Reflective LCD with reduced color shade pattern cycle

Details Reflective LCD with reduced color shade pattern cycle Reflective LCD with reduced color shade pattern cycle

1998-10-08

2001-08-14

Parker, Kenneth (Department: 2871)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

C349S109000, C349S145000

Reexamination Certificate

active

06275274

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective liquid crystal display device which is widely used as display devices for computers, portable information terminals, electronic calculators, electronic organizers, and the like.
2. Description of the Related Art
Reflective liquid crystal display devices have been widely used for various types of portable apparatuses due to their low power consumption. In recent years, with the sophistication of information, a demand for performing color display for such portable apparatuses has increased. As such, reflective color liquid crystal display devices have been actively developed.
FIG. 14
is a schematic plan view of a configuration of a conventional active matrix type reflective liquid crystal display device.
FIG. 15
is a plan view illustrating the conventional display device of
FIG. 14
in more detail.
FIG. 16
is a sectional view illustrating the structure of an amorphous silicon (a-Si) thin film transistor (TFT) which is used as an active element for the reflective liquid crystal display device.
The configuration and the fabrication method of the conventional reflective liquid crystal display device will be described with respect to
FIGS. 14
to
16
.
First, a metal film is formed on a glass substrate by sputtering and patterned by photolithography and etching, to form gate bus lines (scanning lines)
20
and gate electrodes
21
of TFTs
26
.
Then, a gate insulating film
40
, a semiconductor layer
41
, and a contact layer
42
are sequentially formed and patterned so that semiconductor layers
41
and contact layers
42
are at least partially formed at portions above the gate electrodes
21
.
Thereafter, a metal for source bus lines (signal lines)
30
is deposited by sputtering and patterned, to form the source bus lines
30
as well as source electrodes
31
and drain electrodes
32
of the TFTs
26
. Subsequently, the portions of the contact layers
42
located above channel portions of the TFTs
26
are removed.
An interlayer insulating film
50
is formed over the resultant substrate to flatten the uneven top surface of the substrate. Contact holes
33
are then formed through the depth of the interlayer insulating film
50
at positions above the drain electrodes
32
.
Finally, a metal thin film is formed and patterned to form reflective pixel electrodes
60
. The pixel electrodes
60
are in electrical contact with the corresponding drain electrodes
32
via the contact holes
33
.
Thus, an active matrix substrate is fabricated. The resultant active matrix substrate is bonded together with a counter substrate including a counter electrode formed on substantially the entire surface thereof with a predetermined -space therebetween. A liquid crystal material is injected into the space between the substrates and forms a seal therebetween, thereby to complete the reflective liquid crystal display device.
As shown in
FIG. 14
, the illustrated conventional reflective liquid crystal display device employs a pixel arrangement called a delta arrangement, which is advantageous, in general, in the display of video images, static images, and the like. When the pixel electrodes
60
are formed to overlap the adjacent gate bus lines
20
having the interlayer insulating film therebetween, a storage capacitance (Cs) is produced at each of the overlap portions and the area of each pixel electrode
60
increases. This overlap structure therefore serves to increase the amount of reflected light from the display device.
However, the above configuration has the following problem. Since each of the above Cs portions is recognized as part of a pixel region, the resultant pixel region has a shape as shown in
FIG. 18
, which is composed of a pixel portion
60
a
of substantially a rectangular shape (the shape of a pixel electrode obtained when no Cs portion is formed as shown in
FIG. 17
) and an additional pixel portion (extending portion)
60
b
corresponding to the Cs portion extended from the pixel portion
60
a
to a considerable extent. With this shape of the pixel electrodes, when the display screen is divided into sections Q, P, and O defined by vertically dashed lines as shown in
FIG. 18
, and the occupation of the area of red (R) pixels, for example, in the entire area of each divided section (hereinafter, referred to as the area occupation of R pixels, for example) is compared with those of other divided sections, the result of Q>P>O is obtained as will be described hereinbelow.
FIG. 18
illustrates only three rows of pixels as an example, and thus the center section Q among the three is shown as including only one red (R) pixel. It should be noted that since the same pattern of pixel arrangement continues in the vertical direction, if four rows of pixels were taken into consideration, the area occupation of R pixels would have been the same for the three sections Q. The above description regarding the area occupation of R pixels is also applicable to other colors G and B. As shown in
FIGS. 14 and 15
, the TFTs
26
are formed on the right and left sides of each source bus line
30
alternately, and the pixels of the same color are connected to each source bus line
30
.
FIG. 19A
is a simplified illustration of the aforementioned area occupation of R pixels. Referring to
FIG. 19A
, while section Q has a high area occupation of red pixels, section P has a reduced area occupation since only part of the additional pixel portions
60
b
where the red pixels overlap the gate bus lines
20
are included therein, and section O includes no red pixel portions therein.
Thus, as will be observed from
FIG. 19A
, in the conventional reflective liquid crystal display device, a pattern of the sections Q, P, Q, and O constitutes one pattern cycle which corresponds to three pixel regions. This means that one pitch (one pattern cycle) of color shade is three times as large as the pixel pitch. Accordingly, when the pitch of one pixel is several tens of micrometers or more, the difference in the density (i.e., occupation area) of each color is visually recognized as vertical stripes, and a vertical stripe pattern is observed at a pitch three times as large as the pixel pitch, i.e., at a pitch of approximately 0.5 mm. This degrades the display quality. More specifically, when the pixel arrangement shown in
FIGS. 14 and 15
is employed, such a vertical stripe pattern is observed on a screen for image display which has a size of 3 inches diagonally and includes tens of thousands of pixels.
SUMMARY OF THE INVENTION
The reflective liquid crystal display device of this invention includes: a pair of substrates sandwiching a liquid crystal layer therebetween; a plurality of pixel electrodes having a delta arrangement formed on one of the pair of substrates; a plurality of signal lines formed on the one of the pair of substrates, the signal lines having bent portions; a plurality of scanning lines formed on the one of the pair of substrates, the plurality of scanning lines and signal lines being formed to run along peripheries of the plurality of pixel electrodes so as to cross each other; and a plurality of thin film transistors electrically connected to the plurality of signal lines for controlling potentials of the pixel electrodes, wherein at least one of the plurality of thin film transistors is formed at a position where the distance between two adjacent signal lines of the plurality of signal lines is reduced by the bent portion of at least one of the two adjacent signal lines.
In one embodiment of the invention, the at least one of the plurality of thin film transistors includes at least two thin film transistors coupled to one of the plurality of signal lines, and the at least two thin film transistors are each coupled to one side of the one of the plurality of signal lines.
In another embodiment of the invention, the plurality of pixel electrodes are electrically connected to drain electrodes of the at least one of the plurality of thin film transistors via contact holes formed thro

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