Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-06-01
2004-04-20
Flynn, Nathan J. (Department: 2826)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000, C349S044000, C349S038000, C349S039000, C349S048000, C349S054000, C349S055000, C349S041000
Reexamination Certificate
active
06724444
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to liquid crystal display devices and, more particularly, to liquid crystal display devices of the so-called active matrix type with switching elements being disposed in units of picture elements or “pixels.”
2. Description of the Related Art
Liquid crystal display devices are typically designed to perform a display operation by applying an electric field to liquid crystal molecules of a liquid crystal layer that is interposed between a pair of substrates to thereby permit liquid crystals to change in orientation or “alignment” direction and then utilizing resultant optical changes of the liquid crystal layer occurred due to such alignment direction changes.
One typical prior known active-matrix type liquid crystal display device is the one of the twisted nematic (TN) type, wherein the direction of an electric field being applied to its liquid crystal layer is set at a specific direction that is substantially at right angles to surfaces of the substrates with the liquid crystal layer sandwiched therebetween for achievement of the intended display operation by utilization of optical rotary polarization of the liquid crystal layer.
On the other hand, liquid crystal display devices of the in-plane switching (IPS) scheme type are also proposed until today, which are designed to employ a comb-shaped electrode while letting the direction of an electric field applied to liquid crystals be substantially parallel to the substrate surfaces to thereby perform displaying by use of double refractivity of the liquid crystals. Typical ones of such devices have been disclosed, for example, in Japanese Patent Publication No. 21907/1988, U.S. Pat. No. 4,345,249, WO91/10936, and Japanese Patent Laid-Open No. 160878/1994.
This IPS scheme is distinguishable over the traditional TN scheme in that the former offers advantages including but not limited to wider viewing angles and lower load capacitances. Presence of such technical superiority permits the IPS scheme to be adaptable for use with new types of active-matrix liquid crystal display devices which are widely used in place of the TN scheme type ones.
In the IPS scheme, as apparent from M. Oh-e, M. Yoneda and K. Kondo, Journal of Applied Physics, Vol. 82, No. 2, 1997 at pp. 528-535, it is possible to almost perfectly realize the intended in-plane switching operation in cases where liquid crystals are designed to have negative polarity dielectric anisotropy, rather than positive dielectric anisotropy.
Currently available IPS liquid crystal display devices employ optically opaque metal comb-shaped electrodes of stripe shape as provided within the surface of one of the pair of substrates stated supra.
In recent years, another type of IPS scheme has been proposed which employs comb-shaped electrodes made of a transparent conductive material such as indium tin oxide (ITO) in place of opaque metal electrodes, which electrodes are laid out at selected pitches that are less in value than those used in the prior art IPS scheme, and further uses a chosen liquid crystal material whose dielectricity anisotropy is negative in polarity to thereby make it possible by use of only electric fields as created at edge portions of a comb electrode to cause all the liquid crystals existing on or over this transparent comb electrode to offer the required alignment changeability, thus improving both the optical transmissivity and aperture ratio thereof.
The above-noted proposal is disclosed, for example, in S. H. Lee, S. L. Lee and H. Y. Kim, “Asia Display,” 1998 at pp. 371-374 and also in S. H. Lee, S. L. Lee, H. Y. Kim & T. Y. Eom, “SID Digest,” 1999, pp. 202-205.
It is also reported in the above-identified technical documents that with the IPS scheme using in combination certain liquid crystal material of negative dielectricity anisotropy and short-pitch transparent comb electrodes, the transmissivity near in value to that in the TN scheme becomes available while retaining wide view-angle characteristics equivalent to those in the IPS scheme.
A liquid crystal display device of the type employing the above-discussed technology is arranged to comprise a plurality of gate lines (gate lead wires) and multiple drain lines (drain leads) formed on or over a substrate, and also to comprise switching elements' (typically, thin-film transistors, as the rest of the description as will be presented below assumes the use of such thin-film transistors) at cross-over points or “intersections” of the gate and drain lines, wherein more than one common electrode and pixel electrodes driven by the switching elements are disposed adjacent to each other.
A respective one of the thin-film transistors has its gate electrode formed of part of a gate line associated therewith, a drain electrode extended from its associative drain line, the drain electrode overlying the gate electrode with a semiconductive layer (amorphous silicon or “a-Si” layer) being interposed therebetween, and a source electrode for electrical connection to a pixel electrode. Note here that although the drain electrode and the source electrode are functionally interchangeable during an operation of the thin-film transistor, the following explanation will be given fixedly as shown in drawings to be presented later.
FIG. 17
is a diagram showing an enlarged plan view of a main part of a thin-film transistor in one exemplary IPS scheme liquid crystal display device. In
FIG. 17
, “GL” is used to designate a gate line; DL indicates a drain line; ASI denotes a semiconductor layer (amorphous silicon gr “a-Si” layer, also called a-Si island in some cases); PX denotes a pixel electrode; and CT denotes a common electrode. The element SD
1
is a source electrode, whereas the element SD
2
is a drain electrode. In this liquid crystal display device, the pixel electrode PX and common electrode CT are disposed over a thin-film transistor substrate in such a manner that these elements are in close proximity in position relative to each other. The source electrode SD
1
and pixel electrode PX of the thin-film transistor are connected together via a through-hole TH.
In addition,
FIG. 18
is a main part plan view diagram pictorially depicting a thin-film transistor portion in another example of the IPS liquid crystal display device. In
FIG. 18
the same reference characters as those used in
FIG. 17
designate the same functional parts or components. In this liquid crystal display device, the pixel electrode PX is formed directly on the thin-film transistor substrate, whereas the common electrode CT is formed to overlie the pixel electrode PX with a dielectric layer interposed therebetween. The source electrode SD
1
is formed at the same layer level as the pixel electrode PX. The source electrode SDI of the thin-film transistor is connected to the pixel electrode PX via a through-hole TH.
Another structure is available, which is designed so that the common electrode CT is formed directly on the thin-film transistor substrate while letting the pixel electrode PX be formed thereover with a dielectric layer sandwiched therebetween. Still another one is also under consideration, which is arranged so that the shape of at least one of the pixel electrode PX and common electrode CT is bent or flexed in either a longitudinal direction or lateral direction.
SUMMARY OF THE INVENTION
With any one of the liquid crystal display devices shown in
FIGS. 17 and 18
also, the semiconductor layer ASI to be formed over the gate line GL is oversized and extruded from the gate line GL at portions underlying the source electrode SD
1
(i.e. the portions surrounded by circles “A” in FIGS.
17
-
18
). When back-light rays reach and fall on such extruded portions of this semiconductor layer ASI, what is called the photoconduction current—also known as photoconductivity current in some cases—might be generated, resulting in creation of current leakage at the thin-film transistor, or alternatively a potential decrease occurs in signal hold/retention voltage. G
Ashizawa Keiichirou
Hashimoto Yuuichi
Kuriyama Hideki
Nakatani Mitsuo
Tanaka Takeshi
Erdem Fazli
Flynn Nathan J.
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