JP3255992B2 - Display method of active matrix display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of displaying an image in a liquid crystal display device using a thin film transistor (hereinafter referred to as a TFT) as a driving switching element, and in particular, to a gradation display method for obtaining an intermediate color tone or shade. It is about. The present invention particularly relates to a so-called perfect digital gradation display that performs gradation display without externally applying any analog signal to an active element, and a liquid crystal material used is a strong liquid crystal material having excellent high-speed response. A display method for a liquid crystal display device, which is limited to a dielectric liquid crystal or an antiferroelectric liquid crystal, or a so-called polymer liquid crystal (also referred to as a dispersion liquid crystal) in which they are dispersed in a polymer compound (polymer). About.

[0002]

2. Description of the Related Art A liquid crystal composition changes its light transmission amount and refractive index depending on an external electric field. By using this property, an electric signal can be converted into an optical signal and a display can be performed. . As liquid crystal materials, TN (twinstead nematic) liquid crystal, STN (super
(Twinstead nematic) liquid crystal, ferroelectric or antiferroelectric liquid crystal, and more recently, nematic liquid crystal or polymer liquid crystal in which ferroelectric or antiferroelectric liquid crystal is dispersed in a polymer material (also called dispersion type liquid crystal). Is known. It is known that a liquid crystal does not respond to an external voltage in an infinitely short time, but takes a certain time to respond. The value is specific to each liquid crystal material.
msec, several hundred msec for STN liquid crystal, several tens μsec for ferroelectric liquid crystal, and several tens μsec for dispersion or polymer liquid crystal using nematic liquid crystal.
It is 0 msec.

[0003] Among electro-optical devices using liquid crystals, the one that can obtain the best image quality is the one using the active matrix system. In a conventional active matrix type liquid crystal electro-optical device, a thin film transistor (TFT) is used as an active element, an amorphous or polycrystalline semiconductor is used for the TFT, and P
In this case, a TFT of only one of the N-type and the N-type was used. That is, in general, an N-channel TFT
(Referred to as NTFT) is connected in series to the pixel. Then, a signal voltage is applied to the signal lines of the matrix, and when signals are applied from both sides to the TFTs provided at the orthogonal portions of the respective signal lines, the ON / OFF state of the liquid crystal pixels is utilized by utilizing the fact that the TFTs are turned ON. OFF was individually controlled. By controlling the pixels by such a method, a liquid crystal electro-optical device having a high contrast can be realized.

[0004]

However, in the conventional active matrix method, light and darkness and color tone are reduced.
It was extremely difficult to perform gradation display. Conventionally, for gray scale display, a method has been studied which utilizes the fact that the light transmittance of a liquid crystal changes depending on the magnitude of an applied voltage. This is because, for example, an appropriate voltage is supplied from a peripheral circuit between the source and drain of the TFT in the matrix, and a signal voltage is applied to the liquid crystal pixel by applying a signal voltage to the gate electrode in that state. Was to try.

However, in such a method, for example, due to the inhomogeneity of the TFT and the inhomogeneity of the matrix wiring, the voltage actually applied to the liquid crystal pixels differs by at least several% depending on each pixel. Oops. On the other hand, the voltage dependence of the light transmittance of the liquid crystal is extremely non-linear, and the light transmittance changes rapidly at a specific voltage. Therefore, even if the pixel voltage differs by several%, the light transmittance is low. In some cases it was significantly different. Therefore, it has been a limit to achieve 16 gradations in the conventional analog gradation display method. For example, in a TN liquid crystal material, a so-called transition region in which light transmissivity changes has a width of only 1.2 V and has a width of 1 V.
In order to achieve six gradations, it is necessary to be able to control a voltage as small as 75 mV, so that the production yield has been significantly reduced.

[0006] As described above, the difficulty of gradation display is extremely disadvantageous in that the liquid crystal display device competes with a conventional general display device such as a cathode ray tube (CRT). In contrast, the present inventors have found that gradation can be visually obtained by controlling the time during which voltage is applied to the liquid crystal. The details are disclosed in Japanese Patent Application No. 3-169306.

For example, when a TN (twisted nematic) liquid crystal, which is a typical liquid crystal material, is used,
The brightness can be changed by applying various pulse waveforms to the liquid crystal pixels as shown in FIG. That is, the brightness can be increased stepwise in the order of "1", "2",... "15" in FIG.
Display of six gradations is possible. For example, in FIG.
At "1", a pulse of one unit length is applied. In the case of "2", a pulse having a length of 2 units is applied.
At “3”, one unit pulse and two unit pulses are applied, and as a result, a pulse having a length of three units is applied.
At “4”, a pulse having a length of 4 units is applied.
At "5", one unit pulse and four unit pulses are applied, and at "6", two unit pulses and four unit pulses are applied. Further, by providing a pulse of 8 units in length, a pulse of 15 units in length can be obtained as a result.

That is, by appropriately combining four kinds of pulses of 1 unit, 2 units, 4 units, and 8 units, it is possible to display 2 ⁴ = 16 gradations. In addition, 1
By preparing as many pulses as 6 units, 32 units, 64 units, and 128 units,
Advanced gray scale display of 32 gray scales, 64 gray scales, 128 gray scales, and 256 gray scales becomes possible. For example, in order to obtain 256 gradation display, eight types of pulses may be prepared.

Further, in the example of FIG. 1 (A), the duration of the voltage applied to the pixel, the first T _1, the following is 2T _1, so that the next increases geometric progression manner and so 4T ₁ an example is shown arranged in which, for example, as shown in FIG. 1 (B), the first T _1, then 8T _1, the following 2T _1, finally may be 4T _1. Satisfying the geometric progression in this way
By arranging the data in the display device, the load on the device transmitting data to the display device can be reduced.

However, when the TN liquid crystal is used, as a result, the applied voltage is required to have the same accuracy as that of the conventional analog gradation display system. That is, when 5 V is applied to the pixel as an ON voltage and “10” in FIG.
The display looks darker by about 2% than when the same “10” is displayed by applying a voltage of 1V. That is, in such a digital gray scale display system, it is required that there is no variation in TFT as in the conventional analog gray scale display system.

FIG. 3A is a circuit diagram of a typical TFT active matrix. A scanning signal (V
_G ), and the change in the potential V ₁ of the liquid crystal pixel when the data signal (V _D ) is applied is shown in FIG.

[0012] Factors leading to variations in V ₁ was there some, but a large potential drop that occurs when the scanning signal is cut by the parasitic capacitance between the gate electrode and the pixel electrode side wiring of the TFT ([Delta] V), the TFT leakage a voltage drop due to current and liquid crystal leakage current, TFT driving capability is not sufficient if (low mobility) are for impossible is fully charged during the time the scanning signal persists t ₁ This is the variation of the attained voltage.

Since these fluctuations are greatly affected by the characteristics of the TFT, if the variation of the TFT is large, the brightness of the pixel is greatly different. For example, if the parasitic capacitances of the gate electrode and the pixel electrode side wiring are different, ΔV is different, and if the magnitude of the leak current of the TFT is different, the falling speed of the pixel voltage is also different. Low mobility TF such as amorphous silicon TFT (a-Si TFT)
At T, charging variation is also a problem. For these reasons, even when applying the same signal, the pixel potential V ₁ was also ones represented by the solid line in FIG. 3 (B), also obtained dotted like. Of course, such variations are not desirable.

In order to solve such a problem, an auxiliary capacitor is usually provided in parallel with the liquid crystal pixel as shown in FIG. This is usually several times to several tens times the pixel capacity, but naturally, the burden on the TFT increases. Further, since usually only N-type or P-type TFTs are used, the obtained signal waveform is also not good in positive and negative symmetry, which causes deterioration of the liquid crystal.

[0015]

As pointed out above, the need to precisely control the pixel potential is due to the fact that the TN liquid crystal changes the light transmittance according to the effective value voltage.
The same applies to the STN liquid crystal or the dispersion type liquid crystal using the nematic liquid crystal which is the basic material. On the other hand, a ferroelectric liquid crystal or an antiferroelectric liquid crystal shows a very high-speed response, and does not substantially respond to an effective value voltage. Therefore, it has been clarified that, when the above-described digital gradation display is performed, uniform gradation display can be performed even with a TFT having a slight variation. That is, in a ferroelectric liquid crystal or an antiferroelectric liquid crystal, O
This is because when the voltage is continuously applied for 1 msec or more as the N voltage, the same light transmittance is exhibited at 5 V and 5.1 V. Ferroelectric liquid crystal (phenylpyrimidine-based)
FIG. 5 shows an example in which the contrast is controlled by changing the duration of the pulse by using, and gradation display is performed. For example, when a TN liquid crystal is used, such a linear gradation display cannot be obtained even when gradation display is performed by a method in which the pulse width is similarly changed. A similar effect was observed in a material in which a ferroelectric liquid crystal or an antiferroelectric liquid crystal was dispersed in a polymer.

In order to carry out the present invention, for example, a matrix circuit using thin film transistors as shown in FIG. 4 may be assembled. The circuit shown in FIG. 4 is the same as the circuit used in a conventional active matrix display device using a TFT. However, no auxiliary capacitance is provided for the purpose of reducing the load on the TFT.

Since the ferroelectric liquid crystal itself has a memory property, an auxiliary capacitance (capacity inserted in parallel with the pixel capacitance) as required for the conventional active matrix.
Therefore, it is possible to maintain the ON state even when the pixel electrode is discharged. However, such a material having strong ferroelectricity (or antiferroelectricity) tends to cause a so-called "burn" phenomenon and has a lack of reliability. On the other hand, a material having low ferroelectricity (or antiferroelectricity) has few defects relating to display such as "burn", but the pixel capacitance is severely discharged, and a voltage sufficient to maintain display cannot be maintained. In the case of ON state (or OF
F state). Therefore, a storage capacitor is required as in the related art.

Of course, if the discharge of the pixel is sufficiently small, the auxiliary capacity may not be provided. In particular, the presence of an excessive auxiliary capacity takes time for charging or discharging operation,
It is not desirable in practicing the present invention. In order to reduce the discharge of the pixel, for example, it is necessary to sufficiently increase the OFF resistance of the thin film transistor to reduce the leak current and to sufficiently increase the resistance between the electrodes of the pixel itself such as liquid crystal. Especially for the latter purpose, it is effective to cover the pixel electrode with an insulating material such as tantalum oxide or aluminum oxide such as silicon nitride or silicon oxide.

In such a circuit, the ON / OFF of the voltage applied to the pixel is controlled by controlling the gate voltage and the source-drain voltage of each thin film transistor.
It is possible to control OFF. In this example, the matrix is 640 × 480 dots, but for the sake of simplicity, only the vicinity of n rows and m columns is shown. If you expand the same thing up, down, left and right, you will get a complete one. FIG. 2 shows an operation example using this circuit. Here, the operation is shown focusing on only one pixel.

The signal line X _n (scanning line) is connected to the gate electrode of each TFT. And as shown in FIG.
A rectangular pulse signal is applied. On the other hand, the signal line Y _m
The (data line) is connected to the source (or drain electrode) of each TFT, to which a pulse train indicating either a positive or negative state is applied. In a 480 row matrix, this pulse train has one unit of time T
_One item contains 480 pieces of information. In the present invention,
One frame is composed of a plurality of subframes (five in the example of FIG. 2).
It is characterized by being composed of Further, in the example of FIG. 2, each subframe has a different duration. Also
The present invention relates to an active matrix having a thin film transistor.
In the display method of the box type display device, one frame
Are N (N is a natural number of 2 or more) having different durations from each other.
Into N subframes, and the N subframes
Therefore, the absolute value of the voltage applied to the pixel capacitance is constant.
Ri, the shortest duration among the duration was T ₀
At this time, the duration of the N subframes is T ₀ , 2
T ₀ ,... 2 ^N-1 T ₀ and the geometric progression
Array without filling , and display ^2N gradations.
Sign. Further, in the invention, it is preferable that the pixel capacitance be the thin film transistor
Connected to a transistor, and the pixel capacitor is made of a dielectric material.
Liquid crystal material, antiferroelectric liquid crystal material, ferroelectric
Disperse electro-conductive or anti-ferroelectric liquid crystal in polymer
It is characterized by using a material that has been subjected to the above.

In the following, it is assumed that the potential of the counter substrate is constant at 0 for the sake of simplicity. As shown, when V _G was first applied, V _D was positive, so
The pixel potential _VLC becomes positive. At this time, the potential drops by ΔV as described with reference to FIG. 3, and then the potential _VLC of the pixel gradually approaches 0 due to spontaneous discharge. However, paying attention to the transmittance _TLC of the pixel, the transmittance _TLC is kept constant despite the drop in the pixel potential _VLC . If the liquid crystal material has a large spontaneous polarization, the pixel potential V _LC
Descent is not a problem. However, small spontaneous polarization, it is necessary to set the other parameters (V _G, V _D) to tolerate a reduction in pixel potential V _LC in the case of using the memory of the poor liquid crystal material.

As a design point, V _G and V _D may be set based on the TFT having the worst characteristic. For example, V _D is set so that the potential V _LC after the longest subframe duration 16T _{0 in the} case shown in FIG. 2 is +3 V or more, preferably +5 V or more in the pixel having the worst charge retention characteristics. Then, set the appropriate V _G for driving the left V _D.

In FIG. 2, the potential V
_{Although the LC} drop is written to be the same, it must be noted that the longer the subframe duration is, the larger the potential drop is.

[0024] The first pulse V _G is the time from when the applied T
After _0, the second pulse V _G is applied. Since the data signal V _D was also positive in this case, the pixel potential V _LC remains positive. However, a new charge is injected and the potential increases again. The transmittance _{TLC of the} pixel does not change.

Next, after the time 16T ₀ , the third pulse V
_{When G} is applied, the data signal V _D was negative, the pixel potential V _LC is inverted to negative. Then, the transmittance also varies. However, this transition is relatively gentle, and if the applied voltage is 8 V or less, a time of about 50 μsec is required. In contrast, the width of the pulse V _G 30
although μsec or less, since the optical response transition of the liquid crystal is not the pulse V _G, is performed by the pixel potential V _LC, there is no problem at all.

[0026] Then, after a time 2T _0, is applied third pulses V _G, since the data signal V _D at the time was negative, the state of the pixel does not change. More time 8T after _0, 4 th pulse V _G is applied, since the data signal V _D at that time was positive, becomes positive again pixel potential V _LC, also changes the transmittance T _LC of the pixel. Finally, time 4T ₀
Later, with the first pulse V _G of the next frame is applied, one frame is completed. By appropriately combining these five sub-frames, a display of 32 gradations is possible. By the above operation, 1 + 16 +
4 = 21 [gray levels] were obtained.

In the above operation, it is important to determine the optimum minimum time unit T ₀ . As already mentioned,
The optical response time of a ferroelectric (or antiferroelectric) liquid crystal depends on the applied voltage. If the voltage is about 8 V as described above, the response time is 50 μsec. Generally, the optical response time is inversely proportional to the applied voltage. By the way, one frame is 100 msec or less, preferably 30 msec, for the purpose of display characteristics of a moving image and prevention of flicker.
Must be: For example, one frame is 30 ms
c, the maximum gradation frequency is 50 μs from 30 msec.
The limit is 600 gradations divided by sec.
In order for the optical response to be completely performed, several times of the above optical response time is required, and therefore, the number of gradations of about 100 is limited. Such restrictions can be improved by increasing the magnitude of the voltage (or electric field) applied to the liquid crystal and shortening the optical response time, but it is necessary that the withstand voltage of the TFT be improved accordingly.

[Embodiment] In this embodiment, one frame is composed of five subframes as shown in FIG.
This is a digital gradation display of two gradations. The duration of each subframe is 1 ms for the first subframe.
c, the second subframe is 16 msec, the third subframe is 2 msec, the fourth subframe is 8 msec, the fifth
The subframe is 4 ms and one frame is 31 ms
ec.

Crystalline silicon TF formed on quartz substrate
In the active matrix circuit configuration using T, as shown in FIG. 4, no auxiliary capacitance was provided. The TFT used was a single-gate PMOS because the leak current was small and ON / OFF was large. Typically, the leak current is 1 pA or less (gate voltage +15 V, drain voltage −10 V) or less, and the ON / OFF ratio is 7.5 digits or more (gate voltage −15 V / + 15 V, drain voltage −10 V).
V).

The size of the pixel is 20 μm × 60 μm,
The size of the matrix was 1920 × 480. When the charge retention characteristics of each pixel were examined, the data signal was-
The worst voltage when 16 V was applied when 10 V was applied was about -5.5 V.

The width of the scanning signal pulse applied to the matrix was 2 μsec, the pulse height was -15 V, and the data signal was ± 10 V. The liquid crystal material used was a phenylpyrimidine ferroelectric liquid crystal, and the thickness of the liquid crystal cell was 2.5 μm. The active matrix substrate was used as one substrate of the cell, and as the other substrate, a substrate in which an ITO film was formed on the entire surface and a silicon oxide film for preventing short circuit was formed thereon was used. When the voltage applied to the liquid crystal was 5 V or less, it was confirmed that a domain structure was generated in the liquid crystal. Since such a domain structure deteriorates characteristics in performing digital gradation display, it is desired to increase the applied voltage so that a domain does not occur. With the above liquid crystal display device, a display with a maximum contrast ratio of 180 and 32 gradations could be obtained.

[0032]

According to the present invention, in contrast to the conventional analog gradation display using a nematic liquid crystal, a digital gradation display using a ferroelectric or antiferroelectric liquid crystal material or a polymer liquid crystal material thereof is used. It is characterized by performing. Whereas the conventional gray scale display method, whether analog or digital, has limited TFT characteristics due to variations, the present invention consequently sets each parameter according to the worst pixel characteristics. By optimizing, an advanced gradation display can be achieved. As a result, according to the present invention, a very fine gradation display can be performed even with a panel which is conventionally determined to be defective, and the yield can be substantially improved, and the cost can be reduced substantially. .

For example, 256,0 of 640 × 400 dots
When an analog gradation display is performed using a normal nematic liquid crystal on a liquid crystal display device in which 00 TFTs are formed in a 100 mm square, variation in TFT characteristics is about ± 10%.
%, The 16-gradation display was the limit. However, when the digital gradation display according to the present invention is performed, the display is not easily affected by the variation in the characteristics of the TFT elements, so that the display of 64 gradations or more is possible, and the color display has a variety of 16,777,216 colors. Display of subtle colors has been realized.

[Brief description of the drawings]

FIG. 1 shows an example of a driving waveform according to the present invention.

FIG. 2 shows an example of a driving waveform according to the present invention.

FIG. 3 shows an example of a conventional active matrix circuit and its driving waveform.

FIG. 4 shows an example of a matrix configuration according to the invention.

FIG. 5 shows an example of gray scale display according to the present invention. (Vertical axis: pulse duration, horizontal axis: contrast ratio)

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-107381 (JP, A) JP-A-61-69036 (JP, A)

Claims (2)

(57) [Claims] 1. A display method for an active matrix type display device having thin film transistors, comprising: dividing one frame into N (N is a natural number of 2 or more) subframes having different durations; in the frame, the absolute value of the voltage applied to the picture element capacitance is constant, when the shortest duration among the durations and T _0, the duration of N sub-frames, T _0, A display method for a display device, wherein the display is of any one of 2T ₀ ,... 2 ^N-1 T ₀ , and is arranged without satisfying a geometric progression, and 2 ^N gradations are displayed. Wherein Oite to claim 1, before Symbol pixel capacitance is connected to said <br/> TFT, the pixel capacitance induced
A display method for a display device, comprising using a ferroelectric liquid crystal material, an antiferroelectric liquid crystal material, a ferroelectric liquid crystal material, or a material obtained by dispersing an antiferroelectric liquid crystal material in a polymer as an electrically conductive material. .