JPH09292631A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display device capable of performing a display with low power consumption and also capable of performing a halftone display. SOLUTION: A unit cell is constituted of a switching element 10, the pixel electrode 11 connected to the switching element 10, the counter electrode 12 arranged by being opposed to the pixel electrode 11, the ferroelectric thin film 13 provided between the pixel electrode 11 and the counter electrode 12 and the liquid crystal layer 14 provided between the pixel electrode 11 and the ferroelectric thin film 13 or between the counter electrode 12 and the ferroelectric thin film 13, and the area of the pixel electrode 11 is made different from that of the counter electrode 12. As a result, an electric field to be impressed on liquid crystal 14 is made ununiform in a unit display area to be defined by one pixel electrode 11 and one counter electrode 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device capable of displaying halftone with low power consumption.

[0002]

2. Description of the Related Art Liquid crystal display devices are thin and lightweight and can be driven at a low voltage, and are therefore widely used in wristwatches, calculators, word processors, personal computers, small game machines and the like. Recently, the need for a pen-input electronic notebook has increased, and the demand for a portable terminal (PDA) has been increasing.

On the other hand, when multimedia is developed and a plurality of displays are displayed on the same screen, a large screen and high definition are required, and as a result, the amount of display information increases and the driving frequency also increases. Therefore, it is necessary to develop an IC that can operate at higher speed. In addition, an increase in driving frequency causes an increase in power consumption.

Here, an active matrix type liquid crystal display device will be considered as an example of a large screen and high definition liquid crystal display device. In this active matrix type liquid crystal display device, when an image signal is written to pixels arranged in a matrix, a plurality of address lines arranged in the row direction are sequentially scanned from top to bottom and connected to the scanned address lines. All the switching elements that have been turned on are turned on, and the signal from the signal line is written to the pixel electrode through the turned on switching elements. In this case, since all the switching elements in the same row connected to the same address line are turned on, it is necessary to give a desired signal to all the pixels arranged in the same row. That is, even when the same image is displayed in a certain field and the next field, the same image signal must be supplied to the signal line in both fields. This is because the liquid crystal cell in the conventional active matrix type liquid crystal display device has only the switching element 51, the pixel electrode 52, the counter electrode 53 and the liquid crystal layer 54 as basic constituent elements, as shown in FIG. For a pixel in which the switching element 51 is in the ON state, the potential of the pixel electrode 52 changes and the pixel electrode 52
This is also because some electric field is applied between the counter electrode 53 and the counter electrode 53.

As described above, in the conventional active matrix type liquid crystal display device, since it is necessary to write data in all pixels for each field, power consumption increases.

By the way, as a large-screen and high-definition liquid crystal display device, a display device using a ferroelectric liquid crystal is conventionally known. Since this ferroelectric liquid crystal display device has a bistable state and a memory property, it is possible to reduce the number of times of writing an image signal into a pixel, and as a result, it is possible to reduce power consumption. However, since the ferroelectric liquid crystal has bistability, it can basically perform only binary display and cannot be said to be very suitable for halftone display.

[0007]

As described above, in the conventional active matrix type liquid crystal display device,
Since it is necessary to write to all pixels in each field, there is a problem that power consumption increases.

Further, the ferroelectric liquid crystal display device can reduce power consumption, but has a problem that it is difficult to perform halftone display.

Therefore, it is an object of the present invention to provide a liquid crystal display device which is low in power consumption and can perform halftone display.

[0010]

A liquid crystal display device according to the present invention has a basic configuration including a switching element, a pixel electrode connected to the switching element, a counter electrode provided to face the pixel electrode, A ferroelectric thin film provided between the pixel electrode and the counter electrode, and a liquid crystal layer provided between the pixel electrode and the ferroelectric thin film or between the counter electrode and the ferroelectric thin film. By forming a unit cell and making the area of the pixel electrode and the counter electrode different in the unit cell, a unit display in which the electric field applied to the liquid crystal layer is defined by one pixel electrode and one counter electrode It is made non-uniform within the region.

In this case, since the area of the pixel electrode and the area of the counter electrode are different, when a voltage is applied between the pixel electrode and the counter electrode, the electric field increases from the central portion of one electrode toward the peripheral portion. Becomes weak. Further, in the ferroelectric thin film, a domain in which spontaneous polarization is directed in a certain direction is formed, and this domain is held by the hysteresis characteristic of the ferroelectric thin film. Further, the size of this domain is controlled by the size of the electric field applied to the ferroelectric thin film. Therefore, it is possible to reduce the number of times of rewriting display information due to the memory property of the ferroelectric thin film, and to perform gradation display by controlling the size of the domain formed in the ferroelectric thin film. Will be possible.

A liquid crystal display device having a large screen and high definition can be obtained by using the above basic structure and providing the unit cells in a matrix. In this case, gradation display can be obtained on the entire display screen by changing the electric field distribution in each unit display area according to the display image.

It is also possible to provide a plurality of the unit display areas adjacent to each other by using the above-mentioned basic structure and display by the interaction of the plurality of the adjacent unit display areas. In this case, the electric field distribution may be uniform or non-uniform within one unit display area.

In this case, the combined electric field between a plurality of adjacent unit display areas makes it possible to perform a smooth halftone display or a display in which the opposite contrast is emphasized.

With the above basic structure, a plurality of pixel electrodes may be connected to a single switching element.

In this case, since an electric field region having the same strength can be formed for each pixel in each unit display region corresponding to each pixel electrode, as a result, the amplitude of the signal voltage applied to the pixel electrode is reduced. It becomes possible to do. In addition, since domains with uniform polarization directions can be formed in a dispersed manner within one pixel, it is possible to perform uniform display.

Further, the liquid crystal display device according to the present invention comprises a switching element, a pixel electrode connected to the switching element, a counter electrode provided to face the pixel electrode, the pixel electrode and the counter electrode. A unit cell is composed of a liquid crystal layer using a ferroelectric liquid crystal provided between them, and by making the area of the pixel electrode and the counter electrode in the unit cell different, the electric field applied to the liquid crystal layer is made uniform. In the unit display area defined by the pixel electrode and the counter electrode.

Also in this case, like the liquid crystal display device having the ferroelectric thin film described above, the memory property of the ferroelectric liquid crystal can reduce the number of times of rewriting the display information, and the ferroelectric liquid crystal can be reduced. By controlling the size of the domain formed in, it is possible to perform gradation display.

[0019]

DETAILED DESCRIPTION OF THE INVENTION A first embodiment of the present invention will be described below with reference to the drawings.

FIGS. 1A to 1D are explanatory views showing the first embodiment, and show the constitution of a unit cell in a liquid crystal display device.

Reference numeral 10 is a switching element using a thin film transistor, 11 is a pixel electrode, 12 is a counter electrode facing the pixel electrode, 13 is a ferroelectric thin film provided between the pixel electrode 11 and the counter electrode 12, and 14 is a ferroelectric thin film. Between the pixel electrode 11 and the ferroelectric thin film 13 (see FIGS. 1B and 1C) or between the counter electrode 12 and the ferroelectric thin film 13 (see FIGS. 1A and 1D). The liquid crystal layer provided. The areas of the pixel electrode 11 and the counter electrode 12 that form one pixel are different from each other. In FIGS. 1A and 1C, the area of the pixel electrode 11 is smaller than the area of the counter electrode 12, and FIG. In () and (d), the area of the pixel electrode 11 is larger than the area of the counter electrode 12.

In this embodiment, the area displayed by the unit cell shown in FIG. 1 is the unit display area.
Make up a pixel. If this unit display area is defined more accurately, it means an area defined by one pixel electrode and one counter electrode (the same applies to other embodiments). ) Shows the state of the electric field distribution when a voltage is applied between the pixel electrode 11 and the counter electrode 12 in the above configuration. 2 (a) shows the state of the lines of electric force, and FIG. 2 (b) shows the state of FIG.
It shows an equipotential line as seen from the arrow X direction in (a). As can be seen from FIG. 2, the electric field gradually weakens as the distance from the center of the pixel electrode 11 increases. Although FIG. 2 shows the case of FIG. 1A,
The same applies to cases (b) to (d).

As shown in FIG. 1, a ferroelectric thin film 13 is arranged between the pixel electrode 11 and the counter electrode 12.
Therefore, due to the electric field shown in FIG.
In 3, the domain in which the spontaneous polarization is directed in a certain direction is formed. Then, the pixel electrode 11 and the counter electrode 1
The electric field generated between and will be retained by the formation of this domain. However, what should be noted here is that the strength of the electric field is not retained as it is, but converted into the size of the domain in the ferroelectric thin film 13 and retained. That is, if the electric field is strong (the voltage applied between the pixel electrode 11 and the counter electrode 12 is high), the domain becomes large, and conversely, if the electric field is weak, the domain becomes small.

On the other hand, in the liquid crystal layer 14, the size of the regions having different alignment states changes according to the size of the domain in the ferroelectric thin film 13. Therefore, this makes it possible to perform gradation display by the spatial modulation method. Also, during the period when no write operation is performed,
Information written in the pixel is retained as it is by making the potentials of the pixel electrode 11 and the counter electrode 12 the same, or by making these potentials such that the size of the domain in the ferroelectric thin film 13 does not change. It becomes possible to do.

FIG. 3 is a diagram showing changes in the state of spontaneous polarization with respect to the voltage of the ferroelectric thin film 13. As can be seen from the figure, the spontaneous polarization state change shows a hysteresis characteristic,
Two states (state 1 and state 2) whose polarization directions are opposite to each other are taken.

FIG. 4 is a diagram showing the waveform of each part when a thin film transistor is used for the switching element 10 of FIG. 1, and FIG. 4A shows a voltage signal (the voltage signal applied to the gate (address line) of the thin film transistor. Selection signal), FIG. 4 (b)
Shows the voltage signal (image signal) applied to the source (signal line) of the thin film transistor, and FIG. 4C shows the voltage applied between the pixel electrode and the counter electrode. Also, FIG.
(D) shows the state of the domain when different write signals are applied to the pixel electrodes.

The operation of FIG. 4 will be briefly described. The reset operation is performed in the first half (period t1) and the write operation is performed in the second half (period t2) of the period in which the gate is selected. That is, first, the display pixel is set to the state 1 (see FIG. 3) by the reset operation, then the image signal is input by the writing operation, and based on the size of the domain in the ferroelectric thin film 13 determined by the size of the image signal. Display gradation. Note that the writing operation may be performed after the display pixel is set to the state 2 (see FIG. 3) by the reset operation.

FIG. 5 shows an example in which the reset to the state 1 and the reset to the state 2 are combined. Vco
m is the counter electrode potential, and Vsig is the pixel electrode potential. In the first field, resetting to the state 1 is performed in the period t1, so that Vsig-Vcom <Vsatn. Note that Vsatn is the state 1 shown in FIG.
Is the saturation potential of. Then, in the period t2, the image signal is input to perform gradation display. In the next field,
Since the state 2 is reset in the period t3, Vsig-Vcom> Vsatp. Note that Vsatp is the state 2 shown in FIG.
Is the saturation potential of. Then, in the period t4, the image signal is input and gradation display is performed.

Next, regarding the alignment state of the liquid crystal layer, an electric field is applied to the liquid crystal layer because a voltage is generated with respect to the counter electrode in both state 1 and state 2. Therefore, the gradation level of display can be made the same when the domain of state 1 spreads over the entire display area and when the domain of state 2 spreads over it. In this case, the directions of the electric fields are opposite to each other, so that the liquid crystal can be AC-driven by periodically combining the two. When AC driving is performed in this way, the state where no electric field is applied to the liquid crystal layer is when the domain of state 1 and the domain of state 2 have the same area, and normally white displays white in such a state. Will be done.

As a method different from the above, state 1
Alternatively, there is a method in which any one of the states 2 is set to a state in which an electric field is not applied to the liquid crystal layer. This method will be described by taking the configuration of FIGS. 1A and 1D as an example. For example, when the state 1 is set to a state in which no electric field is applied to the liquid crystal layer, the potential of the ferroelectric thin film 13 is shifted to the common side after writing the image signal by the variation of the potential generated by forming the state 1. Good. In this case, an electric field is applied to the liquid crystal layer corresponding to the domain in the state 2 in the form of the fluctuation of the potential caused by the formation of the state 1 conversely. In addition to this, as a method of shifting the variation of the potential, a method of providing the variation of the potential from the common electrode side and a method of providing the variation of the potential from the pixel electrode side are considered depending on the structure.

In this embodiment, since the area of the pixel electrode 11 and the area of the counter electrode 12 are different from each other, when a voltage is applied between the pixel electrode 11 and the counter electrode 12, the area from the center of one electrode to the periphery is reduced. The electric field becomes weaker toward the department. Further, domains are formed in the ferroelectric thin film 13, and the domains are held by the hysteresis characteristics of the ferroelectric thin film 13. Further, the size of this domain is controlled by the size of the electric field applied to the ferroelectric thin film. Therefore, due to the memory property of the ferroelectric thin film 13, it is only necessary to rewrite the data in a necessary area, and it is possible to reduce the number of times of rewriting the display information. Further, by controlling the size of the domain formed in the ferroelectric thin film 13, gradation display can be performed.

Next, a second embodiment of the present invention will be described.

In this embodiment, the unit cells described in the first embodiment are arranged in a matrix. First
As already described in the above embodiment, by providing the ferroelectric thin film and by making the areas of the pixel electrode and the counter electrode different, it is possible to perform gray scale display in the unit display region. Therefore, by arranging such unit display areas in a matrix and changing the electric field distribution in each unit display area according to the display image, gradation display can be obtained on the entire display screen.

FIG. 6 is a diagram showing a concrete circuit configuration example in the present embodiment. FIG. 6A is a diagram showing the configuration of the entire system, and FIG. 6B is a diagram showing a part of the configuration of the liquid crystal panel 21 of FIG. 6A.

Reference numeral 21 is a liquid crystal panel in which unit display areas (unit cells) as shown in FIG.
Is an address line driver for supplying a control signal to the switching element in the liquid crystal panel 21 via the address line 23,
4 is a signal line driver for supplying image signals to the pixel electrodes in the liquid crystal panel 21 via the signal line 25, 26 is an address line counter for sequentially selecting address lines, and 27 is a selection signal for turning on the switching elements. It is a selection signal generating circuit for generating. 28 is a voltage level shift line 29
It is a voltage level shift circuit for shifting the potential of the ferroelectric thin film 13 via the.

In this embodiment, the voltage level shift circuit 28 is provided as described above. However, as described in FIG. 5 and its corresponding description, the polarities of the state 1 and the state 2 are opposite to each other, and the pixel electrode When driving so that the voltage between the common electrode and the common electrode becomes equal, it is not necessary to provide the voltage level shift circuit 28. The voltage level shift circuit 28 is required when a driving method is adopted in which the voltage is shifted so that the voltage is not applied between the pixel electrode and the common electrode in the state 1 or the state 2.

In the example shown in FIG. 6, one of the address lines 23 is
After the line writing is completed, the voltage level shift is performed for each line. Therefore, the ferroelectric thin film is common to the pixel electrodes for one horizontal row. Even if they are common, the polarization state of the ferroelectric thin film is maintained for each unit display area.

FIG. 7 is a diagram showing signal waveforms of respective parts. (A) is a gate voltage waveform supplied to the gate of the switching element through the address line 23, and (b) is supplied to the pixel electrode through the signal line 25. Signal line voltage waveform,
(C) shows the ferroelectric thin film 1 from the voltage level shift circuit 28.
3 is a voltage waveform of the voltage level shift line supplied to No. 3, and (d) is a voltage waveform applied between the pixel electrode and the counter electrode.

In the present embodiment, the ferroelectric thin film is commonly used for each address line 23, and the voltage level is shifted line by line after the writing of one line of the address line 23 is completed. The body thin film may be shared on the entire surface of the liquid crystal panel 21, and the voltage level shift may be performed after all the pixels of the liquid crystal panel 21 are written. In this case, the writing period of the image signal is sufficiently shorter than the period in which the voltage is level-shifted and held.

Next, a third embodiment of the present invention will be described.

In this embodiment, a plurality of unit display areas obtained by the unit cells as shown in FIG. 1, for example, are provided adjacent to each other, and display is performed by the interaction (composite electric field) of the plurality of adjacent unit display areas. It is a thing. The electric field distribution may be uniform or non-uniform within one unit display area.

The unit cell configuration, operating principle, driving method, and the like in this embodiment can be diverted from those described in the first embodiment.
Description is omitted.

When gradation display is performed, the states of the domains in the ferroelectric thin film may differ even if the gradation is the same (see, for example, FIG. 5). In such a case, by controlling the state of the domains between the adjacent unit display areas, it is possible to perform a smooth halftone display or a display in which the opposite contrast is emphasized.

FIG. 8 shows an example of forming domains in this embodiment. In FIG. 8A, although the direction of spontaneous polarization is changed between the adjacent unit display regions, by making the area of the domain of each unit display region equal,
A smooth halftone display can be performed. In this case, one pixel may be configured by the nine unit display areas shown in the figure. In addition, in FIG.
By biasing the unit display areas in which the directions of spontaneous polarization are aligned, it is possible to perform display with enhanced contrast.

By using the combined electric field between the adjacent unit display areas, it is possible to perform display even at the boundary between the adjacent unit display areas. More specifically, when the electric fields of the unit display areas adjacent to each other at the boundary portion are oriented in the same direction, the electric field is strengthened by the combined electric field, and conversely, when the electric fields are oriented in the opposite direction. The electric field will be weakened by the combined electric field. For the same reason, when a plurality of display panels are joined together, an electric field can be formed even in the joined region.

Next, a fourth embodiment of the present invention will be described.

In this embodiment, one pixel (one unit cell)
In addition, at least two or more pixel electrodes are formed, and these two or more pixel electrodes are controlled by the same switching element. In this embodiment,
An area displayed corresponding to each pixel electrode is a unit display area.

As for the operation principle or driving method in the unit display area in this embodiment, the one explained in the first embodiment can be diverted.
Description is omitted.

FIG. 9 shows the structure of a cell for one pixel in this embodiment. In one pixel 44 (one unit cell), a plurality of pixel electrodes 45 (represented by a domain corresponding to each pixel electrode in FIG. 6B) are arranged (the pixel electrode 45 is a ferroelectric thin film (not shown). The pixel electrodes 45 are not electrically connected to the counter electrode (not shown) and an electric field is not generated between them.
Interconnected by Then, all the pixel electrodes 45 in one pixel 44 are controlled by the same switching element 41.

With the above-described structure, it is possible to form an electric field region of equal strength in each unit display region (region obtained corresponding to each pixel electrode), and as a result, it is applied to the pixel electrode. It is possible to reduce the amplitude of the signal voltage. Further, even in the case of performing gradation display, domains with uniform polarization directions can be dispersedly formed in one pixel, and thus it is possible to perform uniform display.

Although FIG. 10 shows another pixel electrode structure, it is possible to perform a display with less jagged feeling by configuring it so that the spatial frequency becomes higher.

Next, a fifth embodiment of the present invention will be described.

The basic structure of this embodiment can be regarded as using a ferroelectric liquid crystal in place of the ferroelectric thin film in the first embodiment. That is, it can be considered that the ferroelectric thin film 13 is removed in FIG. 1 and the ferroelectric liquid crystal is used for the liquid crystal layer 14. Therefore, the configuration, operating principle, driving method, and the like of the unit cell can be the same as those described in the first embodiment, and the description thereof will be omitted. Further, since the basic operation and effect are also the same as those in the first embodiment, the description will be omitted.

By the way, in the present embodiment, it is also possible to leave the ferroelectric thin film 13 in FIG. 1 as it is and use a ferroelectric liquid crystal for the liquid crystal layer 14, which will be described below. The case will be described.

When the ferroelectric thin film 13 is not provided, a reversal current occurs due to insufficient writing due to the memory property of the ferroelectric liquid crystal in the holding period after the voltage is applied to the pixel electrode 11. This depends on the response speed of the ferroelectric liquid crystal and is influenced by the display image in the previous field. Therefore, in order to reduce the influence of such a reversal current, the ferroelectric thin film 13 is used to increase the auxiliary capacitance, and the ferroelectric thin film 13 is made of a material having a high responsiveness.

FIG. 11A shows a voltage waveform of the pixel electrode when a reversal current is generated, and FIG. 11B shows a voltage waveform of the pixel electrode when the above ferroelectric thin film is used.
Note that the period t1 shows a writing period and the period t2 shows a holding period.

[0057]

According to the invention of claim 1 of the present application, the electric field applied to the liquid crystal layer is changed between one pixel electrode and one counter electrode by making the areas of the pixel electrode and the counter electrode in the unit cell different. That is, it is made non-uniform within the unit display area defined by.

Therefore, since the electric field distribution in the unit display area becomes non-uniform, halftone display can be performed, and the electric field is generated by the hysteresis characteristic (memory characteristic) of the domain formed in the ferroelectric thin film. Since the distribution is maintained, the number of times of rewriting image information can be reduced. Therefore, it is possible to obtain a liquid crystal display device which can perform halftone display with low power consumption.

By using the ferroelectric thin film,
It is also possible to achieve the effect of flattening the interface with the liquid crystal layer.

In the invention according to claim 2 of the present application, a plurality of unit display areas are provided adjacent to each other, and display is performed by the interaction of the plurality of adjacent unit display areas. The electric field makes it possible to perform smooth halftone display or display with enhanced contrast.

In the invention according to claim 3 of the present application, since a plurality of pixel electrodes are connected to a single switching element, it is possible to form an electric field region having a certain strength or more for each unit display region. As a result, the amplitude of the signal voltage applied to the pixel electrode can be reduced. In addition, since domains with uniform polarization directions can be formed in a dispersed manner within one pixel, it is possible to perform uniform display.

[Brief description of drawings]

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

FIG. 2 is a diagram showing an electric field distribution in a liquid crystal cell according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a hysteresis characteristic and a domain state of a ferroelectric thin film according to the first embodiment of the present invention.

FIG. 4 is a diagram showing an example of a method for driving a liquid crystal cell according to the first embodiment of the present invention.

FIG. 5 is a diagram showing another example of the driving method of the liquid crystal cell according to the first embodiment of the present invention.

FIG. 6 is a diagram showing an overall configuration of a system of a liquid crystal display device and a part of a liquid crystal panel according to a second embodiment of the present invention.

7 is a diagram showing an example of a driving method of the liquid crystal display device of FIG.

FIG. 8 is a diagram showing a state of domains formed in a ferroelectric thin film according to a third embodiment of the present invention.

FIG. 9 is a diagram showing an example of a cell configuration and a state of a domain (pixel electrode shape) formed in a ferroelectric thin film according to a fourth embodiment of the present invention.

FIG. 10 is a diagram showing another example of a domain state (pixel electrode shape) according to the fourth embodiment of the present invention.

FIG. 11 is a diagram showing a voltage waveform of a pixel electrode according to the fifth embodiment of the present invention.

FIG. 12 is a diagram showing a configuration of a liquid crystal cell according to a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Switching element 11 ... Pixel electrode 12 ... Counter electrode 13 ... Ferroelectric thin film 14 ... Liquid crystal layer

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G09G 3/36 G09G 3/36

Claims (3)

[Claims] 1. A switching element, a pixel electrode connected to the switching element, a counter electrode provided so as to face the pixel electrode, and a ferroelectric thin film provided between the pixel electrode and the counter electrode. And a liquid crystal layer provided between the pixel electrode and the ferroelectric thin film or between the counter electrode and the ferroelectric thin film, and a unit cell is formed. A liquid crystal display device, wherein the electric field applied to the liquid crystal layer is made non-uniform within a unit display area defined by one pixel electrode and one counter electrode by making the areas of the electrodes different. 2. A liquid crystal display device, wherein a plurality of unit display areas according to claim 1 are provided adjacent to each other, and display is performed by interaction of the plurality of adjacent unit display areas. 3. The liquid crystal display device according to claim 1, wherein a plurality of pixel electrodes are connected to a single switching element.