Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-19
2001-03-13
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S046000, C349S111000, C349S147000
Reexamination Certificate
active
06201590
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device with wide viewing angle characteristics of the active matrix system using thin-film transistors.
(2) Description of the Prior Art
Active matrix liquid crystal display devices using active elements as represented by thin-film transistors (TFT) have now been widely used as display terminals of OA equipment since they are light in weight and have a high picture quality equivalent to that of cathode-ray tubes.
The display system of the liquid crystal display devices can be roughly divided into two. One is a system in which liquid crystals are sandwiched by two substrates having transparent electrodes and are driven by a voltage applied to the transparent electrodes, and light incident upon the liquid crystals and transmitted through the transparent electrode is modulated to achieve display. All of the products that are now available are based upon this system. Another one is a system in which liquid crystals are driven by an electric field which is nearly in parallel with the surface of a substrate between two electrodes that are formed on the same substrate, and light incident upon the liquid crystals through a gap between the two electrodes is modulated to provide display. Though no product which is based upon this system has yet been provided, it has a feature of a very wide viewing angle and a promising art in connection with active matrix liquid crystal display devices.
Features of the latter system have been disclosed in, for example, Japanese Patent Laid-Open No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-Open No. 160878/1994.
SUMMARY OF THE INVENTION
A first problem that is to be solved by the present invention will be described below.
In a conventional liquid crystal display device of the latter system, an electric field which is substantially in parallel with the surface of the substrate is generated via thin-film electrodes having thicknesses of about several thousand angstroms, making it difficult to effectively generate the electric field in the liquid crystal layer compared with the former system.
Therefore, an electric field stronger than that of the former system must be generated between the electrodes, resulting in an increase in consumption of electric power and making it necessary to employ, as a driver unit, an LSI having an increased breakdown voltage.
A second problem is that in the former system, a metal material having good light-shielding property has been used as a black matrix (light-shielding film) that covers the portions where undesired light passes through. When this metal material is used for the latter system, however, the electric field between the electrodes is absorbed by the black matrix, making it difficult to generate an effective electric field between the electrodes.
A third problem is that in the former system, the electric field from a video signal line is absorbed by a counter electrode that is formed on nearly the whole surface of a substrate opposed to the substrate on which the video signal line is formed, and the electric field formed by the video signal line does not affect the electric field established between the electrodes.
In the latter system in which no electrode exists on the substrate opposed to the substrate on which the video signal line is formed, however, the electric field formed by the video signal line affects the electric field established between the electrodes, giving rise to the occurrence of crosstalk (particularly in the vertical direction of the screen) in which video information of other rows affect the display and, hence, appearance of striped image called vertical smear.
A fourth problem is that in the latter system in which the counter electrode can be formed linearly, the resistance of the counter electrode from the input end to the other end thereof becomes very larger than that of when the counter electrode is formed in a planar shape in the former system. Therefore, the counter voltage is not sufficiently fed to the terminal pixels, and the counter voltage is distorted by the video signals due to the capacitance at a portion where the counter voltage signal line intersects the video signal line, resulting in the occurrence of crosstalk (particularly, in the horizontal direction of the screen) and appearance of striped image called lateral smear.
A fifth problem is that in the latter conventional system in which the pixel electrode PX and the counter electrode CT are arranged on the same substrate, the opening area that contributes to the display decreases by the amount corresponding to the arrangement of the counter voltage signal line CL compared with that of the former conventional system.
Moreover, an increase in the number of intersecting points of the wirings arranged in the form of a matrix results in increased chances of short-circuiting among the wirings and in an increase in the parasitic capacitance among the signal lines, hindering smooth transfer of signals.
Besides, while the pixel electrode PX in the former system has a planar shape, the pixel electrode PX of the latter system has a narrow strip shape or a line shape, often causing pixels to become defective due to disconnection of the line.
A sixth problem is that the latter conventional system may employ an AC driving method to apply an AC voltage to the liquid crystal layer, e.g., an AC driving method which inverts a drive voltage applied to the liquid crystal layer after every horizontal scanning interval. In this case, when a pulse voltage is applied to the counter voltage signal line CL having a resistance R and a capacitance C, the pulse voltage is distorted. Hereinafter, the sixth problem will be described with reference to
FIGS. 25 and 26
.
FIG. 25
is a diagram of an equivalent circuit of a transmission passage for transmitting a drive voltage applied to the counter voltage signal line CL in a liquid crystal display device of the latter system, and
FIG. 26
is a diagram showing waveforms of a drive voltage applied to the counter electrode CT at each of the points.
The transmission passage for transmitting a drive voltage applied to the counter electrode CT includes chiefly resistors
50
of the counter voltage signal line CL, a resistor
51
of a common bus line CB between a common voltage driver unit
52
and the counter voltage signal line CL, and storage capacitors
53
in the pixels. Therefore, when a liquid crystal layer is driven by an AC drive voltage, the waveform of the drive voltage (pulse voltage) fed to the counter electrode CT from the common voltage driver unit
52
of a common voltage generator and driver unit
103
is distorted.
As will be understood from a counter voltage waveform
54
at point D, a counter voltage waveform
55
at point E, a counter voltage waveform
56
at point F and a counter voltage waveform
57
at point G shown in
FIG. 26
, the distortion of waveform of the drive voltage fed to the counter electrode CT increases with an increase in the distance, from point D, to point G.
As a result, the electric field between the pixel electrode PX and the counter electrode CT in the pixels differs, irregular brightness (irregular display) occurs along the counter voltage signal line CL, and the display quality of the liquid crystal display panel is impaired.
This becomes a serious problem particularly when there is employed an AC drive system which inverts the drive voltage applied to the liquid crystal layer after every horizontal scanning interval.
When the counter voltage signal line CL is broken even at one place, furthermore, it is impossible to drive the liquid crystals since the drive voltage is no longer fed to the counter electrode CT of the pixels after the broken portion, impairing the display quality of the liquid crystal display panel.
According to the constitution of the prior art, furthermore, the thickness differs by a thickness of the signal line depending upon the por
Ashizawa Keiichiro
Kondo Katsumi
Mishima Yasuyuki
Ogawa Kazuhiro
Oh-e Masahito
Antonelli Terry Stout & Kraus LLP
Dudek James A.
Hitachi , Ltd.
Ton Toan
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