JPH04318512A - Thin film transistor type liquid crystal display device - Google Patents

Abstract

PURPOSE:To eliminate a DC component between the drain electrode and counter electrode of the thin film transistor(TFT) type liquid crystal display an to prevent miswriting due to gate pulse delay. CONSTITUTION:A shield electrode 7 is formed on the drain electrode 2 of a TFT across an insulating film. The shield electrode 7 is connected electrically to the counter electrode. Further, the drain electrode 2 and source electrode 4 constitute a main transistor(TR) and a 1st auxiliary electrode 8 and a 2nd auxiliary electrode 10 constitute a subordinate TR. Then when an (n-1)th gate pulse is ON, the voltage on the shield electrode 7 is written in an (n)th picture element electrode through the subordinate TR and when an (n)th gate pulse is ON, the voltage on the drain electrode is written through the main TR. The OFF time of a gate pulse is made earlier than the generation time of the drain voltage of a next line to prevent the miswriting due to the gate pulse delay.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor type liquid crystal display device, and particularly to an electrode pattern and a driving method.

0002

[Prior Art] Conventionally, as a technology in this field, for example,
EID90-6, ED90-35, IE90-15, 1
The one described in ``Development of 0.4-inch color TFT-LCD'' is known. FIG. 8 is a partial cross-sectional view of a conventional thin film transistor (hereinafter referred to as "TFT") described in the above-mentioned document.

As shown in the figure, a gate insulating film 34, a semiconductor layer 35, and an ohmic layer 36 are successively formed on a gate electrode 32, and a source-drain electrode 37, which is a signal electrode, is provided thereon. The position of the pixel electrode 33 may be below or above the source-drain electrode 37, but this only differs in the focus of each company and does not significantly change the overall TFT structure. Finally, a passivation film 38 is provided. Furthermore, in this TFT, the gate electrode has a two-layer structure of Al and Ta to prevent image blurring caused by gate pulse delay.

0004

However, although the thin film transistor type liquid crystal display device having the above structure has an effect on gate pulse delay, some voltage is always applied to the drain electrode into which the video signal is input. The resulting potential fluctuation between the drain electrode and the counter electrode drives the liquid crystal molecules, resulting in light leakage. As a countermeasure for this, it is common practice to form a black mask layer on the counter electrode side to block this light leakage, but since the black mask layer is formed, the aperture ratio is inevitably small. There was a problem with this.

Furthermore, even if this light leakage can be prevented by such measures, it is impossible to prevent potential fluctuations occurring between the drain electrode and the pixel electrode. In other words, the drain electrode is located right next to the pixel electrode to which the voltage on the drain electrode has been written by the gate pulse, and some voltage is always applied to the drain electrode.
There is also a pixel potential fluctuation due to capacitive coupling between the drain electrode and the pixel electrode, and in general, the center value of the positive and negative levels of the pixel electrode potential is caused by the voltage drop caused by the gate-source capacitance of the TFT. Since it is lower than the center value of the positive and negative levels, a DC component voltage is always applied between the drain electrode and the pixel electrode. If a DC component is applied to the liquid crystal, the deterioration will be significant and reliability will be lost.To prevent this, the counter electrode voltage is set to be low in accordance with the voltage drop. When this is done, a DC component is no longer added to the liquid crystal between the pixel electrode and the counter electrode, but a DC component is added to the liquid crystal between the drain electrode and the counter electrode, causing the liquid crystal to deteriorate.

The present invention solves the above-mentioned conventional problems, and
It is an object of the present invention to provide a thin film transistor type liquid crystal display device with a large aperture ratio, little deterioration of liquid crystal, and excellent display quality and reliability.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a plurality of gate electrodes, a plurality of drain electrodes intersecting with the gate electrodes, a thin film transistor provided at the intersection, and a thin film transistor. In a thin film transistor type liquid crystal display device including a thin film transistor substrate having a pixel electrode connected to the pixel electrode and a counter electrode substrate facing the thin film transistor substrate with a liquid crystal in between, the thin film transistor substrate has an insulating film formed on the drain electrode and an insulating film formed on the drain electrode. , a shield electrode formed on the insulating film and into which a voltage similar to that of the counter electrode of the counter electrode substrate is input, and a voltage on the drain electrode is written to the n-th pixel electrode when the n-th gate pulse is turned on. A main transistor that writes the voltage on the shield electrode to the n-th pixel when the (n-1)th gate pulse is turned on is provided.

[0008]

According to the present invention, since the thin film transistor type liquid crystal display device is constructed as described above, the space between the drain electrode and the counter electrode is shielded by the shield electrode. Therefore, even if a potential difference occurs between the drain electrode and the counter electrode, the drain voltage is shielded by the shielding electrode, so no DC component is generated between these electrodes, and the liquid crystal on the drain electrode is not turned on.

Furthermore, when the n-1th gate pulse is turned on, the voltage on the shield electrode is written to the nth pixel electrode, and
When the th gate pulse is turned on, the voltage on the drain electrode is written to the n th pixel electrode.

0010

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a plan view of a thin film transistor substrate in an embodiment of the present invention. As shown in the figure, a main transistor is provided at the intersection of the gate electrode 1 and the drain electrode 2 via the semiconductor layer 3, and the voltage signal on the drain electrode 2 is written to the source electrode 4 by the gate pulse. It has become. The source electrode 4 is electrically connected to the pixel electrode 5 through the first contact hole 6, and the source voltage waveform becomes the pixel voltage waveform as it is.

On the other hand, the same gate electrode 1 and drain electrode 2
A sub-transistor is provided at the intersection of the lines, and the configuration is such that the voltage of the shield electrode 7 can be written into the pixel electrode 5 of the next line by the gate electrode 1. The sub-transistor is formed between the first auxiliary electrode 8 and the second auxiliary electrode 10, and is electrically connected to the shield electrode 7 and the pixel electrode 5 through the second contact hole 9 and the third contact hole 11, respectively.

[0012] Furthermore, the shielding electrode 7 is provided on all the drain electrodes.
It is formed slightly wider than the width of the drain electrode 2. Figure 2
1 is a sectional view of a main transistor portion (A-A' in FIG. 1) of a thin film transistor substrate in an embodiment of the present invention. In this embodiment, the gate electrode 1 is anodized to form a gate anodic oxide film 12. However, this film is mainly for the purpose of preventing short circuit between the gate electrode and the drain electrode, and is not an essential element of the present invention. A gate insulating film 13 is formed over the entire surface, and a semiconductor layer 3 and an ohmic contact layer 15 are further formed on top of this. Further, a drain electrode 2 and a source electrode 4 are formed thereon, and an intermediate insulating film 16 is formed thereon.
A contact hole 6 is formed, and the source electrode 4 and the pixel electrode 5 are electrically connected through the first contact hole 6. and,
Although a passivation film 17 is finally formed, this is also not an essential element of the present invention, so it may be omitted.

Note that if the shield electrode 7 and the pixel electrode 5 are formed at the same time using the same material (eg, ITO), the number of masks will not increase, and the process will not be complicated. FIG. 3 is a sectional view (BB' in FIG. 1) of the sub-transistor portion of the thin film transistor substrate in the embodiment of the present invention. The shielding electrode 7 is electrically connected to the first auxiliary electrode 8 through the second contact hole 9 . A transistor is formed between the first auxiliary electrode 8 and the second auxiliary electrode 10, and the gate electrode 1 serves as its switching gate. And the second
The auxiliary electrode 10 is electrically connected to the pixel electrode 5 through a third contact hole 11. Here, the first auxiliary electrode 8 can be formed simultaneously with the drain electrode 2.

FIG. 4 is an equivalent circuit diagram of one pixel of a thin film transistor type liquid crystal display device according to an embodiment of the present invention. As shown in the figure, there is a main transistor 22 for writing a drain electrode signal at the intersection of the n-th gate electrode 19 and the drain electrode 2, and a main transistor 22 for writing a drain electrode signal is located at the intersection of the n-1th gate electrode 18 and the drain electrode 2. There is a sub-transistor 21 for writing a shield electrode signal. Further, the pixel electrode 5 is connected to these main and sub transistors, and the liquid crystal layer is represented by a parallel circuit of a liquid crystal resistor 23 and a liquid crystal capacitor 24. A counter electrode 25 electrically connected to the shield electrode 7 is provided on the opposite side of the liquid crystal layer.

In the figure, the nth pixel electrode 5 is n-
When the first gate electrode 18 is turned on, the voltage of the counter electrode 25 is written as a shield electrode signal through the sub-transistor 21, and when the n-th gate electrode 19 is turned on, the drain electrode signal is written through the main transistor 22. is written. FIG. 5 is an explanatory diagram showing a method for driving a thin film transistor type liquid crystal display device according to an embodiment of the present invention. Moreover, FIG. 6 is a pixel voltage waveform diagram obtained by this driving method.

First, the drain voltage waveform 26 shown in FIG. 5(a) is inverted between positive and negative every line, and further inverted between positive and negative every frame. This is a commonly used driving method and is an effective method for reducing flicker and brightness gradient. However, this does not mean that the present invention will not be effective unless the inversion is performed for each line.
There is no problem even if the frame is simply inverted.

Next, the counter voltage waveform 27 is set slightly below the center value of the positive and negative levels of the drain voltage. This is because the source, that is, the pixel voltage waveform, shifts down when the gate is turned off due to the parasitic capacitance between the gate electrode and the source electrode of the TFT. This is because a DC voltage is applied to the liquid crystal, causing the liquid crystal to deteriorate. What should be noted here is that due to the low setting of this counter voltage, there is always a DC voltage between the drain electrode 2 and the counter electrode 25.
component is generated. However, in the present invention, since the shield electrode 7 is provided on the drain electrode 2, fluctuations in the drain voltage are shielded by the shield electrode 7, and since the same signal as the counter voltage waveform 27 is input to the shield electrode 7, the drain wiring No potential difference is generated in the upper liquid crystal, and no DC component is generated.

Next, the gate voltage waveform shown in FIG.
It has a short width of τn-1. Below, Figures 5 and 6
A method of driving a thin film transistor type liquid crystal display device according to an embodiment of the present invention will be described with reference to FIG. Regarding the nth pixel, first, when the n-1th gate is turned on at time tn-1, the counter voltage waveform 27 is written to the pixel electrode. Next, when the gate is turned off, the pixel voltage waveform is shifted down due to the parasitic capacitance between the gate electrode and the source electrode of the sub-transistor, and then maintained for a period of Δt. Next, at time tn, when the nth gate is turned on, a drain electrode signal is written, and when the gate is turned off, the pixel voltage waveform is shifted down due to the parasitic capacitance between the gate electrode and the source electrode of the main transistor. Thereafter, the pixel voltage is held until the n-1th gate is turned on in the next frame.

When driving in this way, the pixel electrode potential is inverted every frame, regardless of whether it is positive or negative, so charging and discharging occur in the desired direction from the time the n-1th gate is turned on. Therefore, the drain voltage signal to be written when the nth gate voltage is turned on is quickly reached. In other words, sufficient on-characteristics of the transistor can be obtained.

In addition, when the gate pulse width is set equal to one line as in the conventional case, if a delay occurs in the nth gate pulse, the drain signal of the opposite polarity when the n+1th gate is turned on is changed to the drain signal of the opposite polarity when the nth gate pulse is trailing. However, in this embodiment, as mentioned above, sufficient on-characteristics can be obtained, so if the n-th gate is turned off slightly before the input of the n+1-th drain pulse, such erroneous writing can be avoided. It disappears. The time for turning off the gate pulse early at this time is shown as Δt, but since this value varies depending on the material and structure of the TFT, the size of the liquid crystal display device, etc., it will not be discussed in detail. Also, this Δ
The problem that setting t is not good for the on-characteristics of the transistor because it reduces the write time is solved by using the sub-transistor and the shield electrode.

FIG. 7 is an explanatory diagram of an electrical connection system of a thin film transistor substrate in an embodiment of the present invention. As shown in the figure, the shield electrode 7 is integrally formed on the substrate and a counter voltage signal is applied thereto, so the addition of the shield electrode 7 does not complicate the electrical connection system. Note that the present invention is not limited to the above-described embodiments, and various modifications can be made based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0022]

As explained in detail above, according to the present invention, a shielding electrode is provided on the drain electrode via an insulating film, and a voltage of the same level as that of the counter electrode is input to the shielding electrode. Since the electrode voltage is written to the pixel electrode by the sub-transistor when the previous gate is turned on, the following effects are achieved. (1) Since the DC component between the drain electrode and the counter electrode becomes zero, the liquid crystal does not deteriorate. (2) The drain signal prevents the liquid crystal from turning on and causing light leakage. Therefore, since the black mask layer is not required, the aperture ratio is improved. (3) Sufficient on-characteristics of the transistor can be obtained. If the off time of the gate pulse is set earlier than the time when the drain voltage of the next line is generated, erroneous writing of the next line signal due to gate pulse delay can be eliminated.

[Brief explanation of drawings]

FIG. 1 is a plan view of a thin film transistor substrate in an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a main transistor portion of a thin film transistor substrate in an embodiment of the present invention.

FIG. 3 is a cross-sectional view of a sub-transistor section of a thin film transistor substrate in an embodiment of the present invention.

FIG. 4 is an equivalent circuit diagram per pixel of a thin film transistor type liquid crystal display device according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a method for driving a thin film transistor type liquid crystal display device according to an embodiment of the present invention.

FIG. 6 is a pixel voltage waveform diagram of a thin film transistor type liquid crystal display device according to an embodiment of the present invention.

FIG. 7 is an explanatory diagram of an electrical connection system of a thin film transistor substrate in an embodiment of the present invention.

FIG. 8 is a partial cross-sectional view of a conventional thin film transistor substrate.

[Explanation of symbols]

1 Gate electrode 2 Drain electrode 3 Semiconductor layer 4 Source electrode 5 Pixel electrode 7 Shielding electrode 8 First auxiliary electrode 10 Second auxiliary electrode 16 Intermediate insulation film

Claims (3)

[Claims] 1. A thin film transistor substrate having a plurality of gate electrodes, a plurality of drain electrodes intersecting with the gate electrodes, a thin film transistor provided at the intersection thereof, a pixel electrode connected to the thin film transistor, and a liquid crystal. A thin film transistor type liquid crystal display device comprising a counter electrode substrate sandwiching the thin film transistor substrate and opposing the thin film transistor substrate,
The thin film transistor substrate includes (a) an insulating film formed on the drain electrode, and (b) a shield formed on the insulating film and to which a voltage similar to that of the counter electrode of the counter electrode substrate is input. (c) a main transistor that writes the voltage on the drain electrode to the nth pixel electrode when the nth gate pulse is on, and (d) the voltage on the shield electrode when the n-1th gate pulse is on. 1. A thin film transistor type liquid crystal display device, comprising: a sub-transistor for writing an image into an n-th pixel. 2. The thin film transistor type liquid crystal display device according to claim 1, further comprising a drive circuit that sets the off time of the gate pulse earlier than the time when the drain voltage of the next line is generated. 3. The thin film transistor type liquid crystal display device according to claim 1, wherein the width of the shield electrode is larger than the width of the drain electrode.