Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-10-23
2004-04-06
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S037000, C349S039000, C349S139000
Reexamination Certificate
active
06717628
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display device employing switching elements such as thin film transistors.
2. Related Background Art
Liquid crystal display devices featuring a thin and lightweight flat display are used widely as display devices in a variety of electronic equipment. Among them, active-matrix liquid crystal display devices employing switching elements such as thin film transistors are applied vigorously to a monitor display for a personal computer and a liquid crystal television by virtue of their excellent image properties.
With the development of large-size and high-resolution liquid crystal display devices intended for these applications, one problem that arises is display irregularities. This results from a distortion of a scanning voltage waveform, which occurs due to a CR time constant of scanning lines. That is to say, while a scanning voltage pulse hardly includes the rounding at a feeding end of a scanning line, the degree of the rounding in the scanning voltage pulse waveform increases with increasing the distance from the feeding end. As a result, a variation occurs among feed-through voltages applied to each pixel at the falling edge of the scanning pulse. This variation remains as a DC component in a voltage applied to liquid crystals, which results in perceptible flickering of the image. This DC voltage component also causes problems such as image persistence and stains.
A technology for making the feed-through voltage uniform on a panel so as to solve the above-stated problems is disclosed in JP 10(1998)-39328 A.
FIGS. 15 and 16
show a configuration of the technology.
FIG. 15
is a plan view of a liquid crystal display device, where numeral
201
denotes a liquid crystal panel,
202
denotes a driving circuit for scanning lines, and
203
denotes a driving circuit for image signal lines.
FIGS. 16A
to
16
C are enlarged views of parts A, B and C of
FIG. 15
, respectively.
In
FIGS. 16A
to
16
C, numeral
211
denotes a scanning electrode and
213
denotes an image signal electrode. A thin film transistor
212
functioning as a switching element is provided at a position of intersection of these electrodes. A pixel electrode
215
is connected to the image signal electrode
213
via the thin film transistor
212
. A gate electrode of the thin film transistor
212
is connected to the scanning electrode
211
. Numerals
220
a
,
220
b
and
220
c
denote auxiliary capacitance lines provided beneath an interlayer insulating film (not illustrated), which have overlapping portions
214
a
,
214
b
and
214
c
with the pixel electrode
215
so as to form a storage capacitance.
Compared with an area of the overlapping portion
214
b
in the part B, the overlapping portion
214
a
in the part A is larger, while the overlapping portion
214
c
in the part C is smaller. With this configuration, the storage capacitance formed with the overlapping portion becomes smaller with increasing the distance from a feeding end of a scanning line, so that a variation in the feed-through voltage resulting from the rounding in the scanning voltage waveform can be deleted. Moreover, by forming the auxiliary capacitance lines
220
a
,
220
b
and
220
c
with transparent electrodes, areas of allowing the light to be passed can be made equal among the parts A, B and C.
However, when adapting the above-stated configuration to a liquid crystal display device like an in-plane switching type liquid crystal display device that has a portion not covered with a pixel electrode in a pixel area, an electric field applied to the liquid crystal layer would be disturbed because of a change in area of the storage capacitances. As a result, problems occur in that the display properties deteriorate and the display properties vary from each pixel.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an active-matrix liquid crystal display device in which, although an area of a storage capacitance is altered for each pixel, electric fields applied to display portions can be made uniform irrespective of pixels.
A liquid crystal display device according to the present invention is an improved active-matrix liquid crystal display device including a first substrate, a second substrate, and liquid crystals sandwiched between the first substrate and the second substrate. In this device, the first substrate includes a plurality of pixels, each of which is provided with a scanning electrode, an image signal electrode, a switching element provided at a point of intersection of the scanning electrode and the image signal electrode, a pixel electrode connected to the image signal electrode via the switching element, a counter electrode that activates the liquid crystals present between the counter electrode and the pixel electrode by a potential difference.
In order to fulfill the above-described object, according to the first configuration of the liquid crystal display device, a portion of the pixel electrode is overlapped with a busbar electrically connected to the counter electrode so as to make up a storage capacitance. A shape of the pixel electrode is altered for each pixel so that a value of the storage capacitance becomes smaller from a feeding side to a termination side, and the portion of the pixel electrode overlapped with the busbar so as to make up the storage capacitance is located within the busbar in a plan view of the device.
With this configuration, aperture ratios and electric fields close to the liquid crystal display areas can be made constant irrespective of pixels. Therefore, uniform display free from disturbances and variations in the display properties among the pixels can be obtained. This configuration also has the advantages of facilitating large-sized and high-resolution display, because the time constants of the scanning electrodes and the common electrodes can be both made small, and of facilitating the design and the process thereof by virtue of its simple configuration.
According to the second configuration of the liquid crystal display device, a portion of the pixel electrode is overlapped with the busbar so as to make up a storage capacitance. A shape of the busbar is altered for each pixel so that a value of the storage capacitance becomes smaller from a feeding side to a termination side, and the pixel electrode overlapped with the busbar so as to make up the storage capacitance covers a portion of the busbar where the shape thereof is altered for each pixel in a plan view of the device.
With this configuration, uniform display can be realized in the same manner as in the above device. This configuration also has the features of facilitating large-sized and high-resolution display because the time constant of the scanning electrodes can be made small; being tolerant of a noise in a driving waveform because the storage capacitance can be increased easily; suppressing a break in the electrode, which can improve the yields; and obtaining still higher uniformity because of the effect of electric field shielding.
According to the third configuration of the liquid crystal display device, a portion of the pixel electrode is overlapped with the busbar so as to make up a storage capacitance. The portion of the pixel electrode overlapped with the busbar so as to make up the storage capacitance includes: a commonly shaped portion having a shape common to pixels on a feeding side and pixels on a termination side; and an additional portion. The commonly shaped portion extends beyond the busbar in a plan view of the device and the additional portion is located within the busbar in the plan view of the device. A shape of the additional portion is altered for each pixel, so that a value of the storage capacitance becomes smaller from the feeding side to the termination side.
With this configuration, uniform display can be realized in the same manner as in the above devices. This configuration also has the advantages of: facilitating large-sized and high-resolution di
Fukami Tetsuo
Kimura Masanori
Kumagawa Katsuhiko
Kim Robert H.
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Wang George Y.
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