layer  500  is zero and both the transparent area (T) and the reflective area (R) are in a dark state. The transparent area (T) and the reflective area (R) change from a dark state to the brightest state when an electric field perpendicular to the surface of the active device array substrate  400  is created between the transparent electrode  402  and the common electrode  302  in the liquid crystal layer  500  within the transparent area (T). The vertical electric field aligns the liquid crystal molecules in a direction shown on the left side of FIG. 2B such that the transparent area (T) has an effective phase difference of Î&gt;&gt;/2.  
         [0033]    Since the transparent electrode  402  and the reflector  404  have no electrical connection, the electric field in the reflective area (R) is created through the transparent electrode  402  and the auxiliary electrode  304 . In this embodiment, through the potential difference between the transparent electrode  402  and the auxiliary electrode  304 , an electric field forming an oblique angle with the surface of the active device array substrate  400  is created in the liquid crystal layer  500  within the reflective area (R). This oblique electric field aligns liquid crystal molecules in a direction as shown on the right side of FIG. 2B such that the reflective area (R) has an effective phase difference of Î&gt;&gt;/4.  
         [0034]    Accordingly, phase variation in the transparent area (T) is (Î&gt;&gt;/2−0)=Î&gt;&gt;/2 and thus the phase variation meets the demanded phase variation Â±Î&gt;&gt;/2. Similarly, phase variation in the reflective area (R) is (Î&gt;&gt;/4−0)=Î&gt;&gt;/4 and hence the phase variation meets the demanded phase variation Â±Î&gt;&gt;/4. Ultimately, optimal utilization of light in both the transparent area and the reflective area is achieved.  
         [0035]    [0035]FIG. 3 is a diagram showing the layout of a transflective LCD according the first embodiment of this invention. As shown in FIG. 3, the active device array substrate has a plurality of scanning lines  410  and a plurality of data lines  412  thereon. Neighboring scanning lines  410  and neighboring data lines  412  together form a pixel region  420 . Each pixel region  420  has an active device  414 , a transparent electrode  402  and a reflector  404 . The active device  414  such as a thin film transistor or a diode is able to change state when driven by applied voltage at the scanning line  410  and the data line  412 . The transparent electrode  402  is positioned over a portion of the pixel region  212  to form a transparent area (T) and the reflector  404  is also positioned over a portion of the pixel region  212  to form a reflective area (R).  
         [0036]    The transparent electrode  402  has no electrical connection with the reflector  404 . Hence, the liquid crystal molecules above the reflector  404  (the reflective area (R)) are driven by the oblique electric field between the transparent electrode  402  and the auxiliary electrode  304 . The liquid crystal molecules above the transparent electrode  402  (the transparent area (T)) are driven by the vertical electric field between the transparent electrode  402  and the common electrode  302 .  
         [0037]    [0037]FIGS. 4A and 4B are schematic cross-sectional views of a transflective liquid crystal display according to a second embodiment of this invention. In general, the reflector  404  in the first embodiment can be attached to the active device array substrate  400  in other ways. In the second embodiment, a reflector  416  is attached to the outer surface of the active device array substrate  400 . The reflector  416  is similarly capable of reflecting light from light sources (including front light and external light).  
         [0038]    [0038]FIG. 5 is a diagram showing the layout of a transflective LCD according the second embodiment of this invention. As shown in FIG. 5, the active device array substrate has a plurality of scanning lines  410  and a plurality of data lines  412  thereon. Neighboring scanning lines  410  and neighboring data lines  412  together form a pixel region  420 . Each pixel region  420  has an active device  414  and a transparent electrode  402 . The active device  414  such as a thin film transistor or a diode is able to change state when driven by applied voltage at the scanning line  410  and the data line  412 . The transparent electrode  402  is positioned over a portion of the pixel region  212  to form a transparent area (T) and the potion of the pixel region  212  outside the transparent area (T) is regarded as a reflective area (R).  
         [0039]    In the second embodiment, the liquid crystal molecules above the reflective area (R) is driven by the oblique electric field between the transparent electrode  402  and the auxiliary electrode  304 . Similarly, the liquid crystal molecules above the transparent area (T) are driven by the vertical electric field between the transparent electrode  402  and the common electrode  302 .  
         [0040]    [0040]FIGS. 6A and 6B are schematic cross-sectional views of a transflective liquid crystal display according to a third embodiment of this invention. As shown in FIGS. 6A and 6B, the transflective LCD mainly comprises of an active device array substrate  400 , a facing substrate  300  and a liquid crystal layer  500 . The active device array substrate  400  has a plurality of pixel regions thereon. Each pixel region has an active device (not shown), a reflective electrode  430  and an auxiliary electrode  418 . The active device (not shown) and the reflective electrode  430  are positioned over a portion of the pixel region to form a reflective area (R). The pixel region  212  outside the reflective area (R) is regarded as a transparent area (T). The facing substrate  300  has a plurality of common electrodes  302  thereon. The common electrodes  302  are positioned above the reflective electrode  430 . In addition, the liquid crystal layer  500  is positioned between the active device array substrate  400  and the facing substrate  300 .  
         [0041]    The third embodiment is similar to the first embodiment of this invention in many ways. In the first embodiment, the auxiliary electrode  304  is positioned over the facing substrate  300  and the liquid crystal molecules above the reflective area (R) are driven by the oblique electric field between the auxiliary electrode  304  and the transparent electrode  402 . In the third embodiment, however, the auxiliary electrode  418  is positioned above the transparent area (T) of the active device array substrate  400 . Hence, the liquid crystal molecules above the transparent area (T) are driven by the lateral electric field between the reflective electrode  430  and the auxiliary electrode  418 . In addition, optical films such as a first delay plate  306  and a top polarizing plate  308  may be attached to the outer surface of the facing substrate  300 . Similarly, optical films such as a second delay plate  406  and a bottom polarizing plate  408  are attached to the outer surface of the active device array substrate  400 . The first delay plate  306  is capable of delaying incoming light by Î&gt;&gt;/4 phase and the second delay plate  406  is capable of delaying incoming light by Î&gt;&gt;/4 phase.  
         [0042]    The liquid crystal layer  500  in the third embodiment uses, for example, positive crystals that have a hybrid alignment or oblique alignment so that the fast axis of the liquid crystal molecules is parallel to the electric field provided. The liquid crystal molecules are aligned in a direction as shown in FIG. 6A before the application of any voltage. Thus, the liquid crystal layer  500  produces an overall phase difference of Î&gt;&gt;/4 and both the transparent area (T) and the reflective area (R) are in a dark state. To change the transparent area (T) and the reflective area (R) to a brightest state, the transparent electrode  430  and the common electrode  302  provide an electric field perpendicular to the surface of the active device array substrate  400  in the liquid crystal layer  500  within the reflective area (R). The vertical electric field aligns the liquid crystal molecules in a direction as shown on the left side of FIG. 6B such that the effective phase difference in the transparent area (T) is zero.  
         [0043]    In the third embodiment, the reflective electrode  430  and the auxiliary electrode  418  provides a lateral electric field parallel to the surface of the active device array substrate  400  in the liquid crystal layer  500  of the transparent area (T). The lateral electric field aligns the liquid crystal molecules in a direction as shown on the right side of FIG. 6B so that the transparent area (T) has an effective phase difference of 3Î&gt;&gt;/4.  
         [0044]    Accordingly, phase variation in the transparent area (T) is (3Î&gt;&gt;/4−Î&gt;&gt;/4)=−Î&gt;&gt;/2 and thus the phase variation meets the demanded phase variation Â±Î&gt;&gt;/2. Similarly, phase variation in the reflective area (R) is (0−Î&gt;&gt;/4)=−Î&gt;&gt;/4 and hence the phase variation meets the demanded phase variation Â±Î&gt;&gt;/4. Ultimately, optimal utilization of light in both the transparent area and the reflective area is achieved.  
         [0045]    [0045]FIG. 7 is a diagram showing the layout of a transflective LCD according the third embodiment of this invention. As shown in FIG. 7, the liquid crystal molecules above the reflective area (R) are driven by the vertical electric field between the reflective electrode  430  and the common electrode  302 . Similarly, the liquid crystal molecules above the transparent area (T) are driven by the lateral electric field between the reflective electrode  430  and the auxiliary electrode  418 .  
         [0046]    In the aforementioned embodiments, the positioning of the transparent electrode, the reflective electrode, the common electrode and the auxiliary electrode relative to each other is utilized to provide an electric field having an appropriate direction and strength. Thus, the effective phase difference of liquid crystal molecules above the transparent area (T) and the reflective area (R) may vary in such a way that light utilization in the transmission region (T) and the reflective area (R) are optimized.  
         [0047]    In summary, the transflective liquid crystal display has at least the following advantages: 1. The effective phase difference in the reflective area and the transparent area is controlled by the direction of an applied electric field so that light utilization in the reflective area and the transparent area is optimized. 2. In the third embodiment, there is no need to fabricate the transparent electrode. Nevertheless, the liquid crystal molecules above the transparent area (T) and the reflective area (R) are simultaneously driven. Hence, fabrication process is simplified. 3. Since the transparent electrode pattern on the active device array substrate and the common electrode pattern on the facing substrate need to be modified, production is compatible with existing processes.  
         [0048]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.