KR19990081254A - Thin film transistor liquid crystal display - Google Patents

Abstract

In a liquid crystal display device having a liquid crystal capacitor and a plurality of pixels having switching elements, a storage capacitor having one terminal connected to a switching element of one pixel and the other terminal connected to a switching element of a pixel adjacent to the pixel. To form. In this case, two holding capacitors are connected to the switching elements of the remaining pixels except two pixels on both edges, so that sufficient holding capacity can be secured, parasitic capacitance can be reduced, and the switching element is defective. Defects can be easily repaired.

Description

Thin film transistor liquid crystal display

The present invention relates to a thin film transistor liquid crystal display device.

Currently, an active matrix liquid crystal display (AMLCD) using a thin film transistor as a switching element has been widely spotlighted for its portability and excellent image quality.

Each pixel of the liquid crystal display device receives an image signal transmitted through a switching element and is held by the liquid crystal capacitor until the next signal comes in. At this time, in order to protect the electric charge holding ability of a liquid crystal, a normal holding capacitor is connected in parallel with a liquid crystal capacitor.

The method of forming the storage capacitor can be divided into an independent wiring method and a shear gate method according to its structure.

Next, a thin film transistor liquid crystal display according to the related art will be described with reference to the accompanying drawings.

1 is an equivalent circuit diagram of a unit pixel of a conventional thin film transistor liquid crystal display device of an independent wiring method.

As shown in FIG. 1, the gate line GL transmitting the scan signal and the data line DL transmitting the image signal cross each other, the gate electrode is connected to the gate line GL, and the source electrode There is a thin film transistor TFT connected to the data line DL. The drain electrode of the thin film transistor TFT is connected to one terminal of the liquid crystal capacitor C _LC and the storage capacitor Cst. That is, the liquid crystal capacitor C _LC and the storage capacitor Cst are connected in parallel with each other. The other terminal of the liquid crystal capacitor C _LC is applied with a constant voltage Vcom expressed as a common voltage, and the other terminal of the storage capacitor Cst is connected to the storage electrode line SL formed separately from the gate line GL. . However, this method has a disadvantage in that the number of wirings is increased and the opening ratio is reduced because of forming a separate sustain electrode line.

2 is an equivalent circuit diagram of a unit pixel of a conventional thin film transistor liquid crystal display of a shear gate type.

As shown in FIG. 2, the front gate method is a method in which the front gate line GL is used as one terminal of the storage capacitor Cst. The other configuration is similar to that of the independent wiring method shown in FIG. This method has the advantage of increasing the aperture ratio because it does not form a separate storage electrode line other than the gate line, while the parasitic capacitance of the gate line is increased.

In addition, as in the prior art, when using a gate line of a front end or a separate storage electrode line to form a storage capacitor, a high voltage of about 6 to 12 volts is generally applied between both electrodes of the storage capacitor. At this time, a leakage current flows through the insulating film on both electrodes of the storage capacitor due to this voltage, so that self-discharge or defects in the insulating film may cause insulation breakdown, resulting in a decrease in yield.

An object of the present invention is to form a novel capacitor in a thin film transistor liquid crystal display device.

1 is an equivalent circuit diagram of a unit pixel of a conventional thin film transistor liquid crystal display device of an independent wiring method.

FIG. 2 is an equivalent circuit diagram of a unit pixel of a conventional thin film transistor liquid crystal display of a shear gate type.

3 is an equivalent circuit diagram of a thin film transistor liquid crystal display according to an exemplary embodiment of the present invention;

FIG. 4 is a circuit diagram illustrating a repair method of a bad pixel in the liquid crystal display shown in FIG. 3.

5 is a plan view of a thin film transistor liquid crystal display according to an exemplary embodiment of the present invention;

6 is a cross-sectional view taken along the line VI-VI 'of FIG. 5,

7 to 9 are graphs simulating the voltage retention of the liquid crystal display according to the exemplary embodiment of the present invention.

10 is a graph illustrating a kickback voltage of a liquid crystal display according to an exemplary embodiment of the present invention.

11 is a graph illustrating pixel electrode voltages of defective pixels in a liquid crystal display according to an exemplary embodiment of the present invention.

In order to achieve this problem, the present invention forms a storage capacitor having one terminal connected to a switching element of one pixel and the other terminal connected to the switching element of a pixel adjacent to the pixel.

Then, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention.

FIG. 3 is an equivalent circuit diagram of a thin film transistor liquid crystal display according to an exemplary embodiment of the present invention, and FIG. 4 illustrates a repair method when a switching element of the thin film transistor liquid crystal display shown in FIG. 3 is defective.

As shown in FIG. 3, the gate lines GL are arranged horizontally, and the data lines DL are arranged vertically. The gate terminal of the thin film transistor Q is connected to the gate line GL, the source terminal is connected to the data line DL, and the drain terminal is connected to the liquid crystal capacitor C _LC . The other terminal of the storage capacitor Cst in which one terminal is connected to the drain terminal of the thin film transistor Q is connected to the thin film transistors of neighboring pixels in the neighboring rows of the same row, that is, the gate terminal is connected to the same gate line GL. It is connected to the drain terminal of Q). Here, the other terminal of the liquid crystal capacitor is applied with a constant voltage Vcom indicated by a common voltage.

At this time, the other terminal of the storage capacitor Cst _L formed on the left side of the storage capacitors connected to the drain of the thin film transistor of the pixel at the left end is isolated, and the storage capacitor is not formed at the rightmost pixel. Do not. As a result, each pixel has two storage capacitors, and only the rightmost pixel has one storage capacitor.

In a pixel having such a structure, when there is a defect in a switching element such as a thin film transistor or other active element used for driving purposes, the defective active element Q ₀ , as shown in part I of FIG. The drain terminal is cut using a laser or the like to separate from the other capacitors. Alternatively, the same applies when the terminal is electrically cut by a defect. Then, the data signal V to the pixel voltage of the pixel to the right is not supplied to the liquid crystal capacitor of the pixel, the voltage Vp of the liquid crystal capacitor of the pixel is referred to as the pixel voltage of the pixel positioned on the left side of the pixel V _{L, Suppose R} is given by

V _p = (V _L + V _R + V _com C _LC / C _st ) / (2 + C _LC / C _st )

At this time, C _LC / C _st is smaller than 1, and as C _st increases, C _LC / C _st decreases. In this case, the pixel voltage approaches the arithmetic mean of the voltages of the liquid crystal capacitors of the pixels positioned to the left and right of the pixel. Therefore, the brightness of this pixel approaches the middle brightness of the left and right screens and can be compensated automatically.

This structure is particularly advantageous in monochrome liquid crystal display devices. In the case of a color panel composed of three color filters, in order to achieve the purpose of the automatic compensation, it is preferable to connect holding capacitors between third adjacent pixels, that is, pixels displaying the same color. However, in the case where automatic compensation is not aimed as described above, the color panel may be connected to adjacent pixels on the left and right sides.

FIG. 5 is a plan view of a thin film transistor liquid crystal display according to an exemplary embodiment of the present invention, and FIG. 6 is a cross-sectional view taken along line VI-VI ′ of FIG. 5.

As shown in FIGS. 5 and 6, a gate line 10 for transmitting a scan signal in a horizontal direction is formed on the transparent insulating substrate 100, and a gate electrode 11, which is a branch of the gate line, is formed. On the substrate 100, the storage capacitor electrodes 21 and 22 are formed separately from the gate line 10 in the longitudinal direction.

The gate insulating film 30 covers the gate line 10, the gate electrode 11, and the storage capacitor electrodes 21 and 22.

The channel layer 40 of the thin film transistor made of amorphous silicon is formed on the gate insulating film 30 on the gate electrode 11, and the resistive contact layers 51 and 52 made of doped amorphous silicon are formed on the gate insulating film 30. The electrodes 11 are formed in both directions.

The source electrode 61 and the drain electrode 62 are formed on the doped amorphous silicon layers 51 and 52, respectively. The source electrode 61 is formed in the vertical direction on the gate insulating film 30 and is connected to the data line 60 which transmits an image signal from the outside.

The passivation layer 70 is covered on the source electrode 61, the drain electrode 62, and the data line 60, and the contact hole 71 exposing a part of the drain electrode 62 is formed in the passivation layer 70. The protective film 70 and the gate insulating film 30 are removed together to form a contact hole 72 exposing the storage capacitor electrode 21. Pixel electrodes 80, 81, and 82 are formed in the pixel region defined by the intersection of the gate line 10 and the data line 60 on the passivation layer 70. The pixel electrode 80 is connected to the drain electrode 62 through the contact hole 71 formed in the passivation layer 70, and the pixel electrode 82 overlaps the storage capacitor electrode 22 to form a storage capacitor. The storage capacitor electrode 21 of the adjacent pixel is connected via the contact hole 72.

The operation of the thin film transistor liquid crystal display will be described below.

First, a thin film transistor is conducted by applying a gate-on voltage to the gate electrode 11 connected to the gate line 10 corresponding to the pixel to be displayed, and then a data voltage representing an image signal is sourced through the data line 60. The data voltage is applied to the electrode 61 to transfer the data voltage to the drain electrode 62 through the channel of the thin film transistor. Then, the voltage transmitted to the drain electrode 62 is transmitted to the pixel electrode 82, and an electric field is formed between the two substrates by a potential difference between the pixel electrode 82 and a common electrode (not shown) formed on the opposite substrate. Is formed. The intensity of the electric field is controlled by the magnitude of the data voltage. When the electric field is formed, the liquid crystal molecules between the two substrates move along the direction of the electric field, thereby controlling the amount of light transmitted through the substrate.

In this case, however, the liquid crystal deteriorates when the electric field is continuously applied in the same direction to the liquid crystal material between the pixel electrode and the common electrode of the opposite substrate, and thus the direction of the electric field must be continuously changed. That is, the value of the pixel electrode voltage (data voltage) with respect to the common electrode voltage should be changed in a positive and negative manner.

Such a driving method is called an inversion driving method, and examples of the inversion driving method include a frame inversion, a line inversion, a dot inversion, and a column inversion driving method.

Frame inversion means that the polarity of the pixel electrode voltage with respect to the common electrode voltage is changed in the frame period, line inversion is the polarity of the pixel electrode voltage with respect to the common electrode voltage with the period of one gate line, and dot inversion is common in one pixel period. The polarity of the pixel electrode voltage with respect to the electrode voltage is changed.

On the other hand, when one gate line is selected and a scan signal is transmitted to the gate line, the pixels of the row connected to the gate line are each received by the applied data voltage. This pixel voltage must be maintained by the liquid crystal capacitor and the storage capacitor until the next image signal comes in. In the embodiment of the present invention, the voltage of each pixel becomes the reference voltage of the adjacent sustain capacitor. However, in the case of line inversion or frame inversion, when displaying white or black as a whole, adjacent pixels all have the same voltage. Therefore, there is almost no voltage difference between the two electrodes constituting the storage capacitor, so that only a small leakage current flows through the dielectric between the electrodes of the storage capacitor, thereby improving the reliability of the dielectric between the storage capacitors. The life time of the liquid crystal display device can be increased.

For the liquid crystal display having the storage capacitor according to the embodiment of the present invention, the results of simulating the voltage retention that the storage capacitor can maintain corresponding to the leakage current of the pixel are shown in FIGS. 7 to 9. The cells having a pixel arrangement of 3 * 9 were simulated, and the resistance R _LC of the liquid crystal material was changed in some cases.

In the case of the line inversion driving, the voltage of the adjacent pixel has the same polarity as the voltage of the pixel with respect to the common electrode of the opposing substrate. In contrast, in the case of dot inversion driving, the voltages of the adjacent pixels have opposite polarities with respect to the common electrode voltage. During the simulation, the common electrode voltage was maintained at 5V and the data voltages were applied at 1V and 10V.

The simulated pixel electrode voltage for the pixels in the fifth column of the second row is shown in FIG. 7 shows a liquid crystal display device according to an exemplary embodiment of the present invention, and a dotted line indicates a liquid crystal display device of a conventional independent wiring system. In the case of the liquid crystal display without a separate storage capacitor, the solid line is represented by a thin solid line, which is the same as that of the liquid crystal display according to the exemplary embodiment of the present invention, and is hidden by a graph indicated by a thick solid line. In the case of the line inversion, all data voltages are kept at 10V during the write time for one row, and shown in FIG. 7 based on the fact that the resistance R _LC of the liquid crystal material is 1 * 10 ¹¹ Ω.

As shown in FIG. 7, the pixel electrode voltage drops at a high speed because the leakage current is large. In the case of a conventional independent wiring type liquid crystal display device, the voltage of the pixel electrode decreases slowly because the voltage is maintained to some extent by the storage capacitor. It can be seen that the voltage suddenly decreases. In the case of the liquid crystal display according to the exemplary embodiment of the present invention, the voltage of the adjacent pixels is also rapidly reduced by the same degree of leakage current.

When the leakage current of one pixel is larger than the other pixels, different results are shown, which is shown in FIG. 8. In this case, the resistance R _LC of the liquid crystal material is 5 * 10 ¹⁰ Ω.

Referring to FIG. 8, although the pixel voltage of the liquid crystal display according to the exemplary embodiment of the present invention decreases drastically compared to the conventional independent wiring type liquid crystal display, the voltage decreases slowly compared to the liquid crystal display without the storage capacitor. Able to know.

In the case of dot inversion, it can be seen from FIG. 9 that the liquid crystal display according to the exemplary embodiment of the present invention has a very high voltage retention. When the dot inversion driving is performed, the voltage of the adjacent pixels changes in the opposite direction, and in this case, the voltage difference between the pixel electrodes of the adjacent pixels serving as the two electrodes of the storage capacitor becomes large. In this simulation, the voltage of the odd data line is 10V and the voltage of the even data line is 1V. The resistance R _LC of the liquid crystal material is 1 * 10 ¹¹ Ω, as shown in FIG. 7.

As shown in FIG. 9, the pixel electrode voltage (indicated by the bold solid line) of the liquid crystal display according to the exemplary embodiment of the present invention is much higher than that of a conventional independent wiring type liquid crystal display device or a liquid crystal display device without a storage capacitor. It can be seen that it decreases slowly.

FIG. 10 illustrates a shift of the pixel voltage, that is, a kickback voltage, by capacitive coupling between the pixel electrode and the data line when the gate voltage drops.

In the case of the liquid crystal display according to the exemplary embodiment of the present invention, when the dot inversion driving is performed, the charging time is longer because the voltage difference between the two electrodes of the storage capacitor is large. The kickback voltage is larger than in the case of a liquid crystal display having a common electrode having a conventional structure, and is the same as the case where there is no sustain capacitor electrode. The increase in kickback voltage is caused by the simultaneous drop in voltage of adjacent pixels due to coupling when the gate voltage drops.

When a bad pixel is generated, when the drain electrode of the bad pixel is cut from the capacitor with a laser in a manner as shown in FIG. 4, the voltage of the bad pixel is determined by the voltage of the adjacent pixel electrode as described above. Is shown in. The upper curve shows the pixel electrode voltage of the pixels on the left and right sides of the bad pixels, and the lower curve shows the pixel electrode voltage of the bad pixels. That is, the voltage of the bad pixel is determined by the voltages of the left and right adjacent pixels.

The liquid crystal display according to the present invention was made into a panel having a size of 5.8 "capable of half-color display and a resolution of 234 * 400 (* 3). As a result, crosstalk and flickering were It did not generate | occur | produce and was confirmed to show the display performance of the same grade as a normal liquid crystal display device.

As in the embodiment of the present invention, one electrode of the storage capacitor of one pixel is connected to the switching element of the pixel and the other electrode is connected to the switching element of the adjacent pixel to form the storage capacitor, thereby reducing the parasitic capacitance. When a device is defective, the defect can be easily repaired.

Claims (2)

In a liquid crystal display device including a storage capacitor and a plurality of pixels including a liquid crystal capacitor and a switching element, The first terminal of the storage capacitor of any pixel is connected to the switching element of that pixel and the second terminal is connected to the switching element of the other pixel, The liquid crystal display device is driven in a dot inversion method. In claim 1, And a switching element connected to the second terminal of the storage capacitor is a switching element of a neighboring pixel.