Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-08-02
2003-04-01
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S110000, C349S111000
Reexamination Certificate
active
06542205
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix display device using a thin film transistor (hereinafter referred to as TFT) as a switching element, and more specifically, to a pixel structure of the active matrix display device.
2. Description of the Related Art
A liquid crystal display device with a method of performing driving by using TFTs (TFT driving method) is known as an active matrix display device. In the liquid crystal display device, a voltage applied to liquid crystal can be controlled for each pixel by using TFTs formed over a transparent substrate made of glass or the like, and thus, images are clear. Therefore, the liquid crystal display devices are widely used in OA equipment, TVs and the like.
FIG. 1
shows an equivalent circuit of one pixel in the liquid crystal display device with the TFT driving method. A pixel TFT
102
is arranged at an intersection portion of a gate signal line
100
and a source signal line
101
. A gate terminal of the pixel TFT
102
is electrically connected with the gate signal line
100
. One of input and output terminals of the pixel TFT
102
(a source or a drain terminal) is connected with the source signal line
101
, and the other terminal is connected with a liquid crystal
103
and a storage capacitor
104
.
When the pixel TFT
102
enters an ON state in response to a signal output to the pixel TFT
102
from the gate signal line
100
, a potential of the source signal line
101
is written into the liquid crystal
103
and the storage capacitor
104
, and an electric charge is stored. Even after the pixel TFT
102
enters an OFF state, the electric charge stored in the liquid crystal
103
and the storage capacitor
104
tries to hold the written potential. A necessary value of the storage capacitor
104
is determined in accordance with an off current, a holding time, a parasitic capacitor and the like of the pixel TFT
102
that becomes a switching element.
FIG. 2
shows a cross-sectional structure of one example of a conventional storage capacitor. The storage capacitor is formed by an active layer
201
and a capacitor wiring
203
formed from the same film as that for a gate wiring as electrodes, and a gate insulating film
202
as a dielectric, which is formed between the active layer
201
and the capacitor wiring
203
. By using the gate insulating film
202
as the dielectric, the highly reliable and high-quality storage capacitor can be formed even with the thin thickness.
Further, it is desirable that the active matrix display device is provided with a light shielding film.
FIG. 3
shows a cross-sectional structure of a pixel TFT provided with a light shielding film under the pixel TFT as one such example. The light shielding film
301
and an insulating film
302
are formed on a glass substrate
300
, and an active layer
303
, a gate insulating film
304
, and a gate wiring
305
, which are provided for forming the pixel TFT, are sequentially laminated thereon. The light shielding film prevents light leakage and improves contrast, and there is obtained an effect of reducing an off current of the pixel TFT by shielding the pixel TFT from light. When the off current of the pixel TFT is reduced, the display data holding characteristic is improved, thereby obtaining a satisfactory display.
As a method of improving display quality (image quality) of the conventional active matrix display device and attaining energy saving, miniaturization and high reliability of the display device, the following points are given.
The first point is that, in the active matrix display device, a capacitor element structure is obtained, which can secure a sufficient storage capacitor even if an area for one pixel is reduced with higher resolution. If each pixel is provided with a storage capacitor that can have a large capacitor, the display data holding characteristic is improved, and thus, a satisfactory display can be obtained.
The second point is that, in the active matrix display device, an aperture ratio is not reduced while a sufficient storage capacitor is secured. If each pixel has a high aperture ratio, utilization efficiency of light of backlight is improved. Thus, energy saving and miniaturization of the display device can be attained.
Further, light leakage is prevented and contrast is improved by arranging the light shielding film. In addition, the off current of the pixel TFT is reduced by shielding the pixel TFT from light, which leads to an improvement of the display data holding characteristic.
SUMMARY OF THE INVENTION
Demands for an improvement of performance of an active matrix display device such as for high precision (minuteness of a pixel TFT), securing a sufficient storage capacitor, a high aperture ratio, and a light shielding film, are opposed to each other in a meaning that one demand is satisfied while other demands are neglected. An object of the present invention is to improve performance of the active matrix liquid crystal display device while the above demands are satisfied.
The present inventors contrived the formation of a storage capacitor with the use of a light shielding film in order to satisfy these mutually opposing demands. Further, the present inventors proposed a method of forming a storage capacitor having a large capacitor without lowering an aperture ratio.
FIG. 4A
is a cross sectional view showing an example in which a light shielding film and a capacitor are formed by extending source and drain regions of a pixel TFT. A light shielding film
401
and a dielectric (first insulating film)
402
are formed on a glass substrate
400
. One of the source and drain regions of the pixel TFT, which is connected with a pixel electrode
409
, is widened in order to secure a necessary storage capacitor to thereby form an active layer
403
.
The light shielding film
401
has conductivity and may be connected at the outside of a pixel region so as to have a constant potential such as a COMMON potential or a power supply. In a case where the capacitor of the light shielding film
401
is sufficiently larger than the storage capacitor of the pixel, the light shielding film
401
does not have to be connected at a constant potential as long as the potential variation is sufficiently small. Thus, the storage capacitor is formed by the active layer
403
and the light shielding film
401
.
FIG. 4B
is a diagram in which, in addition to the storage capacitor formed by the light shielding film
401
and the active layer
403
, a capacitor wiring
410
is formed to secure a storage capacitor having a larger capacity. A gate insulating film
404
is formed on the active layer
403
, and a gate wiring
405
and the capacitor wiring
410
are simultaneously formed. The capacitor wiring
410
is connected at the outside of the pixel region so as to have a constant potential such as a COMMON potential or a power supply to thereby form a capacitor with the active layer
403
. In this way, a larger storage capacitor is secured without lowering an aperture ratio. Further, in
FIG. 4B
, the gate insulating film formed under the capacitor wiring
410
is formed to be thin in order to enlarge the storage capacitor.
In
FIGS. 4A and 4B
, the capacitors are formed by the source and drain regions of the pixel TFT and the light shielding film
401
arranged under the regions. However, the capacitor is not needed with respect to the region connected with a source signal line. The reason for that is because a charge in writing in a video signal to the source signal line increases if the capacitor is formed to the source signal line. Thus, as shown in
FIGS. 5A and 5B
, a structure is proposed in which a light shielding film is provided in two layers so as not to form a capacitor with respect to the region connected with the source signal line.
FIG. 5A
shows an example of two layers of the light shielding film. A first light shielding film
501
is formed on a glass substrate
500
, an insulating film
502
is formed thereon for insulation, and a second light shieldi
Ishikawa Akira
Ohtani Hisashi
Shibata Hiroshi
Tanaka Yukio
Dudek James
Semiconductor Energy Laboratory Co,. Ltd.
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