Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2002-03-28
2004-11-02
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C349S042000, C349S043000, C257S059000
Reexamination Certificate
active
06812912
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix display device having a thin-film transistor disposed for each pixel.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter referred as an “LCD”) can be made thin, compact, and lightweight, and has low power consumption. Liquid crystal displays are now used as display devices in various types of electronic equipment such as portable information equipment. LCDs which have a thin-film transistor disposed as a switch element for each pixel are referred to as active matrix type LCDs, and such panels are used as high resolution, high display quality display devices because they can reliably maintain the display content of each pixel.
FIG. 1
shows an equivalent circuit of a pixel of an active matrix LCD. Each pixel is provided with a thin-film transistor (TFT)
1
which is connected to a gate line and a data line and, when the TFT is turned on by a selection signal output to the gate line, data corresponding to the display content is supplied from the data line to a liquid crystal capacitor
2
(Clc) through the TFT. Here, a storage capacitor
3
(Csc) is connected to the TFT in parallel to the liquid crystal capacitor Clc because it is necessary to securely maintain the written display data for a period during which the TFT is selected, data is written and the next TFT is selected.
FIG. 2
shows a plane structure of a pixel section of the TFT formed substrate (a first substrate
100
) of a conventional LCD, and
FIG. 3
shows a sectional structure of the LCD taken along line X—X of FIG.
2
. The LCD has a structure in which liquid crystal is sealed between first and second substrates. In this active matrix LCD, TFTs
1
, pixel electrodes
74
, etc. are arranged in a matrix on the first substrate
100
, and a common electrode
56
to which a common voltage Vcom is applied, a color filter
54
and the like are formed on the second substrate disposed to face the first substrate. The liquid crystal capacitor Clc is driven for each pixel by a voltage applied between the respective pixel electrodes
74
and the opposing common electrode
56
, with liquid crystal
200
between them.
The TFTs disposed for each pixel on the side of the first electrode
100
are so-called top gate TFTs which have a gate electrode
60
disposed on a layer above an active layer
64
, as shown in FIG.
3
. The active layer
64
of the TFT is patterned on a substrate
5
as shown in
FIG. 2
, a gate insulating film
66
is formed to cover the active layer
64
, and the gate line, which also serves as the gate electrode
60
, is formed on the gate insulating film
66
. The active layer
64
has a channel region
64
c
positioned to face the gate electrode
60
, and a drain region
64
d
and a source region
64
s
, in which an impurity is charged, are formed on both sides of the channel region
64
.
The drain region
64
d
of the active layer
64
is connected to a drain electrode
70
, which also serves as the data line, through a contact hole which is formed in an interlayer insulating film
68
to cover the gate electrode
60
.
A planarization insulating film
72
is formed to cover the area above drain electrode and data line
70
, and the source region
64
s
of the active layer
64
is connected to a pixel electrode
74
, which is formed on the planarization insulating film
72
of ITO (indium tin oxide) or the like, through the contact hole.
The source region
64
s
of the active layer
64
also serves as a first electrode
80
of the storage capacitor Csc disposed for each pixel and extends from the contact region with the pixel electrode
74
as shown in
FIG. 2. A
second electrode
84
of the storage capacitor Csc is simultaneously formed of the same layer as the gate electrode
60
as shown in
FIG. 3
, in a distinct region separated from that of the gate electrode
60
by a prescribed gap. A dielectric substance between the first electrode
80
and the second electrode
84
is also served by the gate insulating film
66
. The second electrode
84
of the storage capacitor Csc, which is not independent for each pixel, extends on the pixel region in the line direction in the same way as the gate line
60
as shown in FIG.
2
. To this second electrode
84
is applied a predetermined storage capacitor voltage Vsc.
Thus, a storage capacitor Csc is disposed for each pixel to hold an electric charge corresponding to the display content which must be applied to the liquid crystal capacitor Clc during a TFT non-selection period. As a result, it is made possible to suppress a potential change of the pixel electrode
74
and to maintain the display content.
In applications in which it is required the display device be compact and have a high resolution, the area per pixel must be made small, and, as a consequence, the liquid crystal capacitor Clc per pixel becomes small. Therefore, a storage capacitor Csc such as described above must be provided to ensure that the display data of each pixel is maintained during the unit display period.
However, because the storage capacitor Csc does not function as a display region, reduction of the displayable area per pixel, namely, a reduction in aperture ratio, cannot be avoided in a transparent type LCD. Especially, when the second electrode
84
of the storage capacitor Csc is formed on the same layer as the gate line
60
as shown in FIG.
2
and
FIG. 3
, an insulating space is required to prevent the gate line
60
and the second electrode
84
from being short-circuited. Furthermore, because the second electrode region is formed of the same material as the gate, it is also opaque. As a consequence, aperture ratio is further lowered accordingly and producing a bright display becomes even more difficult.
Still further, a conventional LCD is provided with a black matrix for shielding light between the pixels in order to improve the contrast of the screen in the region between pixels. This matrix is, typically provided, in addition to the above storage capacitor Csc, on the second substrate which is disposed to face the first substrate on which the TFT is formed. The LCD is formed by bonding the first substrate and the second substrate and sealing liquid crystal in the gap between them as described above, and, in order to avoid variations in the aperture ratio of the respective pixels caused by displacement of the bonded substrates, either the black matrix is made to have a larger width or small pixel regions (e.g., pixel electrode) are formed. This further exacerbates the problem of the aperture ratio.
SUMMARY OF THE INVENTION
To address the above problems, it is an object of the present invention to provide an active matrix display device which simultaneously provides adequate storage capacitance and a high aperture ratio.
In order to achieve the aforementioned object, the present invention is directed to an active matrix display device having a thin-film transistor (TFT) and a storage capacitor in respective pixels, wherein the TFT is formed on a substrate as a top gate type for each pixel; a first electrode of the storage capacitor is electrically connected to an active layer of the TFT; and a second electrode of the storage capacitor is formed to partly overlap at least the active layer of the TFT with an insulating layer provided between the active layer and the substrate.
Another aspect of the invention is directed to an active matrix display device which has a TFT, a liquid crystal capacitor and a storage capacitor in respective pixels and drives liquid crystal sealed in a gap between first and second substrates to display data, wherein the TFT is formed on the liquid crystal-opposed side of the first substrate as a top gate type and the storage capacitor is formed in a region formed between a first electrode which is also served by an active layer of the TFT and a second electrode which is disposed with an insulating film held between the active layer of the TFT and the first substrate.
As described above,
Miyajima Yasushi
Yamada Tsutomu
Yokoyama Ryoichi
Yoneda Kiyoshi
Cantor & Colburn LLP
Chang Kent
Sanyo Electric Co,. Ltd.
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