Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-07-10
2001-06-19
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000
Reexamination Certificate
active
06249325
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device and, particularly, to a liquid crystal display device of the active matrix type.
A liquid crystal display device of the active matrix type is equipped with gate signal lines that are arranged in parallel lines in the y-direction and extend in the x-direction, and drain signal lines arranged in parallel lines in the x-direction and extend in the y-direction, these lines being disposed on the surface of the one transparent substrate of a pair of transparent substrates which are opposed to each other with liquid crystals being disposed therebetween. The signal lines are disposed on the surface of the one transparent substrate facing the liquid crystals, and the regions surrounded by these signal lines serve as inspection pixel regions. Every pixel region is equipped with a thin-film transistor, that is turned on by a scanning signal from the gate signal line, and a pixel electrode to which is applied a video signal from the drain signal line via the thin-film transistor that is turned on. Such a liquid crystal display device exhibits a good contrast and is indispensable, particularly, in color liquid crystal display devices.
SUMMARY OF THE INVENTION
Accompanying a recent trend toward fabricating large, high definition liquid crystal display devices, however, the problem of so-called flickering of the display on the screen has become no longer negligible. In liquid crystal display devices having a diagonal size not shorter than 34 cm (13-type) in the display region, in particular, this problem is becoming more serious. The present inventors have pursued the causes of flickering and have discovered the following facts.
First, for a large size display screen, the gate signal lines must be formed so as to be long. Being affected by the resistances and capacitances of the signal lines, therefore, the waveforms of signals input to the scanning signal lines are distorted toward the ends thereof. This distortion in the waveform delays the gate turn-off timing of the thin-film transistor and causes the potential of the source electrode to decrease less due to a voltage that is applied thereto through a capacity across the gate and the source of the thin-film transistor when the gate is turned off. This means that the potential of the source electrode on the end of the line becomes high relative to the input terminal side of the gate signal line. On the other hand, the electrode (common electrode) opposed to the pixel electrodes via the liquid crystals is supplied with a predetermined potential which is uniform over the display surface. Therefore, the voltage applied to the liquid crystals will vary between the input terminal side of the gate signal lines and the end thereof.
Furthermore, in order to avoid a polarization of the liquid crystals, an AC drive has been employed according to which the potential applied to the liquid crystals is periodically inverted. Therefore, the magnitude of voltage applied to the liquid crystals is inverted between the input terminal side and the end of the gate signal line for every one-half period of the AC drive, and so flickering occurs on the screen due to a periodic change in the brightness. In particular, a liquid crystal display device of the 13-type has a display region measuring 20 cm high and 27 cm wide, whereby the gate signal lines have a length of not shorter than 27 cm, and so, with lines of this length, the difference in the voltage that is applied via the capacity across the gate and the source of the thin-film transistors between the input terminal side of the gate signal lines and the end thereof is no longer negligible. ln the liquid crystal display device having gate signal lines not shorter than 27 cm (not smaller than 13-type), therefore, it is no longer possible to completely eliminate flickering by simply adjusting the potential of the common electrode.
In addition, when the signal lines and the thin-film transistors are to be formed by selective etching based upon standard photolithography technology, it is difficult to completely uniformalize the pattern of the thin-film transistor for each of the pixel regions due to distortion in the optical system of the exposing device or due to deflection of light by the transparent substrate. In this case, when the capacity across the gate and the source of the thin-film transistor is no longer uniform due to dispersion in the pattern, the amount of drop of the source potential due to the capacity across the gate and the source of the thin-film transistor at the time the gate is turned off becomes no longer uniform on the screen. In this case too, therefore, flicking occurs on the screen due to a change in the brightness for the same reason as described above.
The present invention was accomplished in view of the above-mentioned circumstances, and its object is to provide a liquid crystal display device which is capable of completely suppressing the occurrence of flickering even when the liquid crystal display device has a large display screen.
Briefly described below are representative examples of the invention disclosed in this application.
A first aspect of the invention involves a liquid crystal display device having a gate signal line formed on a first insulating substrate; a drive circuit electrically connected to said gate signal lines to produce a gate drive voltage; first and second thin-film transistors each having a source electrode, a gate electrode and a drain electrode; a first pixel electrode electrically connected to either the source electrode or the drain electrode of said first thin-film transistor; a second pixel electrode electrically connected to either the source electrode or the drain electrode of said second thin-film transistor; a first video signal line electrically connected to the other one of the source electrode or the drain electrode of said first thin-film transistor; and a second video signal line electrically connected to the other one of the source electrode or the drain electrode of said second thin-film transistor. The gate electrode of said first thin-film transistor is electrically connected to a first portion of said gate signal line; and, the gate electrode of said second thin-film transistor is electrically connected to a second portion which is remoter than the first portion of said gate signal line from said drive circuit. The source electrodes of said first and second thin-film transistors are separated from said drain electrodes by a channel length on said gate electrodes and are opposed thereto by a channel width; the channel length and the channel width of said second thin-film transistor are substantially equal to the channel length and the channel width of said first thin-film transistor; and the electrostatic capacity between said second pixel electrode and said gate signal line is selected to be larger than the electrostatic capacity between said first pixel electrode and said gate signal line.
In the thus constituted liquid crystal display device, the shift of the potential of the pixel electrode toward the forward direction, due to a distorted waveform of a scanning signal on the gate signal line, is canceled by the shift of the potential of the pixel electrode toward the negative direction, due to the jumping voltage through the capacity Cgs, so that an equal voltage is applied to the pixel electrodes on the input terminal side close to the drive circuit and to the pixel electrodes of the end side remote from the drive circuit. This makes it possible to suppress flickering on the screen caused by a change in the brightness.
A second aspect of the invention involves a liquid crystal display device having a gate signal line formed on a first insulating substrate; a drive circuit electrically connected to said gate signal line to produce a gate drive voltage; first and second thin-film transistors each having a source electrode, a gate electrode and a drain electrode; a first pixel electrode electrically connected to either the source electrode or
Hakoda Hidetaka
Itoh Hikaru
Kamei Tatsuo
Kawamura Tetsuya
Nakayama Takanori
Antonelli Terry Stout & Kraus LLP
Hitach, Ltd.
Malinowski Walter J.
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