A liquid crystal display device controls a light transmittance of a liquid crystal using an electric field to thereby display a picture. The liquid crystal display device is largely classified into a vertical electric field applying type and a horizontal electric field applying type depending upon a direction of the electric field driving the liquid crystal.
The liquid crystal display device of a vertical electric field applying type drives a liquid crystal in a TN (Twisted Nematic) mode with a vertical electric field formed between a pixel electrode and a common electrode arranged in opposition to each other on the upper and lower substrates. The liquid crystal display device of a vertical electric field applying type has an advantage of a large aperture ratio but has a drawback of a narrow viewing angle of about 90°.
The liquid crystal display device of a horizontal electric field applying type drives a liquid crystal in an In Plane Switching (hereinafter, referred to as “IPS”) mode with a horizontal electric field between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The liquid crystal display device of the IPS mode has an advantage of a wide viewing angle of about 160°, but has a disadvantage of low aperture ratio and transmittance. Specifically, in order to provide an In Plane Field in the liquid crystal display device of the IPS mode, a distance between the common electrode and the pixel electrode for driving the liquid crystal is wider than a distance between the upper substrate and the lower substrate. In order to obtain an electric field having an adequate intensity, the common electrode and the pixel electrode in the liquid crystal display device of the IPS mode have a wide width. An electric field parallel to the substrate is formed between the pixel electrode and the common electrode of the IPS mode, but the electric field does not affect the liquid crystal in the upper pixel electrodes and the upper common electrodes portion. Accordingly, the liquid crystal in the upper pixel electrodes and the upper common electrodes portion is not driven by the electric field and maintains an initial state. As a result, the liquid crystal in the initial state does not transmit light and thus an aperture ratio and the light transmittance of the IPS mode liquid crystal display device are reduced.
Recently, in order to overcome the disadvantage of the liquid crystal display device of the IPS mode, there has been suggested a liquid crystal display device of a fringe field switching (hereinafter, referred to as “FFS”) type operated by a fringe field. The FFS-type liquid crystal display device includes a common electrode plate and a pixel electrode with an insulating film therebetween at each pixel area. A distance between the common electrode plate and the pixel electrode in the FFS-type liquid crystal display device is wider than a distance between the upper substrate and the lower substrate such that a fringe field of parabolic shape is provided at an upper portion of the common electrode and the pixel electrode. And, the liquid crystal molecules filled between the upper substrate and the lower substrate are all driven by the fringe field, thus the aperture ratio and the light transmittance in the FFS-type liquid crystal display device are improved.
FIG. 1 is a plan view illustrating a thin film transistor substrate included in a related art FFS-type liquid crystal display device, and FIG. 2 is a sectional view of the thin film transistor substrate taken along the I-I′ line in FIG. 1.
Referring to FIG. 1 and FIG. 2, a thin film transistor substrate includes a gate line 2 and a data line 4 that cross each other with a gate insulating film 22 therebetween on a lower substrate 20, and a thin film transistor is provided for each intersection. A pixel area on a thin film transistor substrate is defined by an intersection of the gate line 2 and the data line 4. A common electrode plate 14 and a pixel electrode slit 18 with the gate insulating film 22 and a protective film 28 therebetween are mounted at the pixel area to provide the fringe field F. The common electrode plate 14 is connected to a common line 1 parallel to the gate line 2. Herein, the gate insulating film 22 and the protective film 28 are made from an inorganic insulating material such as Sinx, etc., and have a thickness of about 2000 Å.
The common electrode plate 14 is supplied, via the common line 1 connected to the common electrode plate 14, with a reference voltage (hereinafter, common voltage) for driving the liquid crystal. The common electrode plate 14 comprises a transparent conductive layer, and the common line 1 comprises a gate metal layer along with the gate line 2.
The thin film transistor TFT allows a pixel signal applied to the data line 4 to be charged into the pixel electrode slit 18 and be maintained in response to a gate signal applied to the gate line 2. To this end, the thin film transistor TFT includes a gate electrode 6 connected to the gate line, a source electrode 8 connected to the data line 4, a drain electrode 10 connected to the pixel electrode slit 18, an active layer 24 overlapping with the gate electrode 6 with the gate insulating film 22 therebetween to provide a channel between the source electrode 8 and the drain electrode 10, and a semiconductor pattern 25 including an ohmic contact layer 26 for making an ohmic contact with the source electrode 8, the drain electrode 10 and the active layer 24.
The pixel electrode slit 18 is connected, via a contact hole 12 passing through the protective film 28, to the drain electrode 10 of the thin film transistor TFT, and overlaps the common electrode plate 14. The pixel electrode slit 18 forms an electric field together with the common electrode plate 14 to rotate the liquid crystal molecules arranged in a horizontal direction between the thin film transistor substrate and the color filter substrate due to a dielectric anisotropy. Transmittance of a light transmitting the pixel area is differentiated according to a rotation extent of the liquid crystal molecules, thereby implementing a gray level scale.
The electric field formed between the common electrode plate 14 and the pixel electrode slit 18 includes both a linear-type electric field and a fringe field F of parabolic shaped curvature. A start point and an end point of the fringe field F are near an upper edge of the pixel electrode slit 18. Thus, liquid crystal molecules positioned at the upper edge of the pixel electrode slit 18 can be driven by the fringe field F. In this case, a width of the pixel electrode slit 18 is narrow enough such that the fringe field F is produced both at the common electrode plate 14 and the upper portion of the pixel electrode slit 18. Accordingly, the liquid crystal at both the common electrode plate 14 and the upper portion of the pixel electrode slit 18 can be all driven by the fringe field F.
A storage capacitor Cst stably maintaining a video signal supplied to the pixel electrode slit 18 is formed at an overlapping portion of the common electrode plate 14 and the pixel electrode slit 18.
A liquid crystal display device having an FFS-type thin film transistor substrate is a Normally Black Mode device in which a screen display state is black before an electric field signal is applied to between the common electrode 14 and the pixel electrode 18.
Even though the electric field signal is not applied to the common electrode 14 and the pixel electrode 18, an electric field can be formed between the data line 4 and the pixel electrode 18. When the screen display state is black, a liquid crystal at a pixel area adjacent to the data line 4 is driven owing to the electric field between the data line 4 and the pixel electrode 18, thus causing a light leakage. The pixel area adjacent to the data line 4 with a light leakage is corresponded to a black matrix BM area provided at the upper substrate in order to prevent the light leakage phenomenon. The area corresponding to the black matrix BM area of the upper substrate is a non-aperture area, so that the light leakage effectively reduces an aperture ratio of the FFS-type liquid crystal display device.