JP2001215468A - Liquid crystal element and driving method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems such as variation and shift in a threshold when constituting amplifier type pixel circuits. SOLUTION: Each pixel circuit is constituted of 1st and 2nd switch elements 103a and 103b for supplying a pair of information signals Vdata-a and Vdata-b corresponding to one pixel according to a scanning signal Vgate, 1st and 2nd information signal holding means 104a, 104b for holding a pair of the information signals supplied via these switch elements, a power supply line 108 for supplying a prescribed voltage, 1st and 2nd voltage supply means 105a, 105b for supplying the voltage of the power supply line as that of the value corresponding to each information signal held by the 1st and 2nd information signal holding means, and pixel electrodes 106a, 106b for applying the voltages supplied by these voltage supply means to both electrodes of the liquid crystal part corresponding to the one pixel and driving the liquid crystal part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal element used for a light valve used in a flat panel display, a projection display, a printer and the like, and a driving method thereof.

[0002]

2. Description of the Related Art The most widely used flat displays these days include, for example, "Applied Physics Letters" by M. Schadt and W. Helfrich.
Letters), Vol. 18, No. 4 (issued on Feb. 15, 1971, pp. 127-128), "twisted nemat".
ic) One using liquid crystal is known. In recent years, a liquid crystal element using this type of liquid crystal in combination with an active switch such as a TFT has been developed and commercialized. In this type, a transistor is formed for each pixel, there is no problem of crosstalk, and with the rapid progress of production technology in recent years, a display of 10 to 17 inches class can be produced with good productivity. It is being done. However, the response speed of the twisted nematic liquid crystal itself is the first problem in terms of the ability to express moving images sharply, and the main problem is that the viewing angle is narrow. One major problem is to solve these two problems simultaneously.

In view of such a background, recently, a liquid crystal element using a new mode has been developed. For example, the in-plane mode or the MVA alignment technique is a means for improving the viewing angle, and the OCB mode or the like is a means for improving the response speed.

On the other hand, liquid crystal devices having bistability include Clark and Lagerwell.
wall) (JP-A-56-107).
No. 216, U.S. Pat. No. 4,367,924, etc.)
There is a chiral smectic liquid crystal element. As the liquid crystal having this bistability, chiral smectic C is generally used.
A ferroelectric liquid crystal (hereinafter, also referred to as "FLC") comprising a phase or a chiral smectic H phase is used.
Since the ferroelectric liquid crystal performs inversion switching by spontaneous polarization, it has a very fast response speed, can exhibit a bistable state with memory properties, and has excellent viewing angle characteristics. It has been commercialized as a high-speed, high-definition, large-area simple matrix display element or light valve.

Recently, a chiral smectic antiferroelectric liquid crystal device (hereinafter, also referred to as "AFLC") having three stable states has been proposed by Chandani, Takezoe et al. (Japanese Journal of Applied Physics).
of Applied Physics), Volume 27,
1988, p. 1729). Recently, among these antiferroelectric liquid crystal materials, a V-shaped response characteristic having small hysteresis and advantageous characteristics for gradation display has been discovered (for example, Japanese Journal of Applied Physics).
Applied Physics, Vol. 36, 199
7 years, p. 3586). It has been proposed to realize a high-speed display by using this as an active matrix type liquid crystal element (Japanese Patent Application Laid-Open No. 9-50049).

Thus, the viewing angle is good, the response is fast,
In order to achieve the ultimate goal of a display capable of gradation, OCB and AFLC have been more actively researched and developed than ever before, with OCB and AFLC being promising candidates for high-speed liquid crystal display.

As the speed of the liquid crystal increases, a new color liquid crystal system has been proposed. In general, a conventional color liquid crystal display element has a configuration in which a liquid crystal and a color filter such as RGB provided in parallel on a substrate and a liquid crystal are laminated, and one pixel is composed of an RGB picture element whose transmittance can be independently controlled. RG
In general, the transmittance is controlled in combination with a liquid crystal portion or a polarizing plate for each of the B pixels, and colors are generally expressed by additive color mixture of RGB. As a light source, there is a transmission type using a white light backlight or a reflection type using external light, but the principle of expressing a color space is the same.

One of the drawbacks of such a color liquid crystal display device is that light utilization efficiency is poor. That is, of the light incident on the R filter having a spatially one-third area, the one-third R light as a wavelength region is similarly set to one-third ×
Since one-third = one-ninth G light and similarly one-ninth B light are additively mixed to represent white, use of light before the transmittance of the liquid crystal part is considered Efficiency is only one third at most. This means that the power consumption of the backlight, which accounts for much of the power consumption of the liquid crystal, is wastefully required.
In addition, three picture elements must be independently driven for each pixel, and the higher the definition, the more difficult the pixel design becomes, the lower the aperture ratio, and the more the light use efficiency is further reduced. Furthermore, the driving IC also has three lines (terminals) per pixel line.
In particular, the higher the definition, the heavier the load of mounting. Even from the viewpoint of cost, the configuration requires a driving IC or a color filter having a large number of bits, which is a factor of pressing down the cost of the liquid crystal panel, which is disadvantageous.

[0009] In recent years, based on recognition of such disadvantages, proposals and developments of other color liquid crystal display elements have been activated. Among them, the most actively researched and developed is a backlight color switching method. This method is disclosed in JP-A-56-27198. By switching the color of the illuminating light at a time equal to or lower than the flicker frequency and controlling the transmission state of the panel in synchronization with the switching, color reproduction is realized by temporal additive color mixing. It is also called an RGB field sequential display method (field sequential color method).

[0010]

A first object of the present invention relates to a voltage drop due to reversal of spontaneous polarization Ps of AFLC (or FLC). Figure 12 shows AFLC
2 shows an example of a voltage transmittance characteristic (VT characteristic) of the first embodiment. This VT
The characteristics are the results of measuring the transmittance by applying a voltage of a triangular wave, and the AFLC develops a change in the transmittance according to the VT curve, except during transient switching.
However, driving using an active switch such as a TFT causes a problem of voltage drop due to inversion of spontaneous polarization Ps.

Hereinafter, this problem will be described. FIG. 6 is a circuit diagram of a commonly used TFT array. In this circuit, when trying to sequentially drive N = 1000 gate lines 1 to N at a frame rate of 60 [Hz] in consideration of flicker, the gate-on time assigned to each gate line is approximately 16 [μSec]. ]. AF
Although the LC switches to the F (ferro) phase quickly, the response does not always end at 16 [μSec].
Switching to the AF (antiferro) phase is slow,
Usually, it is on the order of several hundred [μSec] to [mSec]. That is, when the AFLC is driven by using an active switch such as a TFT, the switch is turned on, a certain potential V ₀ is applied, and then the switch is turned off to perform AFLC (or FLC) switching.

This series of operations will be described with reference to FIGS. FIG. 15 shows a state before a liquid crystal having spontaneous polarization Ps [C / cm ² ] is put in a cell and aligned, and is uniformly reset to one ferro state. Spontaneous polarization Ps of liquid crystal
In the macro, the macro spontaneous polarization Ps is canceled by the neighboring spontaneous polarization Ps, and only + Ps and -Ps at both ends can be treated as being electrically present. The area of the electrodes 15 and 16 on both sides of the cell is S [c
m ² ] and the capacitance of the liquid crystal capacitor is C [F], the amount of charge due to spontaneous polarization existing near the electrodes 15 and 16 of this liquid crystal capacitor is ± Ps [C / cm ² ] *
S [cm ² ].

Next, as shown in FIG.
Is turned on to apply the potential V ₀ , but it is assumed that the switching of the liquid crystal has not started yet. At this time, the charge Q existing in the electrodes 15 and 16 itself or in the vicinity of the electrodes 15 and 16 is the sum of the charge q supplied from the power supply 18 through the switch 17 and the above Ps * S, that is, Q = q +
Ps * S. Since the potential is the integral of each charge divided by the capacitance of its location, q + Ps *
S = CV ₀ .

Next, the liquid crystal is switched by the applied electric field.
After finishing the pitching, the other
Suppose you have arrived. The state at this time is shown in FIG. electrode
The amount of charge due to spontaneous polarization existing in the vicinity is q-Ps * S
is there. The potential at this time is V₁Then, q-Ps *
S = CV₁It is. With the switching of the liquid crystal, the electrode 1
The potential between 5 and 16 is V₀To V₁Has changed to
You. If the potential change is ΔV, then ΔV = V₀-V₁=
[(Q + Ps * S)-(q-Ps * S)] / C = 2 * P
s * S / C. In standard AFLC, currently Ps
= 100 [nC / cm ^Two] About and retardation
The optimal liquid crystal cell gap considering the
The cell capacity including the relative dielectric constant is 2 [nF / cm^Two]
It is. That is, ΔV = 2 * 100 [nC / cm^Two]
/ 2 [nF / cm^Two] = 100V. ± 5
The liquid crystal having the VT characteristic of V is changed from the previous state of -5V to + 5V.
Do not apply 105V potential once to drive to the next state.
There is an almost unrealistic problem that must be
Will do. According to the drive IC's theory,
Voltage must be suppressed to about ± 10V
Absent. FIG. 13 shows an information signal according to the signal Gate.
data in the above-described series of operations when data is given.
FIG. 14 shows the liquid crystal in that case.
FIG. 4 is a diagram illustrating an image of the movement of a molecule 14.

To solve this problem, the following approaches can be considered. That is, the spontaneous polarization Ps of the liquid crystal itself is reduced, the speed of the liquid crystal is increased, the rate of switching within the on-time of the TFT is improved, and the capacity of the liquid crystal is increased, or the auxiliary capacitance is increased to effectively increase the liquid crystal capacity. Provided, and electric charges required for switching the liquid crystal are supplied from the TFT in several times.

However, according to the report, there is a general report that reduction of spontaneous polarization is in a trade-off relation with improvement in liquid crystal speed and hinders the development of the AF phase itself. Is not at a satisfactory level yet,
According to this, the current flowing through the TFT during the on-time becomes large, so that a TFT having a higher performance than, for example, a polysilicon TFT is required. In particular, a large-format direct-view type liquid crystal display is very disadvantageous in cost. I will. According to the above, considering that the magnitude of the potential to be applied, which is a problem, must be reduced to about one-tenth, an auxiliary capacitor that is nine times as large as the liquid crystal capacitor must be prepared. Unrealistic in nature. Also,
In order to charge the capacitor whose capacity load has increased tenfold in the on-time, the on-resistance of the TFT must be reduced to one-tenth, which also requires a TFT with high capability. I will. However, this method deviates from the purpose of realizing a display with a fast moving image and a fast response, or introduces a new problem that the frame frequency must be increased several times.

As described above, the above-mentioned problem has not been solved by any one of the approaches or the combination of the above-mentioned solutions.
The first object of the present invention is to solve this problem.

A second object of the present invention relates to hysteresis. According to the conventional method of driving a twisted nematic liquid crystal by using a TFT, the liquid crystal response itself is slow, so that an optical response is provided by applying electric charges by the TFT over several frames. However, if the response of the liquid crystal is accelerated to respond in one frame, the phenomenon that the gradation state of the next frame shifts depending on the state of the previous frame becomes apparent. This phenomenon is called hysteresis. Simple matrix drive, not TFT
Hysteresis appearing in the case of driving will be briefly described separately for a liquid crystal having spontaneous polarization and a nematic liquid crystal having dielectric anisotropy.

First, the case of a liquid crystal having spontaneous polarization will be described.
It is assumed that after the TFT is turned on and a potential is applied, the TFT is turned off and switching of the liquid crystal starts. In the FLC, a current due to the reversal of the spontaneous polarization Ps flows along with the switching. However, if the previous state is different, the reversal amount changes, so that the reversal current differs. The pixel capacitance including the auxiliary capacitance is Cpix
[F], the pixel area is S, and the effective inversion ratio of the spontaneous polarization Ps is α (where 0 ≦ α ≦ 1), and the amount of inversion charge is
2α * Ps [C / cm ² ] * S [cm ² ]. TFT
Is turned on, and the potential given is V ₀ , the potential after the end of the liquid crystal response is Vfinal = V ₀ -2α * Ps * S / C
pix. Since α depends on the previous state, the potential after the end of the switching depends on the previous state.
This is the hysteresis of the liquid crystal having spontaneous polarization.

Next, the case of a liquid crystal having dielectric anisotropy will be described. If the liquid crystal molecules have dielectric anisotropy, the capacitance of the liquid crystal capacitor will be different. Therefore, first, when the TFT is turned on and the pixel electrode is charged with the potential V ₀ , q = Cp
The charge of ixV ₀ accumulates. The TFT is turned off, the liquid crystal starts moving in response to a given voltage, while the charge q is stored, Cpix'Vfina from CpixV ₀
It changes to l. At this time, since the pixel capacitance Cpix at the time when the TFT is turned on and the potential V ₀ is applied is different depending on the previous gradation state, the accumulated charge is also different, and the potential settled in the next frame is also different.
This is the hysteresis of a liquid crystal having dielectric anisotropy. Even in the OCB mode, this hysteresis exists because the liquid crystal has dielectric anisotropy.

As described above, whether the liquid crystal is a nematic liquid crystal or a liquid crystal having spontaneous polarization, there is a problem of hysteresis, and the present invention has a second object to solve the problem.

A third problem relates to a field sequential color system. FIG. 7 shows a driving sequence when driving by the field sequential color system using the standard TFT array of one TFT / pixel shown in FIG. The ON signal Gat is sequentially applied to the gate lines 1 to N.
e (1) to e (N), and in synchronization therewith, Red (red; hereinafter, referred to as "R") information data is placed on the data lines 1 to M, and R information is written into each pixel. Go. At this time, the liquid crystal sequentially responds to each pixel by receiving an information voltage. When the response of the liquid crystal of each pixel with respect to the final gate line N is completed, the entire panel is in the display state based on the R information, so that the R backlight is turned on. When about 5.5 [mSec] has elapsed since the gate line 1 was turned on, R
The backlight is turned off, and the R subframe ends.

This sub-sequence is similarly converted to Green
(Green; hereinafter referred to as “G”) Subsequence, Blu
e (blue; hereinafter, referred to as “B”) subsequence is repeated, and one display field is constituted by these three subsequences. This display field is 1/60
If it is [Sec] or less, the human does not sense flicker,
Temporal additive color mixture is established. 5.5 [mSec] described above
Is 5.5 [mSec] = (1/3) * (1/60)
According to [Sec]. Examples of the backlight that can sequentially turn on RGB individually and responsively include an LED backlight, an organic EL backlight, an inorganic EL backlight, a fluorescent tube backlight having a three-tube RGB structure, and a device using an RGB color rotating plate as a white light source. is there.

The above is the basic field-sequential color system in one TFT system. In this system, a new problem which does not exist in the conventional color filter (hereinafter, referred to as "CF") system appears for the TFT array. . FIG. 18 shows a conventional CF as a driving sequence.
The method (FIG. 10A) and the field sequential color method (FIG. 10B) are shown in comparison. As can be seen from the figure, the time allotted to drive the N gate lines one way is about 1/6 in the field sequential color system as compared to the CF system. That is, in the case of the field sequential color system, for example, in the case of the G subfield, the gate lines 1 to N are sequentially turned on to re-apply the information data to each pixel. Is still displayed in the R subfield. For this reason, the backlight cannot be turned on while the gate lines are sequentially selected and rewritten. That is, the subfield time must be further divided into a writing period (gate selection period + liquid crystal response time) and a lighting period. If an attempt is made to secure the gate selection time as long as possible, the lighting time will not be sufficient, the light will be dark, and a dilemma will occur with the response time of the backlight. In FIG. 7, since the writing period and the lighting period are shown as 1: 1, the time for driving the gate line is reduced to 1/6. That is, the gate line write time is close to 1/3 at best, and is practically 1/4 to 1/6 or less.

The limitation on the gate-on time is a problem due to the delay of the gate wiring. The ON signal is distorted on the side far from the gate IC due to the gate wiring resistance and the parasitic capacitance, and information cannot be written to the pixel capacitor within a predetermined time. This parasitic capacitance has two major factors: the capacitance at the intersection of the gate line and the signal line and the Cgs capacitance of the TFT. Regarding the former, the number of signal lines in the field sequential color system is １ ／ of that in the CF system, so that the parasitic capacitance at the intersection is also １ ／. However, with regard to the latter, the capacity of the TFT is required to be several times that of the conventional one because the pixel area which has been divided into RGB in the CF method is tripled and the on-time of the TFT is shortened. , Thereby increasing Cgs. Therefore, for example, when a 17-inch class SXGA or the like is virtually designed by the field sequential color system, the design breaks down.

To solve this problem, Japanese Patent Application Laid-Open No. 08-0955 discloses a method for reducing the gate selection time to one third.
No. 26, a 2TFT transfer type has been proposed. FIG. 8 shows the panel equivalent circuit, and FIG. 9 shows the driving sequence. The gate lines 1 to N are sequentially turned on to supply R information data to the capacitor C1.
When the writing of data is completed, the gate signal Transfer
When r is turned on, information data is transferred to the liquid crystal capacitor _CLC in the entire panel. Then, on the back of turning on the R backlight and performing R display, writing of the next G field is performed on the capacitor C1. In this method, gate lines 1 to N
Can be sequentially turned on to turn on the backlight, and the writing period can be reduced to 1/3 of the CF system. However, since only share charge connects the capacitor C1 before the liquid crystal capacitor C _LC and the next sub-field information subfield information is given is given in TFT 82, the next gradation display state by the previous state Is affected and hysteresis occurs. Moreover, the potential remains in the capacitor C1 even after finishing transferred from capacitor C1, the use efficiency of the charge supplied from the data IC is poor, for example, a liquid crystal capacitor C _LC
If the capacitance ratio between the capacitor and the capacitor C1 is 1: 1, the output voltage of the data IC is required to be twice as large as that of the related art.

On the other hand, in order to solve the problem of hysteresis, there is a proposal as described in JP-A-09-288261 in which a reset TFT is further added to a liquid crystal capacitor. FIG. 10 shows an equivalent circuit of the panel, and FIG.
Shows a drive sequence. Immediately before the transfer of the information signal data, the reset TFT 11 is turned on over the entire panel by the signal Reset to erase the previous information. However, even this method does not solve the problem of requiring a high voltage for driving the data IC.

Japanese Patent Laid-Open Publication No. 09-80386 proposes an amplifier type 2TFT or 3TFT configuration and a driving method. According to this, the first problem, that is, the problem of driving a high-load liquid crystal can be solved, but there is no description about how to perform gradation control with an amplifier.

On the other hand, as means for realizing such a TFT element, a-Si, p-Si and the like are generally used. However, such devices have a threshold voltage (Vt
Problems of h) and the shift of the threshold voltage due to the DC bias have been pointed out. When the pixel circuit is realized by the amplifier method, the problem relating to the threshold voltage (hereinafter referred to as “Vth problem”) is a display defect such as a variation in gradation display in a plane and a burn-in due to a shift of the threshold value. And the display quality as a display is significantly reduced.

The present invention has been made in view of the above-mentioned prior art, and an object of the present invention is to provide a liquid crystal element and a method of driving the liquid crystal element, in which, first, a high-speed response, a viewing angle, and a gradation property are simultaneously improved. It is to satisfy. Secondly, a liquid crystal having a spontaneous polarization Ps = 100 [nC / cm ² ]
That is, it is driven by an FT array. Third, it is necessary to simultaneously satisfy accurate gradation reproduction without hysteresis and high-speed response. Fourth, the drive frequency of the TFT array when the field sequential color system is applied is reduced, and the time aperture ratio of the backlight is maximized. Further, the present invention aims at avoiding the Vth problem, which is important for solving these problems, thereby providing a liquid crystal display or the like having good display quality.

[0031]

In order to solve these problems, a first liquid crystal element according to the present invention comprises a first liquid crystal element for supplying a pair of information signals corresponding to one pixel in accordance with a scanning signal. Two switch elements, first and second information signal holding means for respectively holding the pair of information signals supplied through the switch elements, a power supply line for supplying a predetermined voltage, First and second voltage supply means for supplying the voltages of the lines as voltages having values corresponding to the information signals held by the first and second information signal holding means; A circuit of each pixel is constituted by a pixel electrode for driving the liquid crystal portion by applying a voltage supplied by a supply unit to both polarities of a liquid crystal portion corresponding to the one pixel. Active Ma It is a Rikusu type liquid crystal element of.

The second liquid crystal element is a first liquid crystal element, wherein each pixel circuit is a switch element for simultaneously supplying voltage to the pixel electrodes from the first and second voltage supply means to all pixels. It is characterized by having.

A third liquid crystal element is the first or second liquid crystal element, wherein the switch element is a thin film transistor.

A fourth liquid crystal element is the liquid crystal element according to any one of the first to third liquid crystal elements, wherein the liquid crystal is a liquid crystal which is switched by an electric field substantially parallel to a surface of a substrate sandwiching the liquid crystal. .

In a fifth liquid crystal element, in any one of the first to third liquid crystal elements, at least one of the liquid crystals is sealed between a pair of transparent substrates, and the pixel electrode of each pixel is connected to each substrate. It has two electrodes provided facing each other on the top,
Each of the pixel circuit portions other than the pixel electrodes is formed on one of the pair of substrates, and one of the two electrodes formed on the other substrate has a corresponding pixel circuit portion. It is characterized by being electrically connected.

A sixth liquid crystal element is the liquid crystal element according to any one of the first to third liquid crystal elements, wherein at least one of the liquid crystals is sealed between a pair of transparent substrates, and one of the liquid crystal elements is provided on one of the substrates except for the pixel electrodes. Are formed, and the pixel electrode of each pixel includes first to third electrodes,
And a second electrode is provided on the one substrate so as to be connected to the corresponding pixel circuit portion. The third electrode is electrically insulated on the other substrate and is provided on the first and second electrodes. The first to third electrodes are provided so as to face each other, thereby forming a capacitance in series with the liquid crystal layer in two stages.

According to a seventh liquid crystal element, in any one of the first to sixth liquid crystal elements, the liquid crystal has spontaneous polarization or dielectric anisotropy.

Further, in the first driving method of the liquid crystal element of the present invention, when driving any one of the first to seventh liquid crystal elements of the present invention, the information signal of each pair is transmitted every predetermined period. It is characterized in that the information signal is supplied while being inverted or exchanged within each pair.

A second liquid crystal element driving method is characterized in that, in the first liquid crystal element driving method, the predetermined period is one frame period.

A third method for driving a liquid crystal element is characterized in that, in the first method for driving a liquid crystal element, the predetermined period is one field period.

In a fourth driving method of the liquid crystal element, when driving the second liquid crystal element of the present invention, a scanning signal and an information signal are applied to the liquid crystal element, and the driving of the liquid crystal element is performed for all pixels of a predetermined frame. A writing scanning step of supplying an information signal to the first and second information signal holding means; and a step of writing all the pixels of the liquid crystal element based on the information signals of all the pixels supplied to the first and second information signal holding means. A pixel writing step of simultaneously supplying a voltage to each pixel electrode.

A fifth driving method of the liquid crystal element is the same as the driving method of the fourth liquid crystal element, except that in the period in which display is performed by all the pixels to which the voltage is supplied, the writing scanning step is performed for the next frame. Is performed.

In the sixth driving method of the liquid crystal element, when any one of the first to seventh liquid crystal elements of the present invention is driven, each scanning signal is applied to a liquid crystal portion driven based on the scanning signal. The information signal of each pair is set and applied so that the direction of the electric field is reversed.

The driving method of the seventh liquid crystal element is as follows.
In the method of driving a liquid crystal element, the information signal of each pair is set and applied so that the direction of the electric field applied to the liquid crystal portion driven based on the scanning signal is reversed for each scanning signal. It is characterized by the following.

The driving method of the eighth liquid crystal element is as follows.
In driving any one of the liquid crystal elements, the information signal of each pair is set and applied so that the direction of the electric field applied to the liquid crystal is reversed for each adjacent pixel.

The driving method of the ninth liquid crystal element is as follows.
The method of driving a liquid crystal element according to any one of the above, is characterized in that each pair of information signals is set and applied so that the direction of the electric field applied to the liquid crystal is reversed for each adjacent pixel.

The tenth liquid crystal element driving method is characterized in that, when any one of the first to seventh liquid crystal elements is driven, the color of the backlight is switched for each frame operation to perform color display. I do.

An eleventh method for driving a liquid crystal element is characterized in that, in any one of the first to ninth methods for driving a liquid crystal element, a color display is performed by switching a color of a backlight for each operation of each frame. I do.

The eighth liquid crystal element corresponds to the first liquid crystal element of the present invention.
Characterized in that it has a function of performing any one of the driving methods of (1) to (11).

In the configuration of the present invention, a pair of information signals are supplied as information signals, and a voltage having a value corresponding thereto is supplied to the pixel from the power supply line by the first and second voltage supply means. Is supplied to both polarities of the liquid crystal portion. Therefore, the difference between the voltages supplied to the two polarities is applied to the liquid crystal portion, and thereby the first and second voltages are applied.
Will be canceled out. That is, in the present invention, when a liquid crystal element is configured by an element that performs an amplifier operation, the variation of the threshold, which has been a problem in the related art, is reduced.
Two sets of so-called amplifier-type circuit configurations are provided for each pixel, and the output of the same circuit, which can be regarded as having the same characteristics, is canceled by inputting the output to both poles to apply an electric field to the liquid crystal. Further, by changing the driving conditions of the two sets of the amplifier-type pixel circuits at regular intervals, the threshold changes in the first and second voltage supply means are made durable to be the same, Thereby, the shift of the threshold value is also canceled.

[0051]

FIG. 1 shows an equivalent circuit (pixel circuit) of one pixel portion in a liquid crystal element according to one embodiment of the present invention.
As shown in the drawing, the circuit supplies a pair of information signals Vdata_a and Vdata_b corresponding to one pixel in accordance with a scanning signal Vgate.
a and 103b, and transistors 103a and 103
3b, holding capacitors 104a and 104b respectively holding information signals Vdata_a and Vdata_b, and a power supply line 1 for supplying a predetermined voltage Vdd.
08, the analog buffer circuits 105a and 105b that supply the voltage of the power supply line 108 as a voltage having a value corresponding to each information signal held by the holding capacitors 104a and 104b, and the transistors 111a and 111b. Are applied to both polarities of the liquid crystal portion 107 corresponding to the one pixel to drive the liquid crystal portion 107.
b.

The pixel selection transistors 103a and 103b are connected to a scanning line 101 for supplying a scanning signal Vgate to them, and are connected to signal lines 102a and 102b for supplying signals Vdata_a and Vdata_b for determining pixel display. 102b are respectively connected,
Further, a storage capacitor 104 for holding each signal voltage
a and 104b are connected. Storage capacity 104a
And 104b are input to analog buffer circuits 105a and 105b, respectively, and a voltage output corresponding to the input voltage is output to the pixel electrode 106b.
a and 106b. Voltages are supplied to the buffer circuits 105a and 105b from a power supply line 108 (voltage Vdd) and a power supply line 109 (voltage Vss). The buffer circuit 105a includes the transistor 1
The buffer circuit 105b includes transistors 110b and 111b. The liquid crystal layer 107 is connected to the outputs of the buffer circuits 105a and 105b via the pixel electrodes 106a and 106b.

In this configuration, the buffer circuit 105a
And 105b, which are the outputs of pixel electrodes 106a and 1
06b are supplied to the buffer circuit 105a
If the threshold values of Vth and 105b are Vth_a and Vth_b, they are given by the following equations.

[0054]

(Equation 1) On the other hand, since the two buffer circuits 105a and 105b in the same pixel are spatially formed very close to each other, their threshold values Vth_a and Vth_b are considered to be substantially equal,
The voltage VLc applied to the liquid crystal layer 107 is given by the following equation.

[0055]

(Equation 2) Therefore, the output voltage to the liquid crystal layer 107 is equal to the threshold Vth
Regardless of the values of _a and Vth_b, it is determined by the signals Vdata_a and Vdata_b supplied from the signal lines 102a and 102b. According to this, since a threshold does not appear in the substantial voltage applied to the liquid crystal layer 107, good display can be obtained on the entire panel even if the threshold of each element varies within the panel. At the same time, since the buffer circuits 105a and 105b are always operating, even if the liquid crystal layer 107 is a liquid crystal having spontaneous polarization (Ps) such as a ferroelectric liquid crystal,
The current accompanying the reversal of the spontaneous polarization is always the buffer circuit 105a.
And 105b from the liquid crystal layer 10
The potential difference between the two electrodes 106a and 106b sandwiching 7 is kept unchanged. The same applies to the case where a liquid crystal having dielectric anisotropy is used.

Note that the dynamic range may be slightly reduced or the linearity may be impaired by the substrate bias effect, but this can be dealt with only by adjusting the signals Vdata_a and Vdata_b accordingly.

FIG. 2 shows a drive pulse sequence for suitably driving the pixel circuit. In this sequence, the signals Vdata_a and Vdata_a
a_b is inverted. As described above, by changing the driving conditions for the two buffer circuits 105a and 105b for each field, the buffer circuit 105
transistors 111a and 11a in transistors a and 105b
The change of the threshold value in 1b becomes the same, so that it is possible to obtain a good display without changing the gradation even in a durable manner.

The cycle for exchanging the driving conditions is as follows.
Appropriately selected depending on the purpose. For example, when this is used in a field sequential color system or the like, it is possible to appropriately select every three fields or the like.

FIG. 3 shows a pixel circuit of a liquid crystal element according to another embodiment of the present invention. This pixel circuit is for transferring image signals held in all pixels to each pixel at once. Switches 212a and 212b are provided between liquid crystal layer 107 and buffer circuits 105a and 105b.
Are provided, and a wiring 213 for controlling these elements is provided.

In this configuration, after storing the image data for all the pixels, a timing pulse is applied to the wiring 213 to turn on the switches 212a and 212b, so that the image information is simultaneously transferred to the pixels in all the pixels. That is, in the embodiment of FIG. 1, the timing of sending an image signal to each pixel is the same as when each pixel is selected, whereas in the present embodiment, the timing is the same for all pixels.

As a method of applying an electric field to the liquid crystal layer 107, as shown in FIG.
A method of forming the conductive column 51 also serving as a spacer can be considered. As shown in FIG. 4A, the pixel circuit and the electrodes 106a and 106b connected to the pixel circuit are arranged on one of two substrates sandwiching the liquid crystal layer 107, and the pixel electrode 41 insulated from an adjacent pixel is arranged on the other. Are arranged, and as a result, as shown in FIGS. 4B and 4C, the liquid crystal layer 1 is formed.
07 are connected in series in two stages. Further, a so-called IPS mode in which the liquid crystal layer 107 is switched by an electric field parallel to the substrate may be used.
In that case, an electrode for applying an electric field to the pixel can be formed over the same substrate.

In each pixel, the direction of the applied electric field can be arbitrarily set by the signals Vdata_a and Vdata_b. Therefore, signals Vdata_a and Vdata_a are set so that the direction of the electric field applied to the pixels is reversed for each selected scanning line or for each adjacent pixel.
By setting a_b, the deterioration of the image quality due to a slight fluctuation of the power supply voltage or the like can be canceled by spatially averaging the images corresponding to the positive and negative voltages.

[0063]

As described above, according to the present invention,
Since two sets of amplifier circuits are provided for each pixel and the outputs are connected to pixel electrodes to apply an electric field to the liquid crystal, the difference between the outputs of each amplifier circuit is applied to the liquid crystal layer. Variations and shifts in threshold voltage, which are a problem when an amplifier-type pixel circuit is made of silicon or the like, can be substantially prevented from affecting display.

Therefore, the high-speed response, the viewing angle and the gradation are simultaneously satisfied, and the spontaneous polarization Ps = 100 [nC / c
m ² ] can be driven by a TFT array, which simultaneously satisfies accurate gradation reproduction without hysteresis and high-speed response, and lowers the driving frequency of the TFT array when a field sequential color system is applied. At the same time, the time aperture ratio of the backlight can be maximized.

[Brief description of the drawings]

FIG. 1 illustrates a liquid crystal device according to an embodiment of the present invention.
FIG. 3 is a circuit diagram illustrating an equivalent circuit of a pixel.

FIG. 2 is a diagram showing a drive sequence of the circuit of FIG.

FIG. 3 is a circuit diagram showing an equivalent circuit of one pixel in a liquid crystal element according to another embodiment of the present invention.

FIG. 4 is a schematic view of a means for applying an electric field to a liquid crystal layer in the circuits of FIGS. 1 and 3;

FIG. 5 is a schematic diagram of another means for applying an electric field to the liquid crystal layer in the circuits of FIGS. 1 and 3.

FIG. 6 is a circuit diagram showing a conventional panel equivalent circuit of one standard TFT / pixel.

FIG. 7 is a diagram showing a standard driving sequence for driving in a field sequential color system using a conventional standard 1 TFT / pixel panel.

FIG. 8 is a circuit diagram showing a panel equivalent circuit of a conventional transfer type 2TFT / pixel.

FIG. 9 is a diagram showing a standard drive sequence for driving in a field sequential color system using a conventional transfer type 2TFT / pixel panel.

FIG. 10 is a circuit diagram showing a panel equivalent circuit of a conventional transfer + reset type 3TFT / pixel.

FIG. 11 is a diagram showing a standard driving sequence for driving in a field sequential color system using a conventional transfer + reset type 3TFT / pixel panel.

FIG. 12 is a diagram illustrating an example of a voltage transmittance characteristic of an AFLC liquid crystal cell.

FIG. 13 shows A in a conventional standard 1TFT / pixel.
FIG. 4 is a diagram illustrating driving of the FLC.

FIG. 14 is a diagram showing an image of switching of a liquid crystal having spontaneous polarization.

FIG. 15 is a diagram for explaining a voltage drop due to switching of a liquid crystal having spontaneous polarization.

FIG. 16 is another diagram for explaining a voltage drop due to switching of a liquid crystal having spontaneous polarization.

FIG. 17 is yet another diagram for explaining a voltage drop due to switching of a liquid crystal having spontaneous polarization.

FIG. 18 is a diagram showing a comparison between a conventional CF system as a driving sequence and a field sequential color system.

[Explanation of symbols]

41: pixel electrode, 51: conductive pillar, 101: scanning line,
102a, 102b: signal line, 103a, 103b, 1
11a, 111b: transistor, 104a, 104
b: storage capacitor, 105a, 105b: analog buffer circuit, 106a, 106b: pixel electrode, 107: liquid crystal layer, 108, 109: power supply line, 212a, 212b: switch, 213: wiring.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 621 G09G 3/20 622M 622 623W 623 3/36 3/36 G02F 1/136 500F Term ( Reference) 2H092 GA14 JA24 JB43 JB63 NA05 PA06 PA13 QA06 QA13 QA14 RA04 RA05 2H093 NA16 NA31 NA65 NC12 NC18 NC34 NC35 NC40 NC43 ND06 ND12 ND32 ND35 ND36 NE06 NF04 NF17 NF20 NG02 NG11 5C006 AC11 AF27 FA12 AF13 FA11 AF27 AA10 BB06 CC03 DD08 EE29 EE30 FF11 GG12 JJ02 JJ03 JJ04 JJ05 JJ06 5C094 AA55 BA03 BA11 BA43 CA19 CA24 EA03 EA04 EA07 GA10

Claims (19)

[Claims] 1. A first and a second switch element for supplying a pair of information signals corresponding to one pixel in accordance with a scanning signal, and the first and second switch elements supplied via these switch elements.
First and second information signal holding means for respectively holding a pair of information signals, a power supply line for supplying a predetermined voltage, and a voltage of the power supply line held by the first and second information signal holding means. First and second voltage supply means for supplying these voltages as voltages having values corresponding to the respective information signals, and voltages supplied by these voltage supply means are supplied to a liquid crystal portion corresponding to the one pixel. An active matrix type liquid crystal element, wherein a circuit of each pixel is constituted by a pixel electrode for driving the liquid crystal portion by applying the voltage to both pole sides. 2. The pixel circuit according to claim 1, wherein each of the pixel circuits includes a switch element for simultaneously supplying a voltage from the first and second voltage supply means to the pixel electrode in all pixels. Liquid crystal element. 3. The liquid crystal device according to claim 1, wherein the switch device is a thin film transistor. 4. The liquid crystal device according to claim 1, wherein the liquid crystal is a liquid crystal that switches by an electric field substantially parallel to a surface of a substrate sandwiching the liquid crystal. 5. The liquid crystal is sealed between a pair of substrates, at least one of which is transparent. The pixel electrode of each pixel has two electrodes provided on each substrate so as to face each other. The other pixel circuit portions are formed on one of the pair of substrates, and one of the two electrodes formed on the other substrate is electrically connected to the corresponding pixel circuit portion. The liquid crystal element according to claim 1, wherein the liquid crystal element is connected. 6. The liquid crystal is sealed between a pair of substrates, at least one of which is transparent, and each pixel circuit portion other than the pixel electrodes is formed on one of the substrates. First to third electrodes, wherein the first and second electrodes are provided on the one substrate so as to be connected to the corresponding pixel circuit portions, and the third electrode is electrically connected to the other substrate. Electrically insulated and opposed to both the first and second electrodes, whereby the first to third electrodes form a capacitance in series with the liquid crystal layer in two stages. The liquid crystal device according to any one of claims 1 to 3, wherein 7. The liquid crystal device according to claim 1, wherein the liquid crystal has spontaneous polarization or dielectric anisotropy. 8. When driving the liquid crystal element according to any one of claims 1 to 7, supply of the information signal is performed while inverting or exchanging the information signal of each pair within each pair at predetermined intervals. A method for driving a liquid crystal element. 9. The method according to claim 8, wherein the predetermined period is one frame period. 10. The method according to claim 8, wherein the predetermined period is one field period. 11. When driving the liquid crystal element according to claim 2, a scanning signal and an information signal are given to the liquid crystal element, and the first and second information signal holding means of the liquid crystal element are supplied to all pixels of a predetermined frame. A writing scanning step of supplying an information signal, and a voltage supply to each pixel electrode of all the pixels of the liquid crystal element simultaneously based on the information signals of all the pixels supplied to the first and second information signal holding means. A method for driving a liquid crystal element, comprising: a pixel writing step. 12. The driving method of a liquid crystal element according to claim 11, wherein the writing scanning step is performed for a next frame during a period in which display is performed by all pixels to which the voltage is supplied. . 13. When driving the liquid crystal element according to any one of claims 1 to 7, each scanning signal is controlled such that the direction of an electric field applied to a liquid crystal portion driven based on the scanning signal is reversed. A method for driving a liquid crystal element, wherein a pair of information signals is set and applied. 14. An information signal of each pair is set and applied so that the direction of an electric field applied to a liquid crystal portion driven based on the scanning signal is reversed for each scanning signal. Item 13. The method for driving a liquid crystal element according to any one of Items 8 to 12. 15. An information signal for each pair is set and applied so that a direction of an electric field applied to the liquid crystal is reversed for each adjacent pixel when driving the liquid crystal element according to claim 1. A method for driving a liquid crystal element. 16. The information signal of each pair is set and applied so that the direction of the electric field applied to the liquid crystal is reversed for each adjacent pixel. 3. The method for driving a liquid crystal element according to claim 1. 17. A method for driving a liquid crystal element according to claim 1, wherein when driving the liquid crystal element according to claim 1, a color display is performed by switching a color of a backlight for each operation of each frame. 18. The method of driving a liquid crystal element according to claim 8, wherein a color display is performed by switching a color of a backlight for each operation of each frame. 19. A liquid crystal element having a function of performing the driving method according to claim 8. Description: