|Liquid crystal display (LCD)|
|Friday, 01 September 2006|
A liquid crystal display device generally includes a lower substrate (array substrate) that includes a thin film transistor, and an upper substrate (color filter substrate) that includes a color filter, and a liquid crystal material interposed between the upper substrate and the lower substrate. A lower substrate of the liquid crystal display panel is formed on an upper surface thereof with gate lines and data lines, which are aligned in cross to each other in order to define pixel regions. An LCD panel includes a plurality of pixels arranged in matrix form. Each pixel is composed of an upper plate, a lower plate and a liquid crystal layer set between the lower plate and the upper plate. A pixel electrode and a thin film transistor, which are switched by a driving signal of the gate line in order to apply a signal of the data line to the pixel electrode, are aligned in each pixel region. A common electrode and pixel electrodes are formed on the opposing surfaces of the color filter and thin film transistor array substrates, respectively, and apply electric fields to the layer of liquid crystal material. The pixel electrodes are formed within liquid crystal cells on the thin film transistor array substrate and the common electrode is formed over the entire surface of the color filter substrate. Optical sheets for directing the light from the back light unit toward the liquid crystal display panel in a vertical direction are arranged on the liquid crystal display module. The liquid crystal display apparatus includes a display unit for displaying an image, and a backlight assembly disposed under the display unit to provide the display unit with a light. The liquid crystal display devices are generally provided with a backlight unit or a surface lighting device since liquid crystals do not illuminate by themselves and providing light from behind the liquid crystal panel is often preferred. The backlight assembly includes a lamp unit and a light guide plate. The lamp unit generates the light. The light guide plate guides the light generated from the lamp unit toward the display unit. The reflection plate covers the lamp to reflect a portion of the light, which advances toward the reflection plate, toward the light guide plate. The light guide plate includes an upper surface, a lower surface and a side surface. A case is provided for the LCD device to enclose the back light unit and the optical sheets as well as the edges of the liquid crystal display panel. The liquid crystal panel includes a color filter substrate separated from a thin film transistor array substrate by a layer of liquid crystal material. The color filter layer is aligned in each of the pixel regions, and a common electrode is aligned at a front surface of the upper glass substrate. In general, these liquid crystal devices are formed such that a pair of substrates, each having electrodes formed thereon are bonded by a ring of sealing material such that the electrodes are oriented parallel to each other, and the liquid crystal is encapsulated in the region enclosed by the pair of substrates and the sealing material.
In general, a liquid crystal display device displays a desired image by individually supplying a data signal to liquid crystal cells arranged in a matrix form and controlling light transmittance of the liquid crystal cells according to image information. A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal is a material phase that has properties between liquid and solid. Liquid crystal has optical anisotropy due to its long-range crystal ordering and fluidity. Liquid crystals have long been utilized for their ability to change their optical orientation in the presence of an electric field. Liquid crystals can dramatically increase the diffraction efficiency of a volume hologram of which they are a part. Together, these properties offer the very desirable possibility of electrically switching the diffraction efficiency of volume holograms for use in a wide variety of optical information processing and display applications. In an LCD panel, a liquid crystal layer having anisotropic permittivity is injected between two substrates of a panel, and light transmittivity of the panel is controlled by applying and controlling an electric field to obtain desired images. A seal member has been generally employed to seal a liquid crystal between two substrates which serves to guard the liquid crystal from contamination due to such as water from the outside of the device and environmental changes. Polymer-dispersed liquid crystals (PDLCs) are formed from a homogeneous mixture of prepolymer and liquid crystals. Liquid crystal converts light that does not contain information into light containing information to display images. The long thin shapes of the liquid crystal molecules can be aligned to have an orientation in a specific direction. The alignment direction of the liquid crystal molecules can be controlled by an applied electric field. In the liquid crystal display, an optical compensatory sheet (phase retarder) is often provided between the liquid crystal cell and the polarizing plate, to prevent the displayed image from undesirable coloring. Due to the optical anisotropy of the liquid crystal molecules, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an electric field applied to a group of liquid crystal molecules in respective pixels, a desired image can be produced by diffracting light. The liquid crystal panel is provided with pixel electrodes and a common electrode for applying electric fields to the respective liquid crystal cells. Liquid crystal cells are defined where the gate and data lines cross each other. Switching devices such as thin film transistors (TFT) are provided to control the voltage applied to the pixel electrode by liquid crystal cells. Each of tile pixel electrodes receives video signals, via a thin film transistor that is a switching device. The pixel electrode, along with the common electrode, drives the liquid crystal cell in accordance with a data signal applied to the TFT. Gate lines and data lines are arranged to intersect in the liquid crystal display, and liquid crystal cells are positioned at the intersection of the gate lines and the data lines. The data lines transmit data signals supplied from a data driver IC to the liquid crystal cells. Via the data lines, data signals are applied to source electrodes of the thin film transistors and then to pixel electrodes to control the light transmittance characteristics of individual liquid crystal cells. The gate lines transmit scan signals supplied from a gate driver IC to the liquid crystal cells. Via the gate lines, scan signals are applied to gate electrodes of the thin film transistors to form a conductive channel.
Liquid crystal display devices are in wide use as display devices capable of reducing the overall size, weight and thickness of electronic apparatuses in which they are employed. In particular, the thin film transistor liquid crystal display (TFT-LCD) employing the TFT as a switching element is most widely used. Generally, thin film transistors (TFT) are used as the switch devices. By the thin film transistor, the liquid display device controls a voltage applied to a liquid crystal which is sealed between the TFT array substrate and the opposite substrate, and can performs an image displaying utilizing an electro-optic effect of the liquid crystal. A thin film transistor array panel is manufactured by photolithography using a plurality of photomasks. The thin film transistors are classified into those using amorphous silicon (a-Si) and those using polycrystalline silicon (p-Si), and those using polycrystalline silicon are classified into those formed by a high temperature process and those formed by a low temperature process. An LCD in which TFTs are respectively formed in unit pixel regions is classified as an amorphous type TFT-LCD and a polycrystalline type TFT-LCD. The polycrystalline type TFT-LCD has advantages in that it is capable of speedier operation at low power consumption. It has also an advantage in that TFTs for pixels and semiconductor devices for drive-circuits can be formed together. Liquid crystal display devices can be classified into an active matrix type in which liquid crystal is driven by switching elements, and a passive matrix type in which liquid crystal is driven without using switching elements. The active matrix type display can be further classified into a type which uses three-terminal elements, such as a thin-film transistor (TFT), as switching elements, and a type which uses two-terminal elements such as a thin-film diode (TFD). Active matrix liquid crystal displays use thin film transistors (TFTs) as switching devices to display pictures corresponding to video signals (e.g., television signal) inputted to a pixel matrix. The TFT substrate includes a plurality of gate lines running in parallel and a plurality of source lines running in parallel in the direction crossing the gate lines at right angles. An active matrix liquid display device typically comprises a TFT array substrate in which gate electrodes and data electrodes are arranged in the form of a matrix . The thin film transistors (TFT) are disposed at intersecting points of the matrix, and an opposite substrate located so as face the TFT array substrate with a gap there between. The passive matrix display method does not use any transistor and does not require a complex circuit.
Liquid crystal display elements adopt a liquid crystal display mode that is suitably selected depending on twist angles of the liquid crystal active-driving-type twisted nematic liquid crystal display mode (the TN mode), the multiplex-driving-type super-twisted nematic liquid crystal display mode (the STN mode), GH (guest host) mode, ECB (electrically controlled birefringence) mode, and OCB (optically compensated birefringence) mode. Currently available LCDs use either the STN birefringence mode or the TN mode. The TN liquid crystal display controls transmitted light quantity using changes in rotary polarization resulting from the orientation changes of the liquid crystal molecules caused by voltage. Twisted nematic (TN) liquid crystal displays have inherently narrow and non-uniform viewing angle characteristics. STN displays are characterized by large twist angles of nematic liquid crystal directors as compared to regular twist nematic liquid crystal displays. STN displays provide voltage-contrast characteristics with sharp cutoff, which are required to obtain high multiplexing ability and contrast ratios. STN displays feature extraordinarily high resolution ability and small pixel size, which enhance information capacity of such displays in displaying numerical information as well as in displaying images. The TN liquid crystal display is currently widely used in computers and measurement devices, but has the problem of slow response speed. The STN liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the response rate. The TN liquid crystal display allows wide manufacturing margins and high productivity. However, it has problems with display performance, especially with viewing angle characteristics. Specifically, when the display surface of the TN liquid crystal display is viewed obliquely, the display contrast ratio lowers considerably. STN displays feature excellent image quality, as compared to regular passive-matrix TN-displays. These advantages are especially pronounced in large displays with high multiplexing level.
Generally, liquid crystal display devices can be classified as a transmissive liquid crystal display device, a reflective liquid crystal display device, or transflective LCD device. The transmissive liquid crystal display device needs a dedicated light source called a backlight, while the reflective liquid crystal display device employs the surrounding light as the light source. The transmissive liquid crystal display devices require high power consumption because they use an artificial light source behind the liquid crystal panel. Typically, the backlight assembly includes a light source, such as a light-emitting diode (LED), a fluorescent lamp, or other device that emits light, and some optical elements to direct the light from the light emitter to the LCD. As the transmissive liquid crystal display (LCD) device uses the backlight as a light source, it can display a bright image in dark surroundings. On the other hand, in the reflective type LCD device, sunlight or artificial light is used as a light source of the LCD device. In reflective LCD devices, incident light first passes through the liquid crystal panel, then is reflected in the liquid crystal panel, and finally the reflected light passes through the color filter to display a color image. Since there is no backlight, the reflective type LCD device has much lower power consumption than the transmissive type LCD device. Reflective liquid crystal display devices take advantage of ambient light, such as natural light or illumination light, but are difficult to view under dark conditions. The transflective LCD device can compensate for the respective shortcomings of the reflective LCD device and the transmissive LCD device. The transflective LCD device can selectively provide a reflective or transmissive mode, depending on the needs of users. Since the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount that varies depending on environmental conditions, the transmission type liquid crystal display devices are generally used as display devices for displaying a multi-color or full-color image. Reflective liquid crystal display devices can be applied in a variety of mobile electronic apparatus because they do not have a light source, such as a back-light, incorporated therein, and thus they consume less power.
The transmissive LCD device using the back light as a light source is relatively heavy and voluminous due to the back light, but it is widely used since it does not use an exterior light source and displays an image independently from the exterior light source. Reflective liquid-crystal panels having a reflective mirror on one substrate use light illuminating the liquid-crystal panel from the outside as an illumination light source. The reflective liquid crystal display device possesses the advantages of making possible further reduction in thickness, weight, and power consumption and also lessening eye fatigue in the long-time use over the transmissive liquid crystal display device. For this reason, reflective liquid-crystal panels have been used as displays for portable information terminals or cellular phones which have an especially low power consumption. In the transflective liquid crystal device, an image is displayed in the transmissive mode using a light source when used in a dark environment. When used in a light environment, an image is displayed using ambient light as in the reflective liquid crystal device, and thus it needs low power consumption. Because of such an advantage, the transflective liquid crystal device is widely used as a display unit in portable devices and other various systems.