JPH0580354A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide a liquid crystal display device in which the fluctuation of impressed voltage caused by the dielectric anisotropy of liquid crystal material is eliminated, which is excellent in gradation control and where flicker or the sticking of an image does not occur. CONSTITUTION:In this liquid crystal display device where plural image signal wirings 4 and plural scanning signal wirings 5 crossing with the plural image signal wirings 4 are provided, and a picture element electrode 8 and a thin film transistor 7 for impressing voltage on the electrode 8 are provided in a matrix state at an intersection between the wirings 4 and 5, and the liquid crystal material held between the electrode 8 and a counter electrode 6 provided to be opposed to the electrodes 8 is driven by alternating voltage; the picture element electrode 8 is coupled to the adjacent scanning signal wiring 5 in terms of capacity and the capacity for holding charges is separately provided from the capacity of the liquid crystal material between the electrodes 8 and 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device,
In particular, it relates to an active matrix type liquid crystal display device using a thin film transistor as a switching element.

[0002]

2. Description of the Related Art In recent years, active matrix type liquid crystal display devices have begun to be used in small TVs, projection TVs, viewfinders, etc., but flicker, image burn-in after fixed image display, uniformity of display screen, gradation. In terms of display performance or price, it still falls short of CRT display devices.

A conventional active matrix type liquid crystal display device will be described below with reference to FIGS. 5 and 6.

FIG. 5 is a diagram showing an example of the configuration of a conventional active matrix type liquid crystal display device, and FIG. 6 is an equivalent circuit per pixel in FIG. 5 and 6
In FIG. 13, 13 is an image signal driving circuit, 14 is a scanning signal driving circuit, 15 is an image signal wiring, 16 is a scanning signal wiring, 1
Reference numeral 7 is a counter electrode wiring, 18 is a thin film transistor, 19 is a pixel electrode, 20 is a capacitance C _LC of liquid crystal material between each pixel electrode 19 and the counter electrode 17, and 21 is a parasitic capacitance C _GD between the gate and drain of the thin film transistor 18. ..

In the active matrix type liquid crystal display device shown in FIG. 5, a plurality of image signal wirings 15 and a plurality of scanning signal wirings 16 are arranged orthogonally to each other, and pixel electrodes are provided at respective intersections of the image signal wirings 15 and the scanning signal wirings 16. The thin film transistors 18 for applying a voltage to the pixel electrodes 19 and the pixel electrodes 19 are formed in a matrix form, and are supplied from the image signal driving circuit 13 by controlling ON / OFF of the thin film transistors 18 by a scanning signal supplied from the scanning signal driving circuit 14. An image is displayed by applying an image signal to the liquid crystal material held between each pixel electrode 19 and the counter electrode 17.

FIG. 7 shows waveforms applied to the pixel shown in FIG. V _G is a scanning signal applied to the scanning signal line 16, and this scanning signal V _G is composed of a voltage V _GH for turning on the thin film transistor 18 and a voltage V _GL for turning off the thin film transistor 18. V _S is the image signal applied to the image signal lines 15, the image signal voltage V _GS is, V _S ⁺ and V _S ^- polarity is reversed in each scanning period (1H) to. V _B is a voltage applied to the liquid crystal material between the pixel electrode 19 and the counter electrode 17.

The operation of the active matrix type liquid crystal display device configured as described above will be described with reference to FIGS.
It will be explained based on. For example, when the scanning signal V _GH is applied to the scanning signal wiring 16 while the positive image signal voltage V _S ⁺ is applied to the image signal wiring 15, the thin film transistor 18 is turned on and the image signal voltage is applied to the liquid crystal material. V _S ⁺
Is applied. Next, the scan signal V _{GL is applied} to the scan signal wiring 16.
Is applied, the thin film transistor 18 is turned off. As a result, the gate-drain capacitance C _{GD of} the thin film transistor 18 reduces the voltage V _B applied to the liquid crystal material by ΔV. The voltage V _B applied to the liquid crystal material is held by the capacitance C _{LC of the} liquid crystal material itself until the next period of the scanning signal V _G. Then, in the next cycle, the image signal voltage V _S is inverted and the image signal voltage V _S ⁻ is changed to the image signal wiring 15.
Applied to the scanning signal line 16 to the scanning signal V _GH.
Is applied, the image signal voltage of V _S ⁻ is applied to the liquid crystal material, and when the scanning signal V _GL is applied to the scanning signal wiring 16 next, the applied voltage V _B to the liquid crystal material decreases by ΔV. ,
This voltage is held. Therefore, the voltage V _B applied to the liquid crystal material has its polarity periodically inverted as shown in FIG.
When the scanning signal voltage V _G changes from V _GH to V _GL , the potential applied to the pixel electrode 19 changes due to the parasitic capacitance C _GD between the gate and drain of the thin film transistor 18, and is applied to the liquid crystal material. Voltage V _B varies. The fluctuation ΔV of the voltage V _B applied to the liquid crystal material is represented by the following formula.

ΔV = C _GD / (C _LC + C _GD ) · (V _GH −V _GL ) In order to correct the fluctuation ΔV of the voltage V _B applied to this liquid crystal material, the voltage applied to the counter electrode 17 is applied to the liquid crystal material. It is preset to the V _BC value which is the center value of the voltage V _B applied to the liquid crystal material, and is adjusted so that the positive voltage and the negative voltage of the voltage applied to the liquid crystal material are targeted. That is, V _BC = S
Adjust so that _SC- ΔV holds. The V _SC value of the voltage V _B applied to the liquid crystal material is the center value of the image signal V _S. However, as described above, the voltage applied to the counter electrode 17 is V _BC which is the central value of the voltage V _B applied to the liquid crystal material.
Even if the value is preset, the effective application of the DC voltage component to the liquid crystal material caused by the fluctuation ΔV is compensated by the dielectric anisotropy of the liquid crystal material (the property that the dielectric constant of the liquid crystal material changes with the applied voltage). Therefore, there is a problem that image ghosting occurs immediately after displaying a flicker or a fixed image.

In order to solve such a problem, for example, in Japanese Unexamined Patent Publication No. 64-91185, the polarity of the scanning signal applied to the thin film transistor 18 is opposite to that of the scanning signal wiring of the next stage for one thin film transistor 18. It has been proposed to reduce the DC voltage component to zero by adding the scanning signal of. Also, in Japanese Patent Laid-Open No. 64-26822, when a part of a pixel electrode driven by a thin film transistor is capacitively coupled with a scanning signal line in the next stage, the thin film transistor in the waveform of a scanning signal for driving this thin film transistor is turned off. to changes V ₁ of the gate voltage, by applying a V ₁ + V ₂ becomes the gate voltage varies in the opposite direction this V ₁ through the capacitive coupling, it is proposed to reduce the DC voltage component. Further, in Japanese Patent Application Laid-Open No. 2-913, a wiring connected to a pixel electrode via a storage capacitor is separately provided, and a first modulation signal is applied to this wiring and a second modulation signal is applied to a counter electrode. However, it has been proposed that the potential difference between the wiring connected to the pixel electrode and the counter electrode is modulated to improve the display image quality / driving reliability and reduce the driving power.

[0010]

However, in the above-mentioned structure, the modulation circuit having the same number of output terminals as the scanning signal wirings is required in addition to the scanning signal driving circuit and the image signal driving circuit. The standard becomes larger. Further, the scanning signal becomes very complicated, and the scale of the scanning signal driving circuit becomes large. Further, the dielectric anisotropy of the liquid crystal material causes the amplitude of the voltage applied to the liquid crystal material to fluctuate, which makes it difficult to control the gradation of the liquid crystal panel. Furthermore, since a large capacitance that also serves to retain charges is connected to the scanning signal wiring, the total load capacitance per scanning signal wiring becomes large, the waveform of the scanning signal is deformed, and a uniform display screen is displayed. There were various problems such as not being able to obtain it.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art. It has a relatively simple structure and eliminates fluctuations in applied voltage caused by the dielectric anisotropy of liquid crystal materials. An active matrix type liquid crystal display device having good gradation controllability and free from flicker and image sticking.

[0012]

In order to achieve the above object, the liquid crystal display device of the present invention is provided with a plurality of image signal wirings and a plurality of scanning signal wirings intersecting the plurality of image signal wirings. Pixel electrodes and thin film transistors for applying a voltage to the pixel electrodes are provided in a matrix at each intersection of the image signal wirings and the scanning signal wirings, and the pixel electrodes and the pixel electrodes are opposed to each other. In a liquid crystal display device in which a liquid crystal material held between electrodes is driven by an alternating voltage, the pixel electrodes are capacitively coupled to an adjacent scanning signal wiring, and charge retention is performed between the pixel electrodes and a counter electrode. The capacity for the liquid crystal was provided separately from the capacity of the liquid crystal material.

[0013]

According to the above structure, the capacitance C _S between the scanning signal wiring and the pixel electrode is generated by the signal of the adjacent scanning signal wiring.
It is possible to correct the voltage fluctuation of the pixel electrode via the signal line, and to invert the polarity of the counter electrode in synchronization with the polarity of the pixel signal to reduce the image signal amplitude. Therefore, the coupling capacitance provided between the pixel electrode and the scanning signal line adjacent to the pixel electrode is relatively small compared to the combined capacitance of the liquid crystal material and the charge storage capacitance. The added capacitance can be reduced, and as a result, there is no deformation of the scanning signal voltage, the DC voltage component of the voltage applied to the liquid crystal material is removed, and there is no image burn-in and no variation in display brightness. A display device can be realized.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of an active matrix type liquid crystal display device in a first embodiment of the present invention, FIG. 2 is an equivalent circuit diagram for one pixel in FIG. 1, 1 is a counter electrode signal drive circuit, 2 Is an image signal driving circuit, 3 is a scanning signal driving circuit, 4 is an image signal wiring, 5 is a scanning signal wiring, 6
Counter electrode wiring 7 TFT, 8 pixel electrode, liquid crystal material between the pixel electrode 8 and the counter electrode 6 is capacitance C _LC, 10 is storage capacitor C _T for charge retention, 11 pixel electrodes 8 and the next stage 9 The additional capacitance C _S between the scanning signal lines 5 and 12 is the parasitic capacitance C _GD between the gate and drain of the thin film transistor 7.
Is. FIG. 3 shows the structure of an active matrix type liquid crystal display device according to the present invention. FIG. 3B is a sectional view taken along the line aa ′ of FIG. In FIG. 3, 5 is a scanning signal wiring, 5'is a scanning signal wiring of the next stage, 4 is an image signal wiring,
Reference numeral 25 is a storage capacitor wiring connected to the counter electrode wiring 6, 7 is a thin film transistor, 27 is a source of the thin film transistor 7, 28 is a drain of the thin film transistor 7, 8 is a pixel electrode, 30 is a portion for forming the storage capacitor 10, 31 is an additional capacitance. 11
Forming part, 32 forming part of the parasitic capacitance C _GD between the gate and drain of the thin film transistor 7, 33 glass substrate, 6 counter electrode, 35 liquid crystal layer, 11 pixel electrode, 37 insulating layer, 38 glass It is a substrate.

FIG. 4 shows the time change of the voltage applied to each terminal of FIG. The scanning signal V _{G (n)} is a waveform applied to the n-th scanning signal wiring 5, and is V ₁ , V 2 as shown in FIG.
_{It is} composed of ternary levels of ₂ and V ₃ , and has a repetitive waveform with a constant cycle. The scanning signal V _{G (n + 1)} is a waveform applied to the _{(n + 1)} th scanning signal line 5 and has a phase delayed by a certain period (1H _{) from} V _{G (n)} . The image signal V _{S (m)} is m
With the waveform applied to the th image signal wiring 4, the polarity is inverted every 1H. V _S ⁺ is a positive voltage, V _S ⁻ is a negative voltage, and V _SC is a central voltage of the image signal V _{S (m)} . The signal V _T applied to the counter electrode 6 is the image signal V _T.
The polarity is inverted every 1H in synchronization with the polarity inversion of _{S (m)} .
V _T ⁺ is a positive voltage, V _T ⁻ is a negative voltage, V _TC
Is the central value of the counter electrode signal V _T. V _B is the pixel electrode 8
Is an effective voltage signal (voltage actually applied to the liquid crystal material) between the counter electrode 6 and the counter electrode 6, and V _BC is a central value.

The operation of the active matrix type liquid crystal display device constructed as above will be described below. First, at time t ₁ , when the image signal V _{S (m)} becomes the positive image signal voltage V _S ⁺ and the voltage of the scanning signal V _{G (n)} becomes V ₁ , the thin film transistor 7 turns on and the pixel electrode 8 The image signal voltage V _S ⁺ is applied to the liquid crystal material, and the difference (V _S ⁺ −V _T ⁻ ) between the voltage applied to the pixel electrode 8 and the voltage applied to the counter electrode 6 is applied to the liquid crystal material. Next, at time t ₂ , when the voltage of the scanning signal V _{G (n)} changes from V ₁ to V ₂ , the parasitic capacitance C _GD between the source and drain of the TFT is _generated.
The voltage V _B applied to the liquid crystal material via
To cancel this, the voltage of the scanning signal V _{G (n + 1)} of the next stage changes from V ₃ to V ₂ via the additional capacitance C _S , and this voltage is applied to the liquid crystal material. By setting so as to satisfy the values of these signals _{(V 1 -V 2) · C} GD = (V 2 -V 3) · C S becomes relation from resulting in time t ₃ to time t ₄ between, the liquid crystal material, V _S ⁺ -V
_T ^- voltage is to be applied.

Next, at time t ₄ , the image signal V _{S (m)}
There negative image signal voltages V _S ^- becomes, the scanning signal V _{G (n)}
Becomes V ₁ and the thin film transistor 7 is turned on, and the image signal voltage V _S ⁻ is applied to the pixel electrode 8, thereby applying the voltage V _S ⁻ −V _T ^{+ to} the liquid crystal material. Next, at the time t ₅ , the voltage V _B applied to the liquid crystal material decreases as at the time t ₂ , but this is canceled for the same reason as described above, and as a result, from the time t ₆ to the time t _7. between the liquid crystal material V _S ^- will be -V _T ⁺ voltage is applied. Here, by making the center voltage V _SC of the image signal V _{S (m)} equal to the center voltage V _TC of the counter electrode signal V _T , the center voltage V _BC of the voltage V _B applied to the liquid crystal material is changed to that of the liquid crystal material. It can be set to almost 0 V regardless of the dielectric anisotropy.

That is, the source of the thin film transistor 7
The amount of charge applied to the pixel electrode 8 when the voltage of the scanning signal V _{G (n)} changes from V ₁ to V ₂ via the drain-to-drain parasitic capacitance C _GD is (V ₁ −V ₂ ) · C. _GD and additional capacity C
_When the scanning signal V _{G (n + 1) of the} next stage via _S changes in voltage from V ₃ to V ₂ , the amount of charge applied to the pixel electrode 8 is (V
₃ -V ₂₎ · from a C _S, the sum of both the amount of charge voltage change of the pixel electrode 8 reached 0 becomes zero. Therefore,
Each voltage is (V ₁ −V ₂ ) · C _GD = (V ₃ −V ₂ ) · C
Set to be _S. Since this formula does not include the capacitance C _LC of the liquid crystal material, regardless of the capacitance of the liquid crystal material,
The voltage V _BC applied to the liquid crystal material can be 0V.

It is desirable that the parasitic capacitance C _GD and the load capacitance C _{S of the} thin film transistor satisfy the relation C _GD ≤C _S. That is, the value of (V ₁ -V ₂ ) is determined by the on / off characteristics of the thin film transistor and is usually 20 V or more. Where (V ₁ −V ₂ ) · C _GD = (V ₃ −V ₂ ) · C
From _S, if the C _GD> C _S, a _{(V 3 -V 2) (V} 1 -
V ₂ ), which requires a high voltage of 40 V or more as the amplitude (V ₁ −V ₃ ) of the scanning signal, resulting in an increase in power consumption of the scanning signal drive circuit 3 and an increase in cost. Cause. Therefore, the parasitic capacitance C _GD and the load capacitance C _{S of} the thin film transistor 7 are C _GD ≦ C
It is desirable to satisfy the relationship _S.

Further, the parasitic capacitance C _GD between the gate and drain of the thin film transistor 7, the capacitance C _S of the capacitive coupling between the pixel electrode 8 and the adjacent scanning signal wiring 5, and the charge retention between the pixel electrode 8 and the counter electrode 6 It is desirable that the capacitance C _T for liquid crystal and the capacitance C _{LC of the} liquid crystal material satisfy the relationship of (C _LC + C _T ) ≧ (C _GD + C _S ). That is, from time t ₃ to time t ₄ in FIG. 4, when the voltage fluctuation of the pixel electrode 8 due to the amplitude V _T 'of the inverted signal V _T applied to the counter electrode 6 is V _PT , V _T ' and V _T ' The relationship of _PT is V _PT = (C _LC +
It is represented by C _T ) / (C _GD + C _S + C _LC + C _T ) · V _T. Here, when the value of C _GD + C _{S is} originally sufficiently smaller than C _LC + C _T , V _PT ≈V _T between time t ₃ and time t ₄ in FIG. Since a waveform having substantially the same amplitude as the inverted signal to be applied is applied to the pixel electrode 8 as well, a voltage of approximately V _S ⁺ −V _T ⁻ is applied to the liquid crystal material. However, when the value of C _GD + C _S cannot be ignored compared to C _LC + C _T , V _PT <V _T , and the amplitude of the waveform applied to the pixel electrode 8 is lower than the amplitude of the counter electrode signal. would be, in its result the liquid crystal material, V _S ⁺ -V _T ^-
A voltage significantly lower than that is applied, and a normal image cannot be obtained. This is the same between time t ₆ and time t ₇ in FIG. Therefore, the parasitic capacitance C _GD between the gate and drain of the thin film transistor 7, the capacitance C _S of the capacitive coupling between the pixel electrode 8 and the adjacent scanning signal line 5, and the capacitance for charge retention between the pixel electrode 8 and the counter electrode 6 It is desirable that C _T and the capacitance C _{LC of the} liquid crystal material satisfy the relationship of (C _LC + C _T ) ≧ (C _GD + C _S ).

In the above embodiment, the additional capacitance C _S is provided between the pixel electrode 8 and the next scanning signal line V _{G (n + 1)} .
The additional capacitance C _S is equal to the pixel electrode 8 and the scanning signal line V of the preceding stage.
_It may be provided between _{G (n-1)} . In this case, the driving waveform input to the gate of the thin film transistor 7 through the scanning signal wiring V _{G (n-1)} is the lowest level of the three levels immediately after the timing when the thin film transistor 7 is in the ON state. To come.

[0022]

As described above, according to the liquid crystal display device of the present invention, in addition to the capacitance of the liquid crystal material, the capacitance for accumulating charge is provided in parallel with the capacitance of the liquid crystal material. The coupling capacitance provided between the signal wirings is relatively small compared to the combined capacitance of the liquid crystal material and the charge storage capacitance, so that the total additional capacitance formed in the scanning signal wiring can be reduced, and as a result In addition, the scanning signal voltage is not deformed, the DC voltage component of the voltage applied to the liquid crystal material is removed, and it is possible to realize a liquid crystal display device having a good gradation display property without image burn-in and display brightness variation.

Further, by inverting the voltage applied to the counter electrode every 1H in synchronization with the pixel signal, the voltage of the image signal can be significantly reduced, and the withstand voltage and consumption of the image signal drive circuit can be reduced. The current can be reduced, and the image signal drive circuit LSI can be downsized and reduced in price.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a liquid crystal display device according to the present invention.

FIG. 2 is a diagram showing an equivalent circuit of one pixel of the liquid crystal display device according to the present invention.

3A is a diagram showing a structure of an active matrix type liquid crystal display device according to the present invention, and FIG. 3B is a diagram showing FIG.
2 is a cross-sectional view taken along the line aa ′ of FIG.

4 is a diagram showing a waveform of each terminal shown in FIG. 2 and a waveform of a voltage applied to a liquid crystal material.

FIG. 5 is a diagram showing a configuration of a conventional liquid crystal display device.

FIG. 6 is a diagram showing an equivalent circuit of one pixel of a conventional liquid crystal display device.

7 is a diagram showing the waveform of each terminal shown in FIG. 6 and the waveform of the voltage applied to the liquid crystal material.

[Explanation of symbols]

1 ... Counter electrode signal drive circuit, 2, 13 ... Video signal drive circuit, 3, 14 ... Scan signal drive circuit, 4, 1
5 ... Image signal wiring, 5, 16 ... Scan signal wiring,
6, 17 ... Counter electrode wiring, 7, 18 ... Thin film transistor, 8, 19 ... Pixel electrode, 9, 20 ... Liquid crystal material capacitance C _LC , 10 ... Charge holding storage capacitance C _T, 11 ... load capacitance C _S, the 12, 21 ... TFT parasitic capacitance C _GD.

Claims (3)

[Claims] 1. A plurality of image signal wirings and a plurality of scanning signal wirings intersecting the plurality of image signal wirings are provided, and a pixel electrode is provided at each intersection of the image signal wirings and the scanning signal wirings. In a liquid crystal display device in which a thin film transistor for applying a voltage to an electrode is provided in a matrix, and a liquid crystal material held between the pixel electrode and a counter electrode provided to face the pixel electrode is driven by an alternating voltage, A liquid crystal display characterized in that the pixel electrode is capacitively coupled to an adjacent scanning signal line, and a charge holding capacitor is provided between the pixel electrode and a counter electrode separately from the capacitor of the liquid crystal material. apparatus. 2. The parasitic capacitance C _GD between the gate and drain of the thin film transistor and the capacitance C _S of the capacitive coupling between the pixel electrode and the adjacent scanning signal wiring satisfy the relation C _GD ≦ C _S. The liquid crystal display device according to claim 1. 3. A parasitic capacitance C _GD between the gate and drain of the thin film transistor, a capacitance C _S for capacitive coupling between the pixel electrode and an adjacent scanning signal line, and a capacitance C for holding a charge between the pixel electrode and a counter electrode. _T , the capacitance C _{LC of the} liquid crystal material is
The liquid crystal display device according to claim 1, wherein a relationship of (C _LC + C _T ) ≧ (C _GD + C _S ) is satisfied.