JPH07325287A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain a liquid crystal display device having excellent uniform display characteristics with small change in the transmission of the panel even when an area having different liquid crystal layer width is present in the panel. CONSTITUTION:The material, film thickness, and dielectric const. of each layer or the size and shape of elements which constitute the liquid crystal pixel are changed. For example, by decreasing the size of a pixel electrode formed on an auxiliary capacitor wire 13, the ratio of the auxiliary Cs to the whole capacitance Ct which constitutes the unit pixel is selected to satisfy 0<Cs/Ct<=0.2. Thereby, changes in the transmittance of the panel can be decreased to about <=2%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pixel structure of a liquid crystal display device.

[0002]

2. Description of the Related Art Heretofore, as one of driving methods of a liquid crystal display device for displaying characters and images using liquid crystal, an active matrix method having a switching element for each display unit is known. FIG. 4 shows a pixel configuration of a conventional liquid crystal display device of this system. 4A is a plan view thereof, FIG. 4B is a sectional view taken along line AA in FIG. 4A, and FIG. 4C is a sectional view taken along line BB in FIG.

A scanning signal wiring 12 and an auxiliary capacitance wiring 13 are formed on an insulating substrate 11, and a switching element 15 mainly composed of a semiconductor and an image signal wiring 16 are sandwiched by an insulating film 14 on these wirings 12 and 13. And the pixel electrode 17 is formed. Further, a counter electrode 19 is formed on these with the liquid crystal layer 18 interposed therebetween. An electrically equivalent circuit per pixel of this liquid crystal display device is shown in FIG. In FIG.
Cgd is a parasitic capacitance, which is generated between the drain terminal of the switching element 15 and the scanning signal line 12. Cl is a pixel capacitance, which is a capacitance between the pixel electrode 17 and the counter electrode 19. Cs is an auxiliary capacitance, which is a capacitance between the pixel electrode 17 and the auxiliary capacitance line 13. Further, T is a switching element 15.

In this pixel, the scanning signal Vg is applied to the scanning signal line 12, the capacitance signal Va is applied to the auxiliary capacitance line 13, the image signal Vs is applied to the pixel signal line 16, and the counter signal Vc is applied to the counter electrode 19. The liquid crystal pixel voltage applied to the liquid crystal layer 18 between the pixel electrode 17 and the counter electrode 19 is defined as Vlc.
FIG. 6 shows a time change of a signal waveform in a driving method (hereinafter referred to as “capacitive coupling driving method”) disclosed in Japanese Patent Laid-Open No. 02-157815, which is one of the methods for driving the liquid crystal display device. . As the scanning signal Vg, the potential Von for turning on the switching element and the switching element O
Four potentials of potential Voff for FF and compensation potentials Vga and Vgb are used. The compensation potentials Vga and Vgb are applied alternately for each auxiliary capacitance line. In the case of displaying a halftone, the signal potential and the liquid crystal pixel voltage Vlc applied to the pixel are calculated by using the above potentials by using (Equation 1), (Equation 2) and (Equation 3).

[0005]

[Equation 1]

[0006]

[Equation 2]

[0007]

[Equation 3]

A ratio between the screen capacitance Ct, which is a denominator in the above (Equation 2) and (Equation 3), and the auxiliary capacitance Cs is defined as an auxiliary capacitance ratio A. This auxiliary capacity ratio A is expressed by (Equation 4).

[0009]

[Equation 4]

However, Cs is an auxiliary capacitance, Cl is a pixel capacitance, and Cgd is a parasitic capacitance generated between the drain terminal of the switching element and the scanning signal wiring. In the conventional liquid crystal display device, the auxiliary capacitance ratio A has a value between 0.6 and 0.65.

[0011]

The panel transmittance of a liquid crystal display device is calculated by (Equation 5).

[0012]

[Equation 5]

Here, Tpanel is the panel transmittance of the liquid crystal display device, Tmat is the transmittance of the panel material constituting the liquid crystal display device, and Tvol is the pixel transmittance determined by the driving conditions. The pixel transmittance Tvol is determined by the voltage Vlc applied to the liquid crystal layer and the liquid crystal layer width Dlc, and the liquid crystal layer width Dlc is 5.0.
In the case of μm, the pixel transmittance Tvol can be approximated by (Equation 6).

[0014]

[Equation 6]

Liquid crystal pixel voltage V approximated by (Equation 6)
FIG. 7 shows the relationship between lc and the pixel transmittance Tvol. Further, when the liquid crystal layer width Dlc changes, the pixel transmittance is determined by the electric field applied to the liquid crystal layer, and therefore the pixel transmittance Tvol in halftone display is calculated by (Equation 7).

[0016]

[Equation 7]

When a single color display is performed on a liquid crystal display device of this type, the same signal voltage is applied to all terminals for each scanning signal wiring or pixel signal wiring, but the liquid crystal layer width Dlc is different within the same panel. If a portion exists, the Tvol and Tpanel values in (Equation 5) are different in that portion. Since the human eyes perceive that the panel transmittance Tpanel is different by 2% or more, it is recognized as unevenness, and therefore it is necessary to realize a liquid crystal display device in which the panel transmittance Tpanel does not change even if the liquid crystal layer width Dlc changes.

An object of the present invention is to provide a uniform display in the active matrix type liquid crystal display device using the above capacitive coupling drive method, in which the change in the panel transmittance is small even when there are portions having different liquid crystal layer widths in the panel. An object of the present invention is to provide a liquid crystal display device having excellent characteristics.

[0019]

A liquid crystal display device according to claim 1 sets a ratio Cs / Ct of an auxiliary capacitance Cs and a total capacitance Ct constituting a unit pixel to 0 <Cs / Ct ≦ 0.2. It is characterized by having done. A liquid crystal display device according to a second aspect is the liquid crystal display device according to the first aspect, in which the auxiliary capacitance line has an electrical configuration shared with the scanning signal line, and a modulation signal is applied to the scanning signal line by superimposing it on the scanning signal. I am trying.

[0020]

According to the configuration of the present invention, the ratio Cs / Ct between the auxiliary capacitance Cs and the total capacitance Ct forming the unit pixel is 0 <
By setting Cs / Ct ≦ 0.2, even when there is a portion where the liquid crystal layer width is different in the panel, the change in the panel transmittance, which is the product of the pixel transmittance and the transmittance of the panel material, is about 2 % Or less, and a liquid crystal display device having excellent uniform display characteristics can be realized.

[0021]

EXAMPLE A liquid crystal display device according to an example of the present invention will be described below. This embodiment is similar to the conventional liquid crystal display device shown in FIG. 4, that is, a pixel electrode 17 arranged in a matrix, a counter electrode 19 opposed to the pixel electrode 17 with a liquid crystal layer 18 in between, and a pixel. An auxiliary capacitance line 13 that forms an auxiliary capacitance Cs with the electrode 17 and a switching element 15 that is connected to the image signal line 16 and the scanning signal line 12 and switches the pixel electrode 17 are provided. An image signal voltage with the polarity of the signal voltage inverted for each field of the display screen is applied to the pixel electrode 17 during the ON period, and a reverse modulation signal is applied to the auxiliary capacitance line 13 for each field during the OFF period of the switching element 15. By applying the voltage, the potential of the pixel electrode 17 is changed, and the change of the potential and the pixel signal voltage are mutually superposed and / or cancelled, so that the voltage is applied to the liquid crystal layer 18. A liquid crystal display device of an active matrix system using the driving method of applying the (capacitive coupling driving method).

The feature of this embodiment is that the auxiliary capacitance Cs is
And the total capacitance Ct forming the unit pixel, that is, the auxiliary capacitance ratio Cs / Ct is set to 0 <Cs / Ct ≦ 0.2, which will be described in detail below. As in the conventional example, the auxiliary capacitance ratio Cs / Ct is set to A. In the conventional liquid crystal display device, a value between 0.6 and 0.65 is adopted as the auxiliary capacity ratio A as described above. Now, as a method of changing the auxiliary capacity ratio A, the following (1 ) And (2).

(1) Insulating film 14, switching element 1
5. The material, film thickness and dielectric constant of the liquid crystal layer 18 are changed. (2) The size and shape of the switching element 15, the auxiliary capacitance line 13, and the pixel electrode 17 are changed. In this embodiment, as an example, a case where the size or shape of the pixel electrode 17 is changed will be described. FIG. 1A is a plan view showing a case where the pixel electrode 17 formed on the auxiliary capacitance line 13 is made small to reduce the auxiliary capacitance ratio A value, and FIG. 1B is made large. FIG. 9 is a plan view showing a case where the auxiliary capacitance ratio A value is increased as a result. In addition, FIG.
The broken line in (b) is a conventional pixel electrode and corresponds to FIG. Further, FIG. 2 shows the liquid crystal layer width Dlc dependence of the panel transmittance Tpanel calculated using the above-described formula in the conventional example.
In this calculation, the pixel transmittance Tvol is 50 when the liquid crystal layer width Dlc is 5.0 μm for each auxiliary capacitance ratio A value.
The compensating potentials Vga and Vgb were set so as to be%. The panel material transmittance Tmat value was set to 1 regardless of the liquid crystal layer width Dlc.

As shown in FIG. 2, when the auxiliary capacitance ratio A is reduced, the change of the panel transmittance Tpanel with respect to the liquid crystal layer width Dlc is reduced, and when the auxiliary capacitance ratio A is set to 0 <A ≦ 0.2, It can be seen that even if the liquid crystal layer width Dlc changes by 10%, the change in the panel transmittance Tpanel can be reduced to about 2% or less. Therefore, for example, as shown in FIG. 1A, the pixel electrode 17 formed on the auxiliary capacitance line 13 is made small and the auxiliary capacitance ratio A is set to 0 <A ≦ 0.2. Thus, even when there are portions having different liquid crystal layer widths, it is possible to realize a liquid crystal display device having excellent uniform display characteristics.

Even in the liquid crystal display device having a structure in which the auxiliary capacitance line and the preceding scanning signal line are shared, the uniform display characteristic is excellent by setting the above auxiliary capacitance ratio A to 0 <A ≦ 0.2. A liquid crystal display device can be realized.
Even when the panel material transmittance Tmat changes depending on the liquid crystal layer width Dlc, by selecting the optimum auxiliary capacitance ratio A value, even if the liquid crystal layer width Dlc changes by 10%, the pixel transmittance T is changed.
Since the change of the panel transmittance Tpanel, which is the product of vol, can be 2% or less, a liquid crystal display device having excellent uniform display characteristics can be realized.

In this embodiment, the auxiliary capacitance ratio A is set to 0 <A by changing the size and shape of the pixel electrode 17.
Although ≦ 0.2 is set, it goes without saying that other methods such as (1) and (2) described above may be used. For example,
FIG. 3 is a diagram corresponding to FIG. 4B, except that the electrode 20 is provided. That is, as shown in FIG. 3, when the electrode 20 is provided so as to face the pixel electrode 17 with the insulating film 14 in between, and the same potential as that of the counter electrode 19 is applied, the pixel capacitance Cl increases and the auxiliary capacitance ratio A decreases. can do.

[0027]

As described above, in the liquid crystal display device of the present invention, the ratio Cs / Ct of the auxiliary capacitance Cs and the total capacitance Ct forming the unit pixel is set to 0 <Cs / Ct ≦ 0.2. By doing so, even when there is a portion having a different liquid crystal layer width in the panel, the change in the panel transmittance, which is the product of the pixel transmittance and the transmittance of the panel material, can be reduced to about 2% or less, and uniform display characteristics can be obtained. It is possible to realize an excellent liquid crystal display device.

[Brief description of drawings]

FIG. 1 is a plan view showing an auxiliary capacitance line and a pixel electrode according to an embodiment of the present invention.

FIG. 2 is a diagram showing a liquid crystal layer width dependence of panel transmittance in the example.

FIG. 3 is a sectional view of a liquid crystal display device showing another embodiment for changing the auxiliary capacitance ratio.

FIG. 4 is a plan view and a cross-sectional view of a liquid crystal display device.

FIG. 5 is an electrical equivalent circuit diagram of the liquid crystal display device.

FIG. 6 is a diagram showing a time change of a signal waveform in the capacitive coupling drive method.

FIG. 7 is a diagram showing a relationship between liquid crystal pixel voltage and pixel transmittance.

[Explanation of symbols]

 13 auxiliary capacitance wiring 17 pixel electrode

Claims (2)

[Claims] 1. A pixel electrode arranged in a matrix, and a counter electrode facing the pixel electrode with a liquid crystal layer interposed therebetween.
An auxiliary capacitance line that forms an auxiliary capacitance Cs between the pixel electrode and the pixel electrode, and a switching element that is connected to the image signal line and the scanning signal line and switches the pixel electrode are provided. By applying an image signal voltage in which the polarity of the signal voltage is inverted for each field to the pixel electrode and applying a reverse modulation signal for each field to the auxiliary capacitance line during the OFF period of the switching element, the pixel electrode An active matrix liquid crystal display device using a driving method of applying a voltage to the liquid crystal layer by varying the potential of the pixel signal voltage and / or superposing the variation of the potential and the pixel signal voltage, The auxiliary capacitance Cs and the total capacitance Ct forming the unit pixel
(Ct = Cs + Cl + Cgd: where Cs is auxiliary capacity,
Cl is a pixel capacitance, and Cgd is a ratio Cs / Ct between a drain terminal of the switching element and a parasitic capacitance generated between the scanning signal line), and is set to 0 <Cs / Ct ≦ 0.2. Liquid crystal display device. 2. The liquid crystal display device according to claim 1, wherein the auxiliary capacitance line has an electrical configuration shared with the scanning signal line, and the modulation signal is applied to the scanning signal line by superimposing it on the scanning signal.