JPH0820642B2 - Liquid crystal display - Google Patents

Description

The present invention relates to a liquid crystal display device. For more information,
The present invention relates to an improvement in a liquid crystal display device that enables a high-definition matrix type liquid crystal display having an improved display brightness, which is required for a high-definition finder display of a camera or a projection type display such as a television.

(B) Conventional technology Conventionally, a matrix type liquid crystal display device has been developed as a display device utilizing the electro-optic effect of liquid crystal for pixel display. This liquid crystal display device basically optically modulates incident light according to a voltage applied between a large number of pixel electrodes arranged in a dot matrix and a counter electrode facing the pixel electrodes. And a liquid crystal layer.

The operation modes of such a matrix type liquid crystal display device include a twisted nematic (TN) mode and a super twisted nematic (ST) according to the type of liquid crystal or the difference in electro-optical properties of the liquid crystal layer.
Many modes have been developed, including N) mode, guest-host (GH) mode, dynamic scattering (DS) mode, and phase transition mode. Further, regarding a method of individually controlling each display pixel composed of the liquid crystal layer and the pixel electrode, (1) simple matrix system, (2)
There are a multi-matrix method, (3) a method in which a non-linear two-terminal element (for example, a diode) is added, and (4) a switching three-terminal element [for example, a thin film transistor (TFT)] in a TFT active matrix method.

Among them, DS mode, [GHHeilmeier et al .: Proc I
EEE 56 1162 (1968)], white-tailed GH mode [DLWhite et al .: J. Appl. Phys. 45 4718 (1974)], cholesteric-nematic phase transition mode [JJWysocki
Other: Proc.SID 13/2 115 (1980)]
The liquid crystal display device combined with the display system called the T active matrix system does not need to use a deflection filter,
The display brightness can be improved.

In the liquid crystal display device based on this combination, as shown in FIG. 4, a so-called signal storage capacitor (C ₁ ) is provided in parallel with the pixel electrode (C ₂ ) connected to the drain electrode of the TFT, and this capacitor is also provided. The capacity of (C ₁ ) is increased to improve the charge retention function.

However, even if such a signal storage capacitor is used, there is a limit in preventing the charge retention function from lowering in principle.
Further, in a highly integrated matrix display device,
It is difficult to provide a signal storage capacitor having sufficient electric capacity for each of a large number of TFTs, because it increases the load on the source driver, the source bus line, and the switching TFT, and is limited in area and manufacturing technology.

In order to solve the above-mentioned problems, the applicant of the present application can prevent the adverse effect on the display operation due to the discharge even when a liquid crystal layer having a small specific resistance is used, thereby using the polarizing filter. We have applied for a new TFT active matrix type liquid crystal display device that can achieve high display brightness (Japanese Patent Application No. 1-95581).

(C) Problem to be Solved by the Invention The present invention is a further improvement of the liquid crystal display device of the above application, and a TFT active matrix liquid crystal display device having a drive system suitable for displaying an image including halftone. Is to provide.

(D) Means for Solving the Problems Thus, according to the present invention, (a) an electrode line arranged in an XY matrix and (b) a source, a drain and a gate are provided, and the source is the above-mentioned. In the electrode line X,
A large number of first switching three-terminal elements each having a gate connected to the electrode line Y, (c) a large number of pixel electrodes corresponding to the respective first switching three-terminal elements, and a liquid crystal driving power source. A liquid crystal layer disposed between the counter electrode and a liquid crystal display section that performs a matrix display operation based on the drain output of the switching three-terminal element, and (d) the source is connected to the pixel electrode, A second switching three-terminal element connected to a common line via a dummy load having a drain having an impedance substantially equal to that of the liquid crystal display section; and a drain of the first switching three-terminal element via a signal storage capacitor Connected to a ground line or an adjacent gate line and connected to the gate of the second switching three-terminal element, (e) before The liquid crystal display device comprising configured capable of applying an anti-phase alternating voltage with each other in the opposing electrode and the common line is provided.

That is, the present invention is characterized by being a liquid crystal display device having a novel drive system in which a sample hold circuit composed of a thin film transistor and a signal storage capacitor is provided for each pixel, and a direct current component is not applied to the liquid crystal layer.

In a general field effect transistor, it is customary to call the electrode on the carrier supply side the source and the carrier sweep side the drain, but in the thin film transistor in the liquid crystal display device of the present invention, the source and drain are The structure is symmetric as will be described later, and a current flows bidirectionally between the channels, so that it cannot be distinguished by the above-mentioned custom. Therefore, in the following description, one closer to the supply source of the signal or the driving voltage is called a source and the other is called a drain.

The liquid crystal display device of the present invention (hereinafter, referred to as the device of the present invention) is different from the above-mentioned DS mode, GH mode, cholesteric-nematic phase transition mode, etc., in that no ionic impurities are used as a liquid crystal layer without using a polarizing filter. This is most effective when a liquid crystal display device having a low specific resistance is used in combination with an operation mode in which the liquid crystal electro-optic effect of its light absorption and light scattering characteristics is used for display. Is a further preferred embodiment.

In particular, according to the device of the present invention, even when a liquid crystal layer having higher conductivity than the conventional one, particularly a liquid crystal layer having a low specific resistance of 10 ⁹ Ωm or less, can be used, the adverse effect on the display operation due to the discharge can be prevented. It is. Therefore, in this invention,
It is a preferable embodiment to use a liquid crystal layer having a low specific resistance of 10 ⁹ Ωm or less.

In the device of the present invention, as a material for the electrode line, a general wiring material such as ITO, Al, Ti, Ni, W, Mo, Cr, p-Si (n ⁺ ) (polycrystalline silicon) can be used. An insulating film made of SiO _x , SiN _x , Ta ₂ O ₅ , Al ₂ O ₃ or the like is used at the intersection of the electrode lines to prevent a short circuit.

In the device of the present invention, the first and second switching three-terminal elements are, for example, thin film transistors (TFT).
Is suitable, and a capacitor element used in a normal active matrix system can be applied as a signal storage capacitor. For example, a TFT composed of a-Si (amorphous silicon), p-Si, Si crystal, CdSe, GaAs, GaP or the like can be used as the first and second switching three-terminal elements. Further, a so-called MOS type transistor array using a Si substrate is also applicable for the reflection type device. As the signal storage capacitor, a capacitor formed by using a conductor similar to the above wiring material as an electrode and using an insulating material similar to the above intersection insulating material is suitable. However, the other electrode of the signal storage capacitor may be connected to an adjacent gate electrode instead of being connected to the ground line. Further, the signal storage capacitor does not have to be provided as an element separate from the first switching three-terminal element, and the one using the internal capacitor component of the first switching three-terminal element, that is, its stray capacitance It may be used.

Incidentally, for example, the formation of the above-mentioned TFT can be carried out according to the method described in JP-A-58-147069.

In addition, at least one of the pixel electrodes and the counter electrode forming the liquid crystal display section is transparent (for example, S is abbreviated as ITO).
nO ₂ doped In ₂ O ₃ film or SnO ₂ film known under the trade name NESA) is used, and in the case of a so-called reflective display device, the other is a metal electrode such as Al, Au. Used.

In the liquid crystal display section of the device of the present invention, there is no problem even if the liquid crystal layer is made of one having a low specific resistance containing ionic impurities, and this constitution is appropriately selected according to the operation mode. For example, when applying DS mode, neutral,
Alternatively, a nematic compound having a weak positive dielectric anisotropy or a weak negative dielectric anisotropy and an ionic impurity are used. As the nematic compound, for example, And / or (in the formula, R and R ′ are each independently a C _{3 to} C ₈ alkyl group; X is a hydrogen atom or a fluorine atom) and the like. The liquid crystal layer is preferably used as a mixed liquid crystal composition containing these nematic compounds and having a negative dielectric anisotropy and a positive conductivity anisotropy as the whole system. on the other hand,
As ionic impurities, (In the formula, m is an integer of _{1 to} 16, R ₁ and R ₂ are hydrogen atoms, methyl groups or benzyl groups) and other compounds (Minezaki et al.
79) Spring lecture 30P-B-13) is mentioned as a suitable one.

In the case of a white tailored GH mode, a cholesteric liquid crystal compound having a positive dielectric anisotropy or a nematic liquid crystal compound having a positive dielectric anisotropy and an optically active compound are exemplified. In this mode, as the dichroic dye used, T. Uchida et al. [T.
Uchida et al .; Mol. Cryst and Liq. Cryst. 63 19 (1981)], the following azo dyes; And anthraquinone dyes are common examples, but other than these dyes, fluorescent dyes such as coumarin-based dyes and other dyes are also applicable.

In the device of this invention, the source side of the second switching three-terminal element is connected to the common line. That is, with such a connection, the potential of the source and the drain of the three-terminal element becomes symmetrical with respect to the potential of the central portion of the channel, and therefore, when the potentials of the source and the drain are exchanged, the operation of the positive and negative is performed.

In the present invention, an AC voltage having a phase opposite to that of the counter electrode is applied to the common line. The opposite-phase alternating-current voltage means that the alternating-current voltage applied to the counter electrode has the same amplitude and the opposite phases. That is, by applying this alternating-phase alternating-current voltage to the common line, the potential relationship between the electrodes of the second switching three-terminal element becomes substantially symmetrical depending on whether the polarity of the opposing voltage is positive or negative, and The impedance of the switching three-terminal element has a constant value regardless of the polarity of the counter voltage, and the voltage applied to the liquid crystal layer of the liquid crystal display section can be positive / negative symmetrical.

Further, in the present invention, the symmetry can be further improved by further inserting a dummy load of the same degree as the liquid crystal layer between the drain of the second switching three-terminal element and the common line.

(E) Action Due to the drain output from the first switching three-terminal element selected by the electrode lines X and Y, charges are stored in the signal storage capacitor and a voltage is applied to the gate of the second switching three-terminal element. A closed circuit is formed, and a voltage is applied from the liquid crystal driving power source to the corresponding pixel electrode portion of the liquid crystal display portion to perform the display operation.

At this time, since the electrode lines X and Y are selected by scanning under a constant short frame frequency, the output time of the first switching three-terminal element is extremely short for one pixel electrode.

However, since the signal storage capacitor is attached,
Even after the first switching three-terminal element is turned off, a voltage is applied to the gate of the second switching three-terminal element until a new signal charge is accumulated in the next field and the on state is maintained. Be done. Since the charge stored in the signal storage capacitor is separated from the liquid crystal display unit via the second switching three-terminal element, the discharge does not substantially consume the charge, and the charge holding time is longer than that in the conventional case. Is also extended. That is, it operates as a sample hold circuit.

On the other hand, in the state where the second switching three-terminal element is kept in the ON state, even if discharge occurs in the liquid crystal layer, the charge from the liquid crystal driving power supply is continuously supplied, so that the discharge does not adversely affect. .

Further, in the device of the present invention, the following drive conditions are selected when performing binary display, that is, monochrome display. That is, when a state in which a voltage is applied to the liquid crystal layer is called ON and a state in which no voltage is applied is called OFF, the impedance V _SW of the second switching element is higher than the impedance Z _{LC of the} liquid crystal layer in the ON state. The level of the signal voltage Z _S1 supplied to the first switching element is selected so that it becomes sufficiently small and vice versa in the OFF state. An AC voltage V _C is applied to the counter electrode because the application of a DC voltage causes electrolysis of the liquid crystal and corrosion of the electrode. AC voltage applied to the counter electrode
V _C is divided by the impedance Z _LC of the liquid crystal layer and the impedance Z _{SW of the} switching element. If both satisfy the above conditions, the voltage V _LC applied to the liquid crystal layer in the ON state becomes almost equal to the voltage V _C applied to the counter electrode, and almost no voltage is applied to the liquid crystal layer in the OFF state. . In this case, the voltage V _LC applied to the liquid crystal layer becomes substantially positive and negative symmetrical, and the DC component can be ignored.

Further, according to the device of the present invention, the source side of the second switching element is connected to the common line, and the potential of the source and the drain of the switching element is symmetrical with respect to the potential of the central portion of the channel. Therefore, the operating point does not change even if the voltage applied to the common electrode and the counter electrode is reversed. Therefore, at this time, the impedance (Z _SW ) of the switching element becomes constant regardless of the polarity of the AC voltage applied to the counter electrode, and the impedance (Z _LC ) of the liquid crystal layer also becomes constant, and Z _SW ≈Z _LC . Therefore, when an AC voltage of amplitude V _C is applied to the counter electrode and a voltage of the same amplitude and opposite phase is applied to the common electrode, the potential relationship of each electrode of the second switching element is positive when the polarity of V _C is positive. And become negative and Z _SW becomes almost equal. Therefore, the voltage applied to the liquid crystal layer becomes substantially positive and negative symmetrical.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

(F) Example FIG. 1 is an equivalent circuit diagram showing the configuration of one display unit of the matrix in the matrix type liquid crystal display device of Reference Example 1.

In the figure, X ₁ , X ₂ ... Denote the data signal bus lines (electrode lines X) in the XY matrix electrodes, Y ₁ , Y ₂ ...
Similarly, each indicates a scanning signal bus line (electrode line Y), and these intersections (addresses) are isolated by an insulating film. A first thin film transistor (TFT ₁ ) is provided near each of the intersections, and its gate is connected to the electrode line Y (Y ₁ ), and its source is connected to the electrode line X (X ₁ ). Then, as shown in the figure, the drain of TFT ₁ is connected to the gate of the second thin film transistor (TFT ₂ ), and a capacitor (C ₁ ) serving as a signal storage capacitor is connected in the middle thereof.

On the other hand, the source of the TFT ₂ is connected to one pixel electrode (a) in the liquid crystal display section (C ₂ ) in which a liquid crystal layer is arranged between a large number of pixel electrodes (a) and a counter electrode (b). The counter electrode (b) is connected to an AC power source (V _C ) for driving the liquid crystal.

In addition, E in the drawing indicates an earth line, and one end of the capacitor (C ₁ ) is connected. In the figure, F indicates a common line, to which the drain of TFT ₂ is connected.

A DS mode-projection type active matrix liquid crystal display device not using a polarizing filter was constructed under the following conditions by employing the above circuit configuration.

1) Liquid crystal display method: Projection type 2) Light source: Metal halide lamp 3) Panel size: Diagonal 3 ″ 4) Panel pixel number: 240 × 384 dots 5) Panel substrate: Corning 7059 glass 1.1t 6) TFT ₁ , TFT ₂ : Amorphous silicon TFT gate material Ta, gate oxide film Ta ₂ O ₅ /
SiN _X semiconductor material a-Si source / drain material by P-CVD n ⁺ a-Si / Ti multilayer film 7) C ₁ : Ta / Ta ₂ O ₅ · SiN _X / Ti 8) C ₂ : ITO / liquid crystal / ITO ( Liquid crystal layer thickness uses 7 μm plastic bead spacers) 10) Ionic impurities: 0.5wt /% 11) AC drive voltage: 60Hz square wave ± 7.5V The specific resistance (ρ) of the above liquid crystal layer is 10 ⁷ Ωm, the pixel area (S) is 100μm square = 10 ^-8 m ² , and the liquid crystal layer Thickness (d) =
7 μm, and therefore the resistance value of the liquid crystal per pixel R _LC =
(Ρ · d) / S = 7 × 10 ⁹ Ω.

In such a device, the TFT ₁ and the signal storage capacitor (C ₁ ) function as a sample hold circuit, and the TFT ₂ controls the current for applying an AC voltage for driving the liquid crystal to the liquid crystal layer at a predetermined position of the liquid crystal display (C ₂ ). Acts as an element (a type of buffer transistor).

In this configuration, the capacitor C ₁ is connected to the gate of the high impedance TFT ₂ and the liquid crystal display (C ₂ )
Since it is not directly connected to, it is difficult to discharge, and the electric charge accumulated there acts to keep the TFT ₂ in the ON state for a longer time than before even after the TFT ₁ is in the OFF state.

Therefore, even when a liquid crystal layer having a low specific resistance and easy to discharge is used, this discharge prevents development that turns off in a shorter time than the time required by the TFT ₂ (usually the frame frequency cycle), The liquid crystal matrix display operation can be performed.

Further, in the above apparatus, a voltage V _C ′ having the same amplitude as the AC voltage V _C applied to the counter electrode but an opposite phase is applied to the common line F. With such a driving method, the operation of the TFT ₂ is V _C
When is positive and when is negative, it becomes almost symmetrical.

Reference Example 2 is shown in FIG. The difference from the above embodiment is that one electrode of the capacitor C ₁ is an adjacent gate line. With such a configuration, the ground line can be omitted.

When an image is displayed on a screen using such a liquid crystal display device, it is possible to obtain a display (comparison in a white display state) with a brightness (100 fL) about twice that of the conventional TN mode using the same light source. Was.

Example 1 In the equivalent circuit shown in FIG. 1 of the liquid crystal display element of Reference Example 1, a dummy load (Z _D ) was further inserted between the common line and the drain of the TFT ₂ to form a liquid crystal display element. .

The dummy load (Z _D ) is a non-doped a-Si film (thickness d = 28 nm, specific resistance ρ = 10 ⁸ Ωm) sandwiched between the drain of the TFT ₂ and the common line. Using. Area (S ') of the part where these overlap
Is 20 μm square, the resistance value is R ′ = 7 × 10 ⁹ Ω.

By providing a dummy load having a resistance value equal to that of the liquid crystal layer between the drain electrode and the common line in this way, the positive and negative symmetry during driving was further improved, and flicker could not be visually recognized.

(G) Effects of the Invention According to the liquid crystal display device of the present invention, even when a liquid crystal layer having a low specific resistance and substantially no charge holding function is used, voltage application to the liquid crystal layer is secured in time. Thus, a desired liquid crystal matrix display can be performed.

Therefore, the DS mode and the white tailored GH mode, which enable gradation display, high contrast display, and high-speed response display without using a polarizing filter, are adopted as the liquid crystal electro-optical mode. A matrix display can be performed.

Further, even when displaying a halftone, the voltage applied to the liquid crystal display section becomes positive and negative symmetry, the DC component becomes small enough to be ignored, flicker occurs, electrolysis of the liquid crystal,
Corrosion of the pixel electrode is suppressed, and good display quality and high reliability can be obtained.

The liquid crystal display device of the present invention is particularly effective as a light valve for a projection type display device that needs to satisfy high-temperature operation and high light utilization efficiency at the same time. It can be effectively used for monitors, LCTVs, viewfinders, etc.
It is also suitable for in-vehicle applications and aircraft display applications. Further, the present invention can be applied not only to a transmission type display device but also to a reflection type display device.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of one display unit in the liquid crystal display device of Reference Example 1, FIG. 2 is an equivalent circuit diagram of Reference Example 2, and FIG. 3 is an equivalent circuit diagram of still another embodiment of the present invention. FIG. 4 is a view corresponding to FIG. 1 showing one display unit of a conventional liquid crystal display device. X ₁ , X ₂ …… Electrode line, Y ₁ , Y ₂ …… Electrode line, TFT ₁ ……
1st thin film transistor, TFT ₂ ... 2nd thin film transistor, C ₁ ... Capacitor (signal storage capacitor), C ₂ ...
... Capacity of liquid crystal display part, a ... Pixel electrode, b ... Counter electrode, V _C , V _C ′ ... AC power supply, E ... Ground line, F ...
… Common line, Z _D …… Dummy load.

Claims (1)

[Claims] 1. An electrode line (a) arranged in an XY matrix, and (b) a source, a drain and a gate, the source being the electrode line X and the gate being the electrode line Y. A plurality of connected first switching three-terminal elements, (c) a plurality of pixel electrodes corresponding to the respective first switching three-terminal elements, and a liquid crystal between a counter electrode connected to a liquid crystal driving power supply. And a liquid crystal display section that performs matrix display operation based on the drain output of the switching three-terminal element, (d) the source is connected to the pixel electrode, and the drain is the impedance of the liquid crystal display section. A second line connected to a common line via a dummy load having an impedance substantially equal to
A switching three-terminal element, wherein the drain of the first switching three-terminal element is connected to a ground line or an adjacent gate line via a signal storage capacitor and connected to the gate of the second switching three-terminal element, (E) A liquid crystal display device configured so that alternating voltages having mutually opposite phases can be applied to the counter electrode and the common line.