The present invention relates to liquid crystal display devices, and, more particularly, to liquid crystal display devices of the lateral electric field type.
A liquid crystal display device of the lateral electric field-driven type comprises a pair of transparent substrates disposed so as to be opposed to each other and to be spaced by a liquid crystal (LC) layer sealed therebetween, and a pixel electrode and a counter electrode are spaced apart from the pixel electrode formed in each of the pixel regions on a surface at the liquid crystal layer side of one of the pair of transparent substrates. A pixel electrode and a counter electrode disposed at each of the pixel regions generates an electric field therebetween so as to control the optical transmissivity of the liquid crystals in the vicinity of the pixel region. Since the liquid crystal display device of the lateral electric field type generates an electric field having a major component extending substantially parallel to a main surface of the transparent substrate between the pixel electrode and the counter electrode for switching the orientations of the liquid crystal molecules, the lateral electric field scheme is also referred to as the xe2x80x9cIn-Plane-Switching (IPS) schemexe2x80x9d also.
An active-matrix liquid crystal display device employing the lateral electric field scheme has a plurality of scan signal lines and a plurality of counter voltage signal lines both of which extend in an xe2x80x9cxxe2x80x9d direction (row direction), while being juxtaposed in parallel in a xe2x80x9cyxe2x80x9d (column) direction (transverse to the x direction), and a plurality of video signal lines which extend in the y direction, while being juxtaposed in the x direction on the surface at the liquid crystal layer side of the one of the pair of transparent substrates. When the scan signal lines, the counter voltage signal lines and the video signal lines are disposed on a surface of one of the pair of transparent substrates, each of the pixel regions is defined as an area surrounded by one of the scan signal lines, one of the counter voltage signal lines disposed adjacent to the scan signal line, and a pair of the image signal lines disposed adjacent to one another. Each of the pixel regions has a switching element, a pixel electrode, and a counter electrode. The switching element operates so as to be turned on by a scanning signal supplied by one of the scanning signal lines. The switching element, when turned on, supplies a video signal being transmitted by one of the video signal lines thereto to the pixel electrode in a pixel region corresponding thereto. A counter voltage for generating a potential difference between the counter electrode and the pixel electrode in the pixel region is transmitted through one of the counter voltage signal lines and is applied to the counter electrode.
A liquid crystal display device having the structure described above is characterized by the fact that it produces an image having a higher contrast ratio and exhibits a remarkably wider viewing angle.
The lateral electric field-type liquid crystal display device referred to above is described in detail in such documents as Japanese Patent Application Laid Open Hei 05-505247 (No. 505247/1993), Japanese Patent Publication Sho 63-021907 (No. 021907/1988), and Japanese Patent Application Laid Open Hei 06-160878 (No. 160878/1994).
The lateral electric field-type liquid crystal display device having the structural features described above utilizes electrodes (e.g. an pixel electrode and a counter electrode) formed of thin conductive films of several thousands Angstrom (xc3x85) (thin film electrodes, hereinafter) and generates an electric field substantially in parallel with a surface of one of the pair of transparent substrates. However, it is difficult to reproduce the aforementioned electric field sufficiently and accurately in the liquid crystal layer lying between the thin film electrodes.
In addition to the aforementioned difficulty, the aforementioned electric field being generated between the pixel electrode and the counter electrode in a pixel corresponding thereto is affected by another electric field appearing from the video signal line adjacent to the pixel.
On the other hand, a black matrix layer (a light shielding film) is formed on a surface of another of the pair of transparent substrates confronting the one of the pair of transparent substrates on which the pixel electrode and the counter electrode are formed. For instance, the black matrix layer is formed on the surface of the substrate which confronts the liquid crystal layer. The black matrix layer may be formed of an insulating material or the like having such a high resistivity that the electric field between the pixel electrode and the counter electrode is generated to be as strong as possible.
The black matrix, being formed of an insulating material or the like, tends to have a higher optical transmissivity than that of metals so that a problem of deterioration of the contrast ratio occurs in an image displaying operation of the liquid crystal display device.
The present invention has been made in view of the technical background mentioned above, and an object of the present invention is to provide an improved liquid crystal display device which is suitable for generating an electric field sufficiently between the pixel electrode and the counter electrode as to control the orientations of the liquid crystal molecules lying therebetween in spite of utilizing a simple structure.
Another object of the present invention is to improve the contrast ratio of an image displayed by the liquid crystal display device.
Some representative aspects and features of the present invention as disclosed herein will be briefly summarized as follows.
The present invention provides a liquid crystal display device comprising: a pair of substrates being superposed on one another with a predetermined space therebetween in which a liquid crystal layer is sealed; at least one video signal line being formed on a surface of one of the pair of substrates confronting the liquid crystal layer; and at least one pixel being formed on the surface of the one of the pair of substrates and having at least one pixel electrode to which a signal is supplied through the at least one video signal line and at least two counter electrodes being spaced from the at least one pixel electrode, one of the at least two counter electrodes being disposed adjacent to the at least one video signal line. The display device is characterized in that: (1) the at least one video signal line and the one of the at least two counter electrodes are spaced from one another by at least two insulating regions disposed along the surface, and (2) one of the at least two insulating regions comprises a material having a higher permittivity than that of the other of the at least two insulating regions.
The present invention also provides another liquid crystal display device comprising: a pair of substrates being disposed opposite to one another a liquid crystal layer being sealed therebetween; at least one video signal line formed on a surface of one of the pair of substrates confronting the liquid crystal layer; and at least one pixel being formed on the surface of the one of the pair of substrates, being arranged adjacent to the at least one video signal line, and having at least one pixel electrode and at least one counter electrode, so as to apply an electric field to the liquid crystal layer therebetween. The display device is characterized in that: (1) at least a surface of the at least one video signal line is covered with an insulating material having a permittivity (a dielectric constant) higher than 6.7.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.