JPS61282821A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To expand an operating voltage range and to easily execute a multi- gradation display by providing at least >=1 layers of voltage regulating layers consisting of insulators between a liquid crystal and picture element electrodes except an oriented film. CONSTITUTION:An auxiliary capacitor is formed by laminating a transparent electrode 2, an insulating layer 3 and picture element electrodes 4 on a transparent substrate 1. The gate electrode 5 on the layer 3 is positioned to face a semiconductor layer 7 via a gate insulating layer 6. A thin film transistor is formed of a drain electrode 9 and a source electrode 8. At least >=1 layers of the voltage regulating layers 10 are provided on the electrodes 4 and the oriented film 11 is laminated thereon. The liquid crystal 20 is packed between the substrate disposed with the thin film transistor and the picture elements consisting of the picture element electrodes into the matrix shape and the transparent substrate 21 on which the transparnet electrode 22 is formed over the entire surface. The permissible voltage range for each one gradation is thereby expanded and the multi-gradation display is easily executed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improvement in a liquid crystal display device that drives a liquid crystal using a thin film transistor, and particularly to a liquid crystal display device capable of displaying multiple gradations using a TN mode liquid crystal. be.

BACKGROUND ART Liquid crystal display devices are thin, can be driven at low voltages, and have low power consumption, and are therefore in high demand in the market as flat display devices. Among these, active matrix type liquid crystal display devices, in which a thin film transistor is installed in each pixel to drive the liquid crystal, have no crosstalk and are essentially static driven, which reduces the number of scanning lines compared to simple matrix type devices. It is possible to increase the number of images and support high-density, high-resolution video display.

Further, in order to make the most of the advantage of low voltage driving of a liquid crystal display device, it is common to use a TN liquid crystal whose operating voltage range is relatively low, about 2 to 3 V, and Δε>0 as the liquid crystal material used.

Normally, a thin film transistor has a gate electrode 5. connected to a gate line corresponding to a scanning line, as shown in FIG. Gate insulating layer 6. A semiconductor layer 7 made of amorphous silicon, polycrystalline silicon, etc., and a source electrode 8 connected to a source line corresponding to a signal line. It consists of a drain electrode 9 connected to the picture element electrode 4, and is provided for each picture element arranged in a matrix.

When a scanning signal is applied to the gate line and the thin film transistor is turned on, if a predetermined signal voltage is applied to the signal line, the signal voltage is applied to the pixel electrode through the source electrode, semiconductor layer, and drain electrode, and the amount of light transmitted through the liquid crystal changes. can be controlled.

However, in the conventional liquid crystal display device, between the picture element electrode and the liquid crystal, there is an alignment film 11 that controls the initial alignment of the liquid crystal, or a film other than the electrode protective layer, which is not shown in FIG. It was not provided to prevent a drop in the effective voltage applied to the liquid crystal.

Problems to be Solved by the Invention As mentioned above, when a TN liquid crystal is used as the liquid crystal, the liquid crystal alone (assuming that an alignment film that controls the initial alignment of the liquid crystal is included) will not be able to handle the problems shown in FIG. As can be seen from the relationship between the applied voltage and the transmittance shown by the dotted line, the transmittance rises steeply with respect to the voltage, and the operating voltage range is approximately 2V to 3V. In this operating voltage range, if you try to control the transmittance of, for example, 20 gradations, the permissible voltage range per gradation is 1v.
/20 is approximately somv. However, considering the characteristics of current thin film transistors, this condition is extremely strict due to the low absolute value of off-resistance, variation in on-current for each element, voltage drop due to pass-through capacitance between gate and drain, etc. It was difficult to control the gradation.

This is because, as shown in Figure 2, there is only the alignment film 11 on the pixel electrode 4 (the thickness of the alignment film is usually about 1000, which is sufficiently thin), so the signal voltage itself is applied almost as is to the liquid crystal. It was for the purpose of being

The present invention has been made in view of this point, and an object of the present invention is to provide a liquid crystal display device capable of displaying multiple levels by improving a relatively simple structure.

Means for Solving the Problems In order to solve the above problems, the present invention provides at least one voltage adjustment layer made of an insulator between the liquid crystal and the picture element electrodes in addition to the alignment film. be.

According to the above-described structure, the present invention consumes a part of the applied voltage in the voltage adjustment layer made of an insulator, intentionally reduces the effective voltage applied to the liquid crystal, and thereby changes the applied voltage and transmittance of the liquid crystal display device. The relationship □ shows smoother monotonic characteristics than in the case where no voltage adjustment layer is provided, and the operating voltage range is expanded. Therefore, the permissible voltage range per gradation increases.

This facilitates multi-gradation display.

Embodiment FIG. 1 is an embodiment of the liquid crystal display device of the present invention, and is a cross-sectional view of one pixel of the substrate on the side containing the thin film transistor, of the two substrates that sandwich the liquid crystal. 1 is a transparent substrate, 2 is a transparent electrode, 3 is an insulating layer, 4 is a picture element electrode, and 2, 3.4 form an auxiliary capacitor. This auxiliary capacitor is not directly related to the essence of the present invention, and there is no problem even if it is absent. At that time, the transparent electrode 2 and the insulating layer 3 will be excluded from FIG. 1. 20 indicates a liquid crystal.

As mentioned in the conventional technology section, thin film transistors are
A gate electrode connected to a gate line corresponding to a scanning line 6.
Gate insulating layer 6. Semiconductor layer 7. Consisting of a source electrode 8 connected to a source line corresponding to a signal line, and a drain electrode 9 connected to a picture element electrode 4, it is provided for each picture element arranged in a matrix, and functions as a switching element. .

A liquid crystal is placed between the substrate on which picture elements mainly consisting of thin film transistors and picture element electrodes are arranged in a matrix, and another transparent substrate 21 on which a transparent electrode 22 is formed on the entire surface, via an alignment film 11. 2o is held.

The voltage adjustment layer 1o, which is the key point of the present invention, is provided interposed between the picture element electrode 4 and the liquid crystal 2o.

As an example of the voltage adjustment layer 10, the present inventors used a two-layer structure consisting of a 5102 film of 2000 layers and a SiNx film of 4400 layers formed by the CVN method. The reason why this voltage adjustment layer 1o is formed by vapor deposition of an inorganic film is because of the ease of controlling the film thickness and its good uniformity, but it goes without saying that the forming method and material are not particularly limited. As the voltage adjustment layer 10, a film made of at least one insulator may be formed to have a desired thickness in consideration of the relationship with the drive voltage.

As the alignment film 11, PIX-540 manufactured by Hitachi Chemical Co., Ltd.
0 was formed by coating with a 1000 person spinner. Orientation film 11
Although it can also be formed by printing, etc., the film thickness can be controlled more easily than when it is formed by vapor deposition.

It tends to have uniformity problems and is unsuitable for use as a voltage adjustment layer.

In the liquid crystal display device having the above structure, the total capacitance Ct of the picture element including the alignment film is expressed as follows.

Here, C□ is the capacity of the alignment film, and CLo is the capacity of the liquid crystal.

C8 is the capacitance of the voltage adjustment layer. Therefore, the ratio of the liquid crystal applied effective voltage vmuc to the externally applied voltage V is obtained from equation (1) as follows. However, in general, the capacitance C of a capacitor is determined by: Here, ε is the dielectric constant of the film, ε.

is the permittivity of vacuum, S is the electrode area, and d is the film thickness.

As an example, the present inventors prototyped a liquid crystal display device under the following manufacturing conditions.

Based on the above production conditions, from equations (2) and (3), for the case where there is a voltage adjustment layer 1o, the effective voltage applied to the liquid crystal Vt,
The calculation result of the ratio of C to the externally applied voltage is shown by the solid line in FIG. It can be seen that the effective voltage applied to the liquid crystal is reduced by about 20% compared to the case where the voltage adjustment layer 1° is not provided under the same conditions as shown by the dotted line in the figure. Therefore, as shown in FIG. 6, when there is a voltage adjustment layer 1o, the change in transmittance with respect to the applied voltage is much more gradual than when there is no voltage adjustment layer 10, and the operating voltage range is also about 2 to 4 V. and 2
It has expanded about twice as much. The permissible voltage range for one gradation is also widened, which makes it very easy to control the gradation, making it possible to absorb, for example, variations in characteristics and poor performance of thin film transistors. Therefore, it is possible to sufficiently cope with images that require considerable gradation, such as television images.

Further, the present invention is even more effective when applied to a negative display liquid crystal display device. This is because, as can be seen from Figures 3 and 4, the dielectric constant of liquid crystal increases as the applied voltage increases, so as the externally applied voltage increases, the externally applied voltage is consumed in layers other than the liquid crystal. Therefore, the effective voltage applied to the liquid crystal decreases. Therefore, in the case of positive display, voltage loss in areas other than the liquid crystal increases as the voltage increases, and the amount of change in transmittance decreases even as the voltage increases, so the amount of light transmitted when voltage is applied is minimized. It has been extremely difficult to reduce the black level and increase the contrast ratio without significantly increasing the applied voltage.

In the case of negative display, the black level corresponds to when no voltage is applied, so by optimizing the thickness of the liquid crystal layer in the initial optical design, it is possible to minimize the amount of light transmitted, and the contrast ratio is also sufficient. It can be secured.

The liquid crystal display device to which the present invention is applied may be color or black and white, and the liquid crystal material used is of course not limited to TN liquid crystal with Δε>0.

Effects of the Invention As described above, according to the present invention, by providing a voltage adjustment layer made of at least one insulator between the picture element electrode and the liquid crystal, the voltage per gradation can be reduced. This widens the tolerance range and makes it easier to control multi-gradation display of a liquid crystal display device.

Further, the present invention is even more effective when applied to a negative display type liquid crystal display device, and a liquid crystal display device capable of displaying multiple gradations and having high contrast can be realized.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a main part of a liquid crystal display device according to an embodiment of the present invention;
Figure 2 is a sectional view of the main part of a conventional liquid crystal display device, Figure 3 is a diagram showing the voltage dependence of the dielectric constant of liquid crystal, and Figure 4 is a diagram showing the voltage dependence of the ratio of the effective voltage applied to the liquid crystal to the externally applied voltage. The figure shown in FIG. 5 shows the voltage dependence of the light transmittance of liquid crystal. Name of agent: Patent attorney Toshio Nakao and one other person
Dimensions, Fig. 3 ■Additionally, 7r knee CV No. 41!

Claims (2)

[Claims] (1) A plurality of gate lines and a plurality of source lines are provided on one main surface of a first transparent substrate, and an area defined by the gate lines and source lines corresponds to a picture element, and each picture element is mainly composed of a thin film transistor and a picture element electrode electrically connected thereto, a transparent electrode is formed on one main surface of a second transparent substrate, and a liquid crystal is disposed between the first and second transparent substrates. A liquid crystal display device, characterized in that the voltage adjusting layer made of at least one insulator is interposed between the picture element electrode of the first transparent substrate and the liquid crystal. (2) The liquid crystal display device according to claim 1, wherein the liquid crystal is a TN liquid crystal with Δε>0 and is of a negative display type.