Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-03-05
2003-06-24
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S038000, C349S042000
Reexamination Certificate
active
06583829
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display element, and in particular to a thin film transistor active-matrix type liquid crystal display element provided with a storage capacity formed in a pixel area by a storage line disposed therein and a pixel electrode.
Liquid crystal display devices are widely used as a high-definition color display device for a notebook computer and a display monitor.
Among liquid crystal display elements, the simple matrix type and the active matrix type are well-known. The simple matrix type liquid crystal display element comprises a pair of opposing substrates, a first group of parallel strip electrodes disposed on an inner surface of one of the substrates, a second group of parallel strip electrodes disposed on an inner surface of the other substrate to intersect the first group of parallel strip electrodes, and a liquid crystal layer sandwiched between the substrates. The active matrix type liquid crystal display element comprises a pair of opposing substrates, a plurality of switching elements each provided for selecting a respective pixel and disposed on an inner surface of one of the substrates, and a liquid crystal layer sandwiched between the substrates.
The active matrix type liquid crystal display element is divided into the so-called vertical electric field type which usually called the TN (the Twisted Nematic) type and the so-called horizontal electric field type which is usually called the IPS (the In-Plane-Switching type). In the so-called vertical electric field type as represented by the TN type, two groups of strip electrodes for selection of pixels are disposed on inner surfaces of a pair of upper and lower substrates, respectively. In the so-called horizontal electric field type, two groups of electrodes for selection of pixels are disposed only on an inner surface of one of a pair of upper and lower substrates.
In the TN type active-matrix liquid crystal display element, liquid crystal molecules are oriented to be twisted through 90 degrees, for example, between a pair of opposing substrates. A pair of polarizers are disposed in front of and behind the liquid crystal display element, respectively, such that their absorption axes are at right angles (the arrangement of crossed nicols) with the absorption axis of a polarizer on the entrance side is parallel with or perpendicular to the rubbing direction of the entrance-side substrate.
In the TN type active-matrix liquid crystal display element, first suppose that no electric field is applied across the liquid crystal layer. The incident light is converted into a linearly polarized light by the entrance-side polarizer, the linearly polarized light propagates along the twist of the liquid crystal molecules, and then all of the linearly polarized light passes through the exit-side polarizer and provides a white display if an azimuthal angle of the transmission axis of the exit-side polarizer is identical with that of the linearly polarized light entering the exit-side polarizer. This is the so-called normally open mode.
Next, suppose that an electric field is applied across the liquid crystal layer. The direction (director) of a unit vector representing the average direction of orientations of axes of liquid crystal molecules of the liquid crystal layer is perpendicular to the surface of the substrate, the azumuthal angle of the linearly polarized light entering the liquid crystal layer is not changed, and as a result, it is identical with that of the absorption angle of the exit-end polarizer and consequently, the liquid crystal display element provides a black display. For further detail, see “Basics and Application of Liquid Crystal,” Industrial Research Association, Tokyo, 1991.
As described above, the TN type liquid crystal display element includes a pair of opposing transparent insulating substrates made of material such as glass, a liquid crystal layer sandwiched between the opposing glass substrates, a group of scanning signal lines (hereinafter also called gate signal lines) extending in the x direction and arranged in the y direction on the liquid-crystal layer side surface of one of the substrates in a system of rectangular co-ordinates, and a group of drain lines (hereinafter also called video signal lines) insulated from the gate signal lines, extending in the y direction, and arranged in the x direction.
A pixel area is formed in an area surrounded by two adjacent gate lines and two adjacent drain lines, and all the pixel areas form a display area. Formed in each pixel area are a thin film transistor (TFT) serving as an active element (a switching element), for example, and a transparent pixel electrode.
A scanning signal supplied to a gate line switch on a thin film transistor which, in turn, supplies a video signal to a pixel electrode from a drain line.
A capacitance formed by a dielectric of the lighted liquid crystal is not sufficient to maintain the ON (lighted) state of the pixel which has been driven by the thin film transistor during a specified period of time, and therefore an additional capacitance is provided to each pixel so as to replenish the shortage of the capacitance. The additional capacitance has been provided by overlaying a portion of the pixel electrode on the gate line with an insulating layer interposed therebetween.
On the other hand, a storage method is suggested which disposes a storage line in the pixel area and forms a capacitance (a storage capacitance) using a pixel electrode, the storage line and an insulating layer therebetween.
Storage type liquid crystal display devices are disclosed in Japanese Patent Application Laid-open No. Hei 11-231341 (laid-open on Aug. 27, 1999), Japanese Patent Application Laid-open No. 2000-221527 (laid-open on Aug. 11, 2000), and Japanese Utility Model Registration No. 2536632 (registered on Feb. 21, 1997).
FIG. 13
shows an equivalent circuit of a storage type liquid crystal display element. In the storage type liquid crystal display element, a plurality of gate lines GL and a plurality of drain lines DL are disposed to intersect one another in a display area AR on one of a pair of opposing substrates, a thin film transistor TFT is disposed at each of intersections of the gate lines GL and the drain lines DL, and a plurality of storage lines STL are disposed between and parallel with the gate lines GL. Reference character ITO
2
denotes a common electrode disposed on an inner surface of the other substrate.
A source electrode (the roles of source and drain may be exchanged in a driving cycle) of the thin film transistor TFT is connected to the pixel electrode ITO
1
, a liquid crystal capacitance Clc is formed between the pixel electrode ITO
1
and the common electrode ITO
2
with a liquid crystal therebetween serving as a dielectric, and a storage capacity Cstg is formed between the pixel electrode ITO
1
and the storage line STL.
The common electrode ITO
2
is supplied with a specified voltage (preferably a fixed voltage) from a common-electrode driving circuit CDR, and the storage lines STL are supplied with a specified voltage (preferably a fixed voltage) from a storage-line driving circuit STR. Reference character GDR denotes a gate line driving circuit, DDR is a drain line driving circuit, CONT is a display control circuit for supplying display signals and timing signals.
FIG. 14
is a plan view of essential parts of a unit pixel and its vicinity formed on a substrate SUB
1
of the storage type liquid crystal display element. In
FIG. 14
, certain of the shapes and dimensions are exaggerated for clarity.
FIG. 15
is a schematic cross-sectional view of the storage type liquid crystal display element taken along line XV—XV of FIG.
14
.
In
FIG. 14
, each pixel is disposed in an area surrounded by two adjacent gate lines GL and two adjacent drain lines intersecting the gate lines GL, and is provided with a thin film transistor TFT at their intersection. The thin film transistor TFT is comprised of the gate line GL, an insulating film (not shown to avoid undue co
Ohgiichi Kimitoshi
Ootsu Ryouichi
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