JPH0453294B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for driving optical shutter arrays, image display devices, etc. using liquid crystals, and more specifically relates to bistable liquid crystals, particularly ferroelectric liquid crystals. The present invention relates to a method of driving a liquid crystal using an active matrix configuration.

[Prior Art] Conventionally, liquid crystal display elements have been used to display images or information by configuring a group of scanning electrodes and a group of signal electrodes in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels. ,well known. The driving method for this display element is a time-sharing method in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. However, this display element and its driving method had major and fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, most of them are in practical use as display elements because of their relatively high response speed and low power consumption.
“Applied physics Letters”, vol.18, No.4
(1971.2.15), P.127-128 “Voltage−
Dependent Optical Activity of a Twisted
TN shown in “Nematic Liquid Crystal”
This type of liquid crystal has a structure (helical structure) in which the molecules of the nematic liquid crystal, which have positive dielectric anisotropy in the absence of an electric field, are twisted in the thickness direction of the liquid crystal layer. ), forming a structure in which the molecules of this liquid crystal are aligned parallel to each other on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in optical modulation.
When a display element is constructed using this type of liquid crystal with a matrix electrode structure, in the region where both the scanning electrode and the signal electrode are selected (selected point), there is a A voltage higher than the threshold is applied, and no voltage is applied to areas where neither the scanning electrode nor the signal electrode is selected (non-selected points), so the liquid crystal molecules maintain a stable alignment parallel to the electrode surface. . By arranging linear polarizers above and below such a liquid crystal cell in a cross Nicol relationship, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. becomes. However, when a matrix electrode structure is configured, there may be an area where scanning electrodes are selected and signal electrodes are not selected, or an area where scanning electrodes are not selected and signal electrodes are selected (so-called "half-selected points"). The finite electric field becomes strong. If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large, and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. This is why it works. However, in this method, when the number of scanning lines (N) is increased, the time during which an effective electric field is applied to one selected point while scanning the entire screen (one frame) (duty ratio) is 1/ It decreases at a rate of N. For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, resulting in a decrease in image contrast and crosstalk. It has become an unavoidable drawback. This phenomenon is caused by liquid crystals that do not have a bistable state (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is essentially an unavoidable problem that arises when driving (that is, repeatedly scanning) using the temporal accumulation effect. In order to improve this point, voltage averaging method, dual frequency drive method, multiple matrix method, etc. have already been proposed.
Either method is insufficient, and the current situation is that efforts to increase the screen size and density of display elements have reached a plateau due to the inability to sufficiently increase the number of scanning lines.

[Problems to be Solved by the Invention] An object of the present invention is to provide a novel method for driving a bistable liquid crystal, particularly a ferroelectric liquid crystal element, which solves all the problems of conventional liquid crystal display elements as described above. It's about doing.

That is, the present invention has a fast voltage response speed and a ferroelectric liquid crystal that has state memory by applying electric fields in two directions using an active matrix to drive it into two states, bright and dark. The object of the present invention is to provide a method for driving a ferroelectric liquid crystal that displays images on a large screen and at high speed.

[Means for solving the problem] and [Operation] The method for driving a liquid crystal element of the present invention uses a field effect transistor (hereinafter referred to as "FET") having a gate terminal and first and second terminals of a channel. ,
A pixel electrode connected to the first terminal of the FET, a counter electrode opposite to the pixel electrode, and a first electric field sandwiched between the pixel electrode and the counter electrode, A ferroelectric liquid crystal that produces one stable alignment state and produces a second stable alignment state by applying a second electric field of opposite polarity to the first electric field between the pixel electrode and the counter electrode. Liquid crystal elements are arranged along a plurality of rows and columns, and counter electrodes are divided into rows in parallel to the arrangement direction of the liquid crystal elements arranged along the rows to form a plurality of divided counter electrodes, and scan signals are generated. A driving method for driving a liquid crystal device in an active matrix, in which the second terminals of FETs of liquid crystal elements arranged along a plurality of rows and columns are connected to a line, and the gate terminal is connected to a display signal line. , the potential of the second terminal of the FET is always kept constant, and a scanning signal is applied to the divided opposing electrodes to give a potential whose absolute value of the potential difference with the pixel electrode exceeds the threshold value of the liquid crystal. After performing a refresh operation in which a first electric field is formed between a pixel electrode of a liquid crystal element on a counter electrode to which a signal is applied and a counter electrode to align the liquid crystal in a first alignment state, the scanning signal is The FET is connected to the gate terminal of the FET of the selected liquid crystal element on the counter electrode to which the applied voltage is applied.
A display signal is applied to the second terminal of the liquid crystal element to apply a potential that makes the gate on, and a potential that makes the gate off is applied to the gate terminal of the FET of the unselected liquid crystal element. Apply display signals, synchronize with these display signals,
A scanning signal is applied to the opposite electrode to which the scanning signal is applied, the potential difference with the second terminal of the FET is opposite in polarity to the potential difference in the refresh operation, and the absolute value thereof exceeds the threshold of the liquid crystal. applying a second electric field to form a second electric field, and in synchronization with the second electric field forming operation, refreshing the plurality of liquid crystal elements on other preceding divided counter electrodes. This is a characteristic feature.

The ferroelectric liquid crystal used in the driving method of the present invention includes:
A substance that takes either a first optically stable state or a second optically stable state depending on an applied electric field, that is, a bistable state with respect to an electric field, is used, particularly a liquid crystal having such a property.

As the ferroelectric liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable.
Of these, chiral smectate C phase (SmC ^* )
Also, H-phase (SmH ^* ) liquid crystal is suitable. For more information on this ferroelectric liquid crystal, please refer to “LE JOURNAL DE
PHYSIOUE LETTERS” 36 (L-69) 1975,
“Ferroelectric Liquid Crystals”; “Applied
physics Letters” 36 (11) 1980, “Submicro
Second BistabIe Electrooptic Switching in
Liquid Crystals”; “Solid State Physics” 16 (141) 1981
The ferroelectric liquid crystals disclosed in these documents can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials,
In order to maintain the temperature state such that the liquid crystal compound becomes the SmC ^* phase or the SmH ^* phase, the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

FIG. 1 schematically depicts an example of a ferroelectric liquid crystal cell. 1 and 1′ are In ₂ O ₃ , SnO ₂ and
A substrate (glass plate) coated with a transparent electrode such as ITO (Indium-Tin Oxide), between which a liquid crystal molecular layer 2 is oriented perpendicular to the glass surface.
SmC ^* phase liquid crystal is sealed. A thick line 3 represents a liquid crystal molecule, and this liquid crystal molecule 3 has a dipole moment (P ⊥ 4) in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied to the liquid crystal molecules 3, the helical structure of the liquid crystal molecules 3 is unraveled, and the orientation direction of the liquid crystal molecules 3 can be changed so that all dipole moments (P⊥) 4 are directed in the direction of the electric field.Liquid crystal molecules 3 has an elongated shape and exhibits refractive index anisotropy in its long and short axis directions. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, The liquid crystal optical modulation element whose optical properties change depending on the polarity of applied voltage is
easily understood. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules unravels (non-helical structure) even when no electric field is applied, as shown in Figure 2. The dipole moment P or P' takes either an upward direction 4a or a downward direction 4b. As shown in FIG. 2, when an electric field E or E' with a different polarity above a certain threshold value is applied to such a cell for a predetermined time, the dipole moment will move upward 4a or The direction is changed from the downward direction 4b, and accordingly, the liquid crystal molecules are aligned in either the first alignment state 5 or the second alignment state 5'.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. The second point will be explained with reference to FIG. 2, for example. When the electric field E is applied, the liquid crystal molecules are aligned in the first alignment state 5, and this state remains stable even when the electric field is turned off. Furthermore, when an electric field E' in the opposite direction is applied, the liquid crystal molecules are aligned to the second alignment state 5' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. or,
As long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such a fast response speed and bistability, it is preferable that the cell be as thin as possible, and generally 0.5μ to 20μ, particularly 1μ to 5μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this type is disclosed in US Pat.
It is proposed in the specification of No. 4367924.

The present invention constitutes an active matrix.
It is based on the fact that an FET structure element such as a TFT (thin film transistor) can be used as either the drain or the source by reversing the voltages applied to the two terminals of the channel. In the present invention, among the two terminals of the channel,
For convenience, the terminal connected to the pixel electrode was referred to as a first terminal, and the terminal connected to the other scanning signal line was referred to as a second terminal. In the present invention, any element having an FET structure such as an amorphous silicon TFT or a polycrystalline silicon TFT may be used as the element constituting the active matrix.
Further, it is also possible to perform the same operation even when using a bipolar transistor other than the FET structure.

In an N-type FET, where V _D is the drain voltage, V _G is the gate voltage, V _S is the source voltage, and V _P is the threshold voltage between the gate and source, V _D > V _S , and V _G > V _S + V _P. It becomes conductive when V _G <V S +V P, and becomes non-conductive when V G <V _S +V _P.

For P-type FET, V _D <V _S and V _G <V _S +
It becomes conductive when V _P and becomes non-conductive when V _G > V _S +V _P.

Whether the FET is P-type or N-type, which terminal of the FET acts as the drain and which acts as the source is determined by the direction of voltage application. That is, for N type, the lower voltage serves as the source, and for P type, the higher voltage serves as the source.

In the case of ferroelectric liquid crystals, the cross-Nicol state placed above and below the cell determines which is in the "bright" state and which is in the "dark" state in response to positive and negative voltages applied to the liquid crystal cell. It can be freely set depending on the direction of the polarization axes of the pair of polarizers and the long axis of the liquid crystal molecules.

Since the present invention controls the electric field applied to the liquid crystal cell by controlling the voltage between the terminals of each element of the active matrix to perform display, the voltage level of each signal is determined according to the following example. Regardless, it can be implemented as long as the potential difference between each signal is maintained relatively.

[Example] Next, a specific example of a method for driving a ferroelectric liquid crystal using an active matrix of the present invention is shown in FIGS. 3 to 7.
This will be explained based on the diagram.

FIG. 3 is a circuit diagram of an active matrix, FIG. 4 is an explanatory diagram showing addresses of corresponding pixels, and FIG. 5 is an explanatory diagram showing an example of display of corresponding pixels.

6 is a scanning electrode group, and 7 is a display electrode group.

FIG. 6a shows a scanning signal, which is an electrical signal applied to each selected scanning electrode and an electrical signal applied to other scanning electrodes (unselected scanning electrodes) at phases t ₁ , t ₂ . It shows.
FIG. 6b shows electrode signals which are display signals and are applied to selected display electrodes and unselected display electrodes in phases t ₁ , t _{2 .} . . , respectively.

In FIG. 6, the horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode groups 6 are sequentially and periodically selected. First, as shown in FIG. 6a, the electric signal applied to the selected scanning electrode is -V C during "bright" refresh, and +V _C during "dark" writing.
V _C.

Further, the electrical signals applied to the unselected scanning electrodes are all 0 as shown in FIG. 6a. On the other hand, the electric signal applied to the selected display electrode is 0 or +V _G when the scan electrode is -V _C , and is +V _G when the scan electrode is +V _C , as shown in FIG. 6b. Further, the electric signal applied to the unselected display electrodes is 0 as in the case of scan electrode 0. In the above, each voltage value is set to a desired value that satisfies the following relationship.

At the same time, "dark" is written on the scanning electrode m=q line with the signal line of the display electrode n=l, and at the same time, the scanning electrode m=q+
In one line, display electrode n=1 to M (M is the number of display lines)
If you want to refresh the entire line to "light".

V _S +V _LC <V _Cn (m=q) V _Go −V _P >V _LC +V _S (n=l) V _S −V _LC >V _Cn (m=q+1) V _Go =0 (n=1 to M ) (m=q, n≠l) V _Cn =0 (m≠q) However, each symbol represents the following items.

V _Cn : Counter electrode (scanning signal) voltage V _Go : Gate electrode (display signal) voltage V _S : Source or drain electrode (common terminal) voltage V _LC : Absolute value of threshold voltage of ferroelectric liquid crystal V _P : Gate, The operation above the threshold between sources is repeatedly written from q=1 to N.

Among the pixels when such an electric signal is applied, for example, the writing operation of the pixel in FIG.
As shown in the figure. In FIG. 7, the horizontal axis represents time, and the vertical axis represents display states of ON (dark) upper side and OFF (bright) lower side. That is, as is clear from FIGS. 6 and 7, pixels P _N,N , P _N,N+1 , P _N,N+2 at the intersection of the scanning line and display line selected at phase t ₁ exceeds the threshold −V _LC −V _LC >V _C −V _S is applied,
In Figure 4, pixels P _N,N , P _N,N+1 , P _N,N+2 are "bright"
will be refreshed. Then, at phase t ₂ , the pixel at the intersection of the selected scan line and the display line
For P _N,N , P _N,N+2 , the voltage exceeding the threshold V _LS V _LC <V _C
-V _S is applied, and the pixels P _N,N and P _N,N+2 switch to "dark". At the same time as this "dark" writing in phase t ₂ , pixels on the scan line also selected in phase t ₂ are written P _N+1,N , P _N+1,N+1 ,
At P _N+1,N+2 , as in the case of phase t ₁ , a voltage −V _LC >V _C −V _S exceeding the threshold −V _LC is applied,
Pixels P _N,N , P _N+1,N+1 ′, P _N+1,N+2 are refreshed to "bright". After phase _t3 , by repeating each of the above operations, predetermined writing is sequentially performed on each scanning line. Therefore, on the selected scanning electrode line, the liquid crystal elements of each pixel are first aligned to one alignment state, and the pixel is turned OFF (bright), and then on the same scanning line selected again, the display starts. Depending on whether an electrode is selected or not, if selected, the liquid crystal molecules align to the other alignment state;
The pixel becomes ON (dark).

On the other hand, as shown in FIG. 7, on unselected scanning lines, the voltages applied to all pixels are:
Neither voltage exceeds the value voltage. Therefore, the liquid crystal molecules in each pixel other than on the selected scanning line maintain the orientation corresponding to the signal state (Q _N-1 ) at the time of the previous scan without changing the orientation state. That is, when a scanning electrode is selected, a signal for one line is written, and the signal state can be maintained until the next selection after one frame is completed. Therefore, even if the number of scanning electrodes increases, the actual duty ratio does not change and the contrast does not deteriorate at all.

In FIG. 5, scanning electrodes C _N , C _N+1 , C _N+2 ',...
Among the pixels formed at the intersections of the display electrodes G _N , G _N+1 , G _N+2 ′, ..., the pixels in the shaded areas correspond to the "dark" state, and the pixels shown in white correspond to the "bright" state. It shall be. Now, if we pay attention to the display on the display electrode G _N in Figure 5, the pixels corresponding to the scanning electrodes C _N and C _N+2 are in the "dark" state, and the other pixels are in the "bright" state. . By each operation of the phases t ₁ to _{t 4} , the fifth
The display pattern of the figure is completed.

In the method for driving a ferroelectric liquid crystal of the present invention, the arrangement of the scanning electrode and the signal electrode is arbitrary. For example, it is also possible to arrange the pixels in a line as shown in FIG. 8a and b. When placed, it can be used as a shutter array.

Next, in the embodiment described above, preferred specific values for driving DOBAMBC as a ferroelectric liquid crystal are as follows: For example, input frequency o=1×10 ⁴ to 1×10 ⁶ mm ³ 10<|V _G |<60V (peak value) 0.3<|V _S |<10V (peak value).

FIG. 9 is a cross-sectional view showing the structure of the FET in the TFT used in the present invention, and FIG.
Figure 11 is a cross-sectional view of a ferroelectric liquid crystal cell using
Figure 12 is a perspective view of the TFT substrate, Figure 12 is a plan view of the TFT substrate, Figure 13 is a partial cross-sectional view taken along line A-A' in Figure 12, and Figure 14 is a cross-sectional view taken along line B-B' in Figure 12. It is a partially cutaway sectional view, and each of the figures shown above shows one embodiment of the present invention.

FIG. 10 shows one specific example of a liquid crystal element that can be used in the method of the invention. A semiconductor film 16 (amorphous silicon doped with hydrogen atoms) formed on a substrate 20 of glass, plastic, etc. via a gate electrode 24 and an insulating film 22 (such as a silicon nitride film doped with hydrogen atoms);
A TFT composed of two terminals 8 and 11 in contact with this semiconductor film 16 and a pixel electrode 12 (ITO; Indnium Tin Oxide) connected to the terminal 11 of the TFT are formed. Furthermore, an insulating layer 13 (polyimide, polyamide, polyvinyl alcohol, polyparaxylylene, SiO, SiO ₂ ) and a light shielding film 9 made of aluminum, chromium, or the like are provided on this. A counter electrode 21 (ITO; Indnium Tin Oxide) and an insulating film 2 are provided on the substrate 20', which is the counter substrate.
2 is formed.

The aforementioned ferroelectric liquid crystal 23 is sandwiched between the substrates 20 and 20'. Also, this board 20
A sealing material 25 for sealing the ferroelectric liquid crystal 23 is provided around the ferroelectric liquid crystal 23 and 20'.

Polarizers 19 and 19' in a crossed Nicol state are arranged on both sides of a liquid crystal element having such a cell structure, and an observer A can see the display state by the reflected light _I1 from the incident light _Ip . Similarly, a reflective plate 18 (diffuse reflective aluminum sheet or plate) is provided behind the polarizer 19'.

Further, in each of the above figures, the terms "source electrode" and "drain electrode" are used only when current flows from the drain to the source. In the function of FET, it is also possible for the source to function as the drain.

[Effects of the Invention] By using the method for driving the ferroelectric liquid crystal of the present invention having the above structure, it is possible to display a large screen with a large number of pixels in the active matrix and to display clear images at high speed. .

[Brief explanation of the drawing]

1 and 2 are perspective views schematically showing a ferroelectric liquid crystal used in the method of the present invention, and FIG. 3 is a circuit diagram of a matrix electrode used in the method of the present invention.
FIG. 4 is an explanatory diagram showing addresses of corresponding pixels, FIG. 5 is an explanatory diagram showing display examples of corresponding pixels, and FIGS. 6 a and b.
7 is an explanatory diagram showing the electrical signals applied to the scanning electrode and display electrode, FIG. 7 is an explanatory diagram showing the write operation to each pixel, and FIGS. 8 a and b are wiring diagrams showing an example of the active matrix circuit and pixel arrangement. Figure 9 is a cross-sectional view showing the configuration of FET in TFT.
Figure 0 is a cross-sectional view of a ferroelectric liquid crystal cell using TFT.
Figure 11 is a perspective view of the TFT board, Figure 12 is the TFT
A plan view of the substrate, FIG. 13 is a partial sectional view taken along the line A-A', and FIG. 14 is a partial sectional view taken along the line B-B'. 1, 1'...Substrate coated with transparent electrode, 2
...Liquid crystal molecule layer, 3...Liquid crystal molecule, 4...Dipole moment (P⊥), 4a...Upward dipole moment, 4b...Downward dipole moment, 5...
...first orientation state, 5'...second orientation state, 6
(C _N , C _N+1 , C _N+2 )...scan electrode group (scan electrode),
7 (G _N , G _N+1 , G _N+2 )...Signal electrode group (signal electrode), 8... Source electrode (drain electrode), 9...
Light shielding film, 10... n ⁺ layer, 11... drain electrode (source electrode), 12... pixel electrode, 13...
Insulating layer, 14...Substrate, 15...Light shielding film directly under semiconductor, 16...Semiconductor, 17...Transparent electrode in gate wiring section, 18...Reflector, 19, 19'...
Polarizing plate, 20, 20'... Transparent substrate such as glass or plastic, 21... Counter electrode, 22... Insulating film, 23... Ferroelectric liquid crystal layer, 24... Gate electrode, 25... Sealing material, 26...Thin film semiconductor, 2
7... Gate wiring, 28... Panel board, 29...
...Gate portion having a light blocking effect, 1' to M'... Scanning electrode, 1 to N... Display electrode, L... Common electrode,
LC...Liquid crystal, FET...Field effect transistor.

Claims (1)

[Claims] 1. A field effect transistor (hereinafter referred to as "FET") having a gate terminal and first and second terminals of a channel, a pixel electrode connected to the first terminal of the FET, and a pixel electrode connected to the first terminal of the FET. Opposing counter electrodes;
A first stable alignment state is created by applying a first electric field between the pixel electrode and the counter electrode, which is sandwiched between the pixel electrode and the counter electrode, and the first electric field between the pixel electrode and the counter electrode is A liquid crystal element consisting of a ferroelectric liquid crystal that generates a second stable alignment state by applying a second electric field of opposite polarity is arranged along a plurality of rows and columns, and a counter electrode is arranged along the rows. The liquid crystal elements arranged along a plurality of rows and columns are divided into rows parallel to the arrangement direction of the liquid crystal elements, forming a plurality of divided counter electrodes, and connected to scanning signal lines.
A driving method in which the second terminals of the FETs are connected in common and the gate terminals are connected to the display signal line for active matrix driving of a liquid crystal device, in which the potential of the second terminals of the FETs is always kept constant, and A scanning signal that gives a potential whose absolute value of potential difference with the pixel electrode exceeds the threshold value of the liquid crystal is applied to the pixel electrode of the liquid crystal element on the counter electrode to which the scanning signal is applied and the gap between the counter electrode. After performing a refresh operation to align the liquid crystal to a first alignment state by forming a first electric field at , said FET
A display signal is applied to the second terminal of the liquid crystal element to apply a potential that makes the gate on, and a potential that makes the gate off is applied to the gate terminal of the FET of the unselected liquid crystal element. Apply display signals, synchronize with these display signals,
A scanning signal is applied to the opposite electrode to which the scanning signal is applied, the potential difference with the second terminal of the FET is opposite in polarity to the potential difference in the refresh operation, and the absolute value thereof exceeds the threshold of the liquid crystal. applying a second electric field to form a second electric field, and in synchronization with the second electric field forming operation, refreshing the plurality of liquid crystal elements on other preceding divided counter electrodes. Characteristic driving method of liquid crystal device.