Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2003-10-08
2004-11-30
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S039000, C349S044000, C349S111000
Reexamination Certificate
active
06825891
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to the technical field of active-matrix driving electro-optical devices, and more particularly to the technical field of electro-optical devices including, in a laminate structure of a substrate, a capacitive electrode and a capacitive line for adding a capacitor to a pixel electrode, and a thin-film transistor (TFT) for switching a pixel.
2. Description of Related Art
Currently, in a TFT-driven active-matrix electro-optical device, when a TFT is supplied at the gate thereof with a scanning signal through a scanning line, the TFT is turned on, thereby supplying a pixel electrode through a source and a drain of the TFT with an image signal that is provided through a data line to a source region of a semiconductor layer. Since the image signal is supplied to each pixel electrode through each TFT for an extremely short period of time, a storage capacitor is typically added to each pixel electrode (in parallel with a capacitor of a liquid crystal) to hold the voltage of the image signal supplied through the TFT for a period of time substantially longer than the time of the on state of the TFT.
The storage capacitor is typically formed of a capacitive electrode that is at a pixel-electrode potential and is extended from a conductive polysilicon layer forming the drain region of the TFT connected to the pixel electrode, a dielectric layer, and a capacitive line being at a predetermined potential and including an electrode portion that is opposed to the capacitive electrode with the dielectric layer interposed therebetween. The capacitive line is fabricated of the same conductive layer as that forming the scanning line (a conductive polysilicon layer, for example), and is typically arranged laterally to run in parallel with the scanning line.
SUMMARY OF THE INVENTION
There is generally a strong market demand for a higher-definition display image in electro-optical devices. To achieve higher definition, the pixel pitch can be made finer while the aperture ratio of the pixel is increased (i.e., an aperture area of the pixel through which image light is transmitted is expanded with respect to a non-aperture area of each pixel through which no image light is transmitted).
In accordance with the above-mentioned conventional art in which the scanning lines and the data lines are respectively juxtaposed in an image display area, the non-aperture area of each pixel where the scanning line and the capacitive line are arranged becomes narrower as the aperture ratio of the fine-pitched pixel increases. As the pixel pitch becomes finer, it becomes more difficult to embed a capacitor having a sufficiently larger capacitance and to impart sufficiently higher conductivity to the scanning line and the capacitive line. When a capacitor having a sufficiently large capacitance is not available or a scanning line and a capacitive line having a sufficient conductivity is not produced, the electro-optical device suffers from cross-talk or ghosting in an image display thereof, thereby degrading the image quality. As the aperture ratio of the fine-pitched pixels increases, the degradation of image quality becomes more pronounced. In other words, an attempt to improve image quality creates another problem that is difficult to resolve.
In comparison with the problem, the present invention has been developed, and it is an object of the present invention to provide an electro-optical device which increases the pixel aperture ratio while increasing the capacitance of a storage capacitor (or controlling a reduction in the capacitance of the storage capacitor), and providing a high-quality image display free from cross-talk and ghosting.
To achieve the above object, the electro-optical device of the present invention can include, on a substrate, scanning lines and data lines that intersect each other, thin-film transistors, each connected to one of the scanning lines and one of the data lines, and pixel electrodes respectively connected to the thin-film transistors. The present invention can also include a storage capacitor laminated above each of the scanning lines and including a pixel-potential capacitive electrode connected to the pixel electrode and being at a pixel-electrode potential, a fixed-potential capacitive electrode at a predetermined potential, and a dielectric layer interposed between the pixel-potential capacitive electrode and the fixed-potential capacitive electrode.
In accordance with the electro-optical device of the present invention, the storage capacitor can be laminated on the scanning line (with an interlayer insulator interposed therebetween, for example), and can includes the pixel-potential capacitive electrode connected to the pixel electrode and being at the pixel-electrode potential, the fixed-potential capacitive electrode at the predetermined potential, and the dielectric layer interposed between the pixel-potential capacitive electrode and the fixed-potential capacitive electrode. The storage capacitor is thus produced in a region overlapping the scanning line on the substrate in a plan view by making use of the pixel-potential capacitive electrode formed in the region overlapping the scanning line and the fixed-potential capacitive electrode opposed to the pixel-potential capacitive electrode. Unlike in the conventional art, the fixed-potential capacitive electrode (or the capacitive line) is not arranged laterally in parallel with the scanning line, and the use of the scanning line and the fixed-potential capacitive electrode (or the capacitive line) does not expand the non-aperture area of each pixel, because the scanning line and the fixed-potential capacitive electrode do not run alongside and in parallel with each other. In other words, by forming the fixed-potential capacitive electrode (or the capacitive line) overlapping the scanning line on the substrate, the aperture area of each pixel is expanded while a formation area of the fixed-potential capacitive electrode (or the capacitive line) is expanded at the same time. The capacitance of the storage capacitor is thus relatively increased. A sufficiently wide line width is achieved, thereby imparting sufficient conductivity to the scanning line and the fixed-potential capacitive electrode (or the capacitive line). As a result, the electro-optical device has a high aperture ratio of the fine-pitched pixel while providing an improved image quality for a presented image free from cross-talk and ghost.
In one embodiment of the electro-optical device of the present invention, the thin-film transistor can include a gate electrode formed of part of the scanning line and located over the channel region thereof. This embodiment provides a so-called top gate thin-film transistor which includes the scanning line having a storage capacitor laminated thereon on the substrate.
In another embodiment of the electro-optical device of the present invention, the thin-film transistor includes a gate electrode formed of part of the scanning line and located below the channel region thereof. This embodiment provides a so-called bottom gate thin-film transistor which includes the scanning line having a storage capacitor laminated thereon on the substrate.
In yet another embodiment of the electro-optical device of the present invention, the gate electrode of the thin-film transistor can be formed of the same conductive layer as the conductive layer forming the scanning line. In accordance with this embodiment, a portion of the scanning line running in a linear or comb-like configuration and fabricated of a conductive polysilicon layer, a metal layer or an alloy layer is over the gate insulator of each thin-film transistor and functions as a gate electrode.
In yet another embodiment of the electro-optical device of the present invention, the gate electrode of the thin-film transistor can be formed of a conductive layer different from the conductive layer forming the scanning line. In accordance with this embodiment, an island gate electrode connected directly or via
Chowdhury Tarifur R.
Oliff & Berridg,e PLC
Schechter Andrew
Seiko Epson Corporation
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