JP3382221B2 - Liquid crystal display - Google Patents

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 an active matrix type liquid crystal display device.

[0002]

2. Description of the Related Art Liquid crystal display devices (LCDs) are widely used in large wall televisions, notebook computers, mobile phones, etc. because of their large size, thinness, light weight, high definition and low power consumption. Demand is expected to increase further. Under such circumstances, particularly for portable information equipment applications, long-term battery driving is essential, and a liquid crystal display device with lower power consumption is desired.

FIG. 17 is a diagram showing a configuration example of a conventional active matrix type liquid crystal display device using a thin film transistor (TFT). In the display area 511, the unit pixels 512 are arranged in a matrix corresponding to each intersection of the plurality of scanning lines 513 and the plurality of signal lines 514.
The scan line (gate line) 513 is provided to the scan line driver circuit 515,
The signal line 514 is connected to the signal line driver circuit 516. The scan line driver circuit 515 supplies a scan signal (gate signal) to the scan line 513 at a frequency of 60 Hz, and a pixel signal (display signal) supplied from the signal line 514 through a thin film transistor connected to the selected scan line 513. signal)
Is applied to the liquid crystal layer. Since image sticking occurs in liquid crystal when a DC voltage is continuously applied, AC is usually used for image signals.

FIG. 18 shows the unit pixel 512 shown in FIG.
3 is a diagram showing an equivalent circuit of FIG. The thin film transistor 531 is connected to the intersection of the scan line 513 and the signal line 514, and the auxiliary capacitor 535 is connected in parallel to the liquid crystal layer 532 sandwiched between the pixel electrode 533 and the counter electrode 534.

Considering the power consumption P by the signal line drive circuit of the liquid crystal display device, the signal line load capacitance is C, the number of gate lines is n, the frequency for refreshing the signal (refresh frequency) is f _ref , and the image signal is When the voltage is V, it is expressed as P = Cnf _ref V ² (1).

In order to reduce the power consumption, from equation (1),
It is necessary to reduce any of the signal line load capacitance C, the number of gate lines n, the refresh frequency f _ref , and the image signal voltage V. Among them, C is composed of a capacitance between the signal line and the counter electrode, and n is the display definition. Therefore,
It is difficult to significantly reduce these values without reducing the display quality such as contrast and definition. Further, the image signal voltage V needs to have a certain value or more in order to perform gradation display by using a liquid crystal material, and reducing this leads to a reduction in contrast.

On the other hand, the refresh frequency fref
Can be smaller than the frame frequency f _fr as long as the required image signal can be displayed. Usually, the refresh frequency f _ref is equal to the frame frequency f _fr ,
Reducing the refresh frequency f _ref in the case of a moving image leads to a decrease in responsiveness, but in the case of a still image in which the image signals are the same over a plurality of frames, lowering the refresh frequency reduces power consumption. It is possible. However, in order to realize this, a mechanism for holding an image signal for a long time, that is, a memory property is required.

As a liquid crystal display device having a memory property,
A type using a liquid crystal material having a memory property such as a ferroelectric liquid crystal and a cholesteric liquid crystal, and a type in which a circuit for giving a memory property is provided in each pixel are well known. However, the former has a problem that it is difficult to perform multi-gradation display and a problem that the strength against impact is weak. Further, with regard to the latter, there is a problem that it is difficult to make the pixel structure finer and it is disadvantageous for high definition.

As a liquid crystal display device that solves the above-mentioned problems, as shown in FIG. 19, a ferroelectric capacitor 536 using a ferroelectric thin film layer is formed in each pixel to improve the memory property of the ferroelectric substance. A method of displaying by using the method has been proposed. Since this method uses the memory property of the ferroelectric substance itself, there is no restriction on the liquid crystal material, and twisted nematic (TN) liquid crystal which has been widely used from the past can be used. Therefore, multi-gradation display is possible, and since it does not have a complicated circuit structure, high definition can be supported. However, in this method, since the pixel structure is the serial capacitance structure of the ferroelectric substance and the liquid crystal, there is a problem that the magnitude of the image signal voltage applied to the pixel becomes larger than the conventional one. Hereinafter, this problem will be described.

FIG. 20 shows an image signal waveform (FIG. 20) of the liquid crystal display device having the structure shown in FIG.
20 (a)) and the scanning signal waveform (FIG. 20 (b)). Driving is performed in the following sequence. First,
In the reset period Tr, signals are supplied to all scanning lines to turn on all thin film transistors, and a signal Vr for resetting the polarization of the ferroelectric substance is applied from each signal line (reset). Next, in the writing period Tw, the image signal Vw is written (writing) in each pixel while performing line-sequential scanning. Finally, in the holding period Th, signals are supplied to all the scanning lines similarly to the resetting, and the holding signal voltage Vh for holding the display state is applied (holding) while the thin film transistors are turned on.

The refresh frequency f _ref becomes f _ref = 1 / (Tr + Tw + Th) (2) due to the reset period Tr, the writing period Tw and the holding period Th.

Here, the partial pressure Vf of the ferroelectric layer and the partial pressure Vlc of the liquid crystal layer are calculated. When the pixel signal voltage is Vsig, the charge of the liquid crystal layer is Qlc, the charge of the ferroelectric layer is Qf, and the hysteresis characteristic function of the ferroelectric is F (Vf), then Vf + Vlc = Vsig (3) Qf = F (Vf (4) Qlc = Clc.Vlc = -Clc (Vf-Vsig) (5). Also, the relation of Qf = Qlc (6) holds.

By solving the above simultaneous equations, the partial pressures of the liquid crystal layer and the ferroelectric layer can be obtained. In addition, this is a coordinate system in which the partial voltage Vf of the ferroelectric substance is plotted on the horizontal axis and the electric charge Qf is plotted on the vertical axis. It is also possible to solve.
Hereinafter, equation (5) will be referred to as a load straight line.

FIG. 21 shows a hysteresis characteristic of a ferroelectric substance and a load line in three states of reset, write and hold. Considering the above formula,
Image signal voltage Vsig = Vr at reset, image signal voltage Vsig = Vw at writing, ferroelectric voltage at writing (voltage applied to ferroelectric layer) Vfw, liquid crystal output voltage at holding (to liquid crystal layer) It can be seen that the applied voltage) Vlc = Vout is obtained as the amount shown in each figure.

Referring to FIG. 21, the voltage Vr at reset,
It can be seen that the writing voltage Vw and the liquid crystal output voltage Vout are greatly affected by the liquid crystal capacitance Clc. For example, if the liquid crystal capacitance Clc is increased, the absolute value of the slope of the load capacitance is increased, so that the voltages Vr and Vw are reduced, but at the same time, the liquid crystal output voltage Vout during holding is reduced. Conversely, if the liquid crystal capacitance Clc is reduced, the liquid crystal output voltage Vout at the time of holding is increased, but the voltages Vr and Vw are increased.
Will become bigger.

Next, the signal voltage required at the time of writing is estimated using the formula. As shown in FIG. 22, the hysteresis characteristic function F (Vf) of the ferroelectric substance is assumed to be a parallelogram model. Vc is a threshold voltage for polarization reversal of the ferroelectric substance and is called a coercive voltage.

A straight line a in FIG. 22 is expressed as Qf = F (Vf) = CfVf + Qr (7). Since the load curve b is expressed by the equation (5), the signal voltage Vsig and the liquid crystal voltage Vlc are expressed by the equations (3), (5), (6) and (7) as follows: Vsig = ((Cf + Clc) Vf + Qr ) / Clc (8) Vlc = (CfVsig + Qr) / (Cf + Clc) (9)

The signal voltage Vsig necessary for polarization reversal is considered to be when the ferroelectric voltage Vf is equal to Vc. Considering this, the signal voltage Vw at the time of writing is Vw = ((Cf + Clc) Vc + Qr) / Clc (10). When the bias voltage during holding is equal to the coercive voltage Vc, the liquid crystal output voltage Vout during holding is Vout = (CfVc + Qr) / (Cf + Clc) (11) from the equation (9). When Qr is eliminated from the equations (10) and (11), Vw = ((Cf + Clc) (Vc + Vout) -CfVc) / Clc (12).

Here, the signal voltage Vw at the time of writing is redefined with reference to the bias voltage, and this is defined as the input voltage ΔV.
If w, then ΔVw = Vw−Vc = ((Cf + Clc) / Clc) Vout (13).

As a result, a large image signal voltage represented by the equation (13) is required to obtain the voltage required to drive the liquid crystal. In the case of a still image, if the contribution of lowering the refresh frequency is larger than the contribution of increasing the image signal voltage, it can be seen from equation (1) that the power consumption is lower. On the other hand, in the case of a moving image, it is difficult to lower the refresh frequency without lowering the display quality, so the image signal voltage increases,
On the contrary, power consumption increases.

[0021]

As described above, in the liquid crystal display device utilizing the memory property of the ferroelectric substance, the image signal voltage Vsig needs to be higher than the liquid crystal output voltage Vout applied to the liquid crystal layer. However, there is a problem that power consumption increases when displaying a moving image in which the refresh frequency cannot be lowered.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a liquid crystal display device capable of lowering a driving voltage when displaying a moving image and reducing power consumption. And

[0023]

A liquid crystal display device according to the present invention is provided corresponding to an intersection of a plurality of scanning lines provided in a horizontal direction and a plurality of signal lines provided in a vertical direction, A liquid crystal display device having a plurality of pixel portions each including a liquid crystal layer sandwiched between a pixel electrode and a counter electrode, wherein the pixel portion includes a ferroelectric capacitor having a first end connected to the pixel electrode, and a first capacitor. An end is connected to the signal line, a second end is connected to a second end of the ferroelectric capacitor, and a first switching element controlled by a scanning signal from the scanning line and the ferroelectric capacitor are provided. And a second switching element connected in parallel.

According to the present invention, since the second switching element is connected in parallel with the ferroelectric capacitor,
When the switching element is turned off (off state), the capacitance component of the ferroelectric capacitance is equivalently connected in series to the liquid crystal part, and the second switching element is turned on (on). State), the ferroelectric capacitor is equivalently short-circuited by the second switching element. Therefore, when displaying a still image, the second
It is possible to apply most of the image signal voltage to the liquid crystal layer at the time of displaying a moving image by turning on the second switching element while displaying the moving image while turning off the switching element of The driving voltage at the time of displaying a moving image can be lowered, and the power consumption can be reduced.

In the above invention, the second switching element is turned on when the display signal is different between consecutive display frames based on the comparison result of the display signals supplied to the pixel section between consecutive display frames. By further providing the control unit for controlling the state, it is possible to easily discriminate between the still image display and the moving image display, and it is possible to automatically perform the accurate control of the second switching element.

Further, in the above invention, the control unit is
It is possible to control ON / OFF of the second switching element in units of a plurality of pixel units connected to the same scanning line, and when the display signal supplied to a certain pixel unit is different between consecutive display frames, It is preferable that the second switching element included in the pixel portion and another pixel portion connected to the same scan line as the pixel portion be selectively turned on. In this way, by controlling the second switching element for each same scanning line, it is possible to perform still image display and moving image display in a single screen in a mixed manner on a scanning line basis.

Further, in the above invention, by supplying the pixel section with a display signal having a smaller amplitude when the second switching element is in an on state than when the second switching element is in an off state, an image signal for displaying a moving image is displayed. The voltage becomes lower than that when displaying a still image, and it is possible to reduce power consumption.

[0028]

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

(Embodiment 1) First, a basic embodiment of the present invention will be described. FIG. 1 is a diagram showing a configuration example of an active matrix type liquid crystal display device according to an embodiment of the present invention.

The basic structure is similar to that of the prior art shown in FIG. That is, in the display area 11,
The unit pixels 12 are arranged in a matrix corresponding to respective intersections of the plurality of scanning lines 13 provided in the horizontal direction and the plurality of signal lines 14 provided in the vertical direction, and the scanning lines (gate lines) are arranged. Reference numeral 13 is connected to the scanning line drive circuit 15, and signal line 14 is connected to the signal line drive circuit 16. In the present embodiment, a switch line (conduction control line) 17 is connected to each unit pixel 12, and a predetermined selection signal is supplied from the switch line drive circuit 18 to each unit pixel 12 via this switch line 17. It is supposed to be done.

FIG. 2 is a diagram showing an equivalent circuit of the unit pixel 12 shown in FIG.

A thin film transistor (main transistor) 31 is arranged at the intersection of the scanning line 13 and the signal line 14, and the gate electrode of the thin film transistor 31 is the scanning line 13.
The source electrode of the thin film transistor 31 is the signal line 14
, Respectively. One end of a ferroelectric capacitor 32 using a ferroelectric thin film is connected to the drain electrode of the thin film transistor 31, and the pixel electrode 34 is connected to the other end of the ferroelectric capacitor 32. The counter electrode 35 is arranged at a position facing the pixel electrode 34 with the liquid crystal layer 33 interposed therebetween, and a potential difference between the potential of the pixel electrode 34 and the potential of the counter electrode 35 is applied to the liquid crystal layer 33.

Further, a thin film transistor (parallel transistor) 36 is connected in parallel to the ferroelectric capacitor 32, and its conduction state (ON / ON) is established by the switch line 17 connected to the gate electrode of the thin film transistor 36.
Off) is controlled. That is, in the parallel circuit including the ferroelectric capacitor 32 and the thin film transistor 36, the on-resistance component of the thin film transistor 36 is substantially dominant when the thin film transistor 36 is in the conductive state (on state), and the thin film transistor 36 is in the non-conductive state (off state). 2), the capacitance component of the ferroelectric capacitor 32 becomes substantially dominant.

Although any liquid crystal material can be used as the liquid crystal material of the liquid crystal layer 33, typically, a twisted nematic (TN) type liquid crystal, a super twisted nematic (STN) type liquid crystal, a cholesteric type liquid crystal, Guest-host type liquid crystal, polymer dispersed liquid crystal and the like can be used. As the nematic liquid crystal, for example, a fluorine nematic liquid crystal or a cyano nematic liquid crystal can be used.

Next, the driving method of this embodiment will be described.

The drive waveform differs between the moving image mode and the still image mode. In terms of time, each frame time (T _fr =
Every 1 / f _fr = 16.7 ms), it is judged which mode is the moving image mode or the still image mode. Also,
In terms of space, it is determined whether the mode is the moving image mode or the still image mode for the entire display area or for each scanning line. Whether to select the moving image mode or the still image mode may be fixed by a switch or the like. Alternatively, a display signal for one frame or several frames may be stored in the frame memory so that two or more consecutive frames may be displayed. If the image signal (display signal) is the same, it is judged as a still image,
The moving image mode may be switched to the still image mode. On the contrary, when the image signals are different between two or more consecutive frames, it may be determined as a moving image and the still image mode may be switched to the moving image mode.

Next, an example of the driving method of this embodiment will be described with reference to the timing charts shown in FIGS. FIG. 3 is a timing chart in the still image mode, and FIG. 4 is a timing chart in the moving image mode. The signal waveforms shown in FIGS. 3 and 4 correspond to the image signal Vsi supplied to the thin film transistor 31 via the signal line 14.
g, a scanning signal (gate signal) Vg supplied to the thin film transistor 31 via the scanning line (gate line) 13, and a switch signal Vsw supplied to the thin film transistor 36 via the switch line 17, respectively.

As shown in FIG. 3, in the still image mode,
The switch signal Vsw is always in the off state, and the image signal waveform Vsig and the gate signal waveform Vg have the same waveforms as those in the conventional technique as shown in FIGS.

Specifically, first, in the reset period Tr, a signal Vr for supplying a signal to all the scanning lines 13 in the still image mode to turn on the thin film transistor 31 and resetting the polarization of the ferroelectric capacitor 32 is set. Signal line 14
From the thin film transistor 31 (reset). Next, in the writing period Tw, the scanning line 13 in the still image mode is sequentially selected, and the image signal Vw is sequentially written to each unit pixel from the signal line 14 through the thin film transistor 31 (writing). After that, in the holding period Th,
A signal is supplied to all the scanning lines 13 in the still image mode to turn on each thin film transistor 31, and a holding signal voltage Vh for holding the pixel state is supplied to each unit pixel from the signal line 14 via the thin film transistor 31. (Retention).

Here, the refresh frequency f _ref is determined from the equation (2) by the reset period Tr, the writing period Tw and the holding period Th. The frame frequency must be an integral multiple of the refresh frequency, and the sum of the reset period Tr and the write period Tw is one frame period Tfr.
It is shorter than (1 / f _fr = 16.7 ms). Therefore, in the still image mode, the power consumption is almost the same as that of the liquid crystal display device using the conventional ferroelectric thin film.

On the other hand, as shown in FIG. 4, in the moving image mode, the driving method is the same as that of the conventional liquid crystal display device as shown in FIGS. So in video mode,
The switch signal Vsw is always on and the display signal Vsi
g is rewritten every one frame period Tfr. First,
In the still image mode, the gate line is unselected for the time corresponding to the reset period. Next, the gate lines in the moving image mode are sequentially selected, and the image signal Vw ′ is written in each pixel. After one frame period, the same gate line is selected again and a new image signal is written in each pixel. Vou in still image mode
In order to apply the voltage of t to the liquid crystal layer, it is necessary to use the voltage represented by Vw of the formula (12), but in the moving image mode, Vou
A voltage Vw '(Vw'<Vw) equal to t may be used.

As described above, in this embodiment, since no voltage is applied to the capacitor formed of the ferroelectric thin film when displaying a moving image, the input signal voltage can be reduced, and the voltage can be lowered as compared with the conventional liquid crystal display device. Therefore, a liquid crystal display device with low power consumption can be obtained.

(Second Embodiment) This embodiment is the same as the first embodiment.
The present invention relates to a more specific structure of the liquid crystal display device having the structure shown in FIG. 5 is a plan view showing a configuration example of a unit pixel region of the liquid crystal display device according to the present embodiment, and FIG. 6 is a cross-sectional view taken along the line AB of FIG. In addition,
The equivalent circuit of this embodiment is almost the same as that shown in FIG.

The steps of manufacturing a substrate (array substrate) on which thin film transistors and the like are formed are as follows.

First, 200 nm of SiO _x was deposited on the alkali-free glass substrate 101a by the CVD method to form the undercoat layer 102. Next, Ti was formed in a thickness of 100 nm and then Pt was formed in a thickness of 200 nm by a sputtering method.
Then, lead zirconate titanate (PZ
T) A crystalline film having a thickness of 1 μm was formed. Further, a Pt film having a thickness of 200 nm was formed by the sputtering method. Then, the Pt film was etched to form a pattern of the ferroelectric capacitor upper electrode 105. Subsequently, the PZT film was etched to form an island pattern of the ferroelectric thin film 104. Further, the Pt / Ti layer is etched to form the base electrode pattern 1
03 was formed. In this way, a ferroelectric capacitor was first formed. As the ferroelectric material, it is possible to use an organic ferroelectric material as well as an inorganic ferroelectric material such as a lead oxide ferroelectric material or a bismuth layer structure oxide ferroelectric material.

Next, a MoW alloy film is formed to a thickness of 300 nm,
This was patterned to form the gate electrode 106a and the gate wiring of the main thin film transistor, the gate electrode 106b and the switch wiring of the parallel thin film transistor. In addition to MoW, a high melting point metal material such as MoTa alloy, Au, or Cu can be used.

Next, a SiO _x film of 350 nm and a SiN _x film are formed.
A gate insulating film 107 having a film thickness of 50 nm is formed, and further, an amorphous Si film 50 nm and SiN _x 200 nm are formed.
Was formed. Each of these films was continuously formed using the CVD method. Subsequently, SiN _x was patterned to form channel stopper layers 109a and 109b. Further, the amorphous Si layer was patterned to form the channel layers 108a and 108b.

Next, Mo film 100 nm and Al film 300 n
m are continuously formed and patterned to form the source and drain electrodes 110a and 11a of the thin film transistor.
0b, 110c and 110d were formed.

Next, a photosensitive resin layer 111 having a thickness of 3 μm was formed. At this time, the photosensitive resin layer 11 is exposed by mask exposure.
The uneven pattern layer 112 was formed on the surface of No. 1. continue,
This photosensitive resin layer was patterned by etching to form a passivation layer. After that, an Al film was formed to a thickness of 300 nm by a sputtering method, and this was patterned to form a pixel electrode 113. Furthermore, a polyimide film
nm to form an alignment film 114a.

Next, a substrate (counter substrate) on which a counter electrode and the like are formed was manufactured as follows. Array substrate 101
A chrome film was formed on the alkali-free glass substrate 101b similar to that of a, and this was patterned to form the black matrix layer 118. Then, the color filter layer 117
Was formed. Further, an ITO film to be the counter electrode 116 was formed to a thickness of 100 nm. Finally, a polyimide film having a thickness of 10 nm was formed as the alignment film 114b.

The array substrate manufactured as described above and the counter substrate are opposed to each other, and a spacer 119 is sandwiched between the both substrates so as to be evenly spaced by 5 μm with a seal. A liquid crystal layer 115 was injected. Thus, the cell of the liquid crystal display device was completed.

Finally, the scanning line drive circuit, the signal line drive circuit and the switch line drive circuit were arranged around the display area, and a predetermined signal was sent to the signal line, the scanning line and the switch line to realize the display operation.

As the main thin film transistor and the parallel thin film transistor, in addition to the amorphous silicon TFT whose channel layer is made of amorphous silicon, a polysilicon TFT whose channel layer is made of polycrystalline silicon may be used. When using a polysilicon TFT, it is possible to simultaneously form the scanning line drive circuit, the signal line drive circuit, and the switch line drive circuit around the display area when forming the TFT in the display area on the array substrate. Is.

(Embodiment 3) FIG. 7 is a plan view showing a structural example of a unit pixel region of a liquid crystal display device according to this embodiment, and FIG. 8 is a sectional view taken along the line AB of FIG. . In addition,
The basic equivalent circuit configuration is the same as that shown in FIG.

In the present embodiment, as a thin film transistor connected in parallel to a ferroelectric capacitor, an electrolytic mobility transistor (Metal-Ferroelectric-Semiconductor Field Effect T) having a gate structure of metal-ferroelectric-semiconductor is used.
ransistor: MFS-FET) is used.

The steps of manufacturing a substrate (array substrate) on which thin film transistors and the like are formed are as follows.

First, 200 nm of SiO _x was deposited on the non-alkali glass substrate 201a by the CVD method to form the undercoat layer 202. Next, Ti was formed in a thickness of 100 nm and then Pt was formed in a thickness of 200 nm by a sputtering method.
Then, lead zirconate titanate (PZ
T) A crystalline film having a thickness of 1 μm was formed. Further, a Pt film having a thickness of 200 nm was formed by the sputtering method. Then, the Pt film was etched to form a pattern of the ferroelectric capacitor upper electrode 205. Subsequently, the PZT film was etched to form an island pattern of the ferroelectric thin film portion 204a of the MFS-FET and the ferroelectric thin film 204b serving as the capacitance of the ferroelectric capacitor. Further, the Pt / Ti layer was etched to form a gate electrode portion 203a of the MFS-FET and a base electrode pattern 203b of the ferroelectric capacitor.

In this way, first, the ferroelectric thin film capacitor was formed, and the gate electrode portion and the ferroelectric thin film portion of the MFS-FET were formed. As the ferroelectric material, it is possible to use an organic ferroelectric material as well as an inorganic ferroelectric material such as a lead oxide ferroelectric material or a bismuth layer structure oxide ferroelectric material.

Next, a MoW alloy film is formed to a thickness of 300 nm,
This was patterned to form a gate wiring, a gate electrode 206a of a thin film transistor, and a switch wiring. The switch wiring was connected to the gate electrode 203a of the MFS-FET. In addition to MoW, MoTa alloy, A
A refractory metal material such as u or Cu can be used.

Next, a SiO _x film of 350 nm and a SiN _x film are formed.
A gate insulating film 207 having a film thickness of 50 nm is formed, and further, an amorphous Si film 50 nm and SiN _x 200 nm are formed.
Was formed. Each of these films was continuously formed using the CVD method. Subsequently, SiN _x was patterned to form channel stopper layers 209a and 209b. Further, the amorphous Si layer was patterned to form channel layers 208a and 208b.

Next, Mo film 100 nm and Al film 300 n
m are continuously formed, and these are patterned to form source / drain electrodes 210a and 210a of the thin film transistor.
b, and source / drain electrodes 21 of MFS-FET
0c and 210d were formed simultaneously.

Next, a photosensitive resin layer 211 having a thickness of 3 μm was formed. At this time, the photosensitive resin layer 21 is exposed using mask exposure.
An uneven pattern layer 212 was formed on the surface of No. 1. continue,
This photosensitive resin layer was patterned by etching to form a passivation layer. After that, an Al film was formed to a thickness of 300 nm by a sputtering method and patterned to form a pixel electrode 213. Furthermore, a polyimide film
nm to form an alignment film 214a.

Next, a substrate (counter substrate) on which the counter electrode and the like are formed was manufactured as follows. Array substrate 201
A chrome film was formed on an alkali-free glass substrate 201b similar to that of a and patterned to form a black matrix layer 218. Then, the color filter layer 217
Was formed. Further, an ITO film to be the counter electrode 216 was formed to 100 nm. Finally, a polyimide film having a thickness of 10 nm was formed as the alignment film 214b.

The array substrate manufactured as described above and the counter substrate are opposed to each other, and a spacer 219 is sandwiched between the two substrates so that they are evenly spaced by 5 μm with a seal. The liquid crystal layer 215 was injected. Thus, the cell of the liquid crystal display device was completed.

Finally, the scanning line drive circuit, the signal line drive circuit and the switch line drive circuit are arranged around the display area.

Next, the driving method of this embodiment will be described with reference to the timing charts shown in FIGS. The basic operation is the same as that of the first embodiment, but in this embodiment, the voltage waveform Vsw applied to the parallel thin film transistor via the switch line is different from that of the first embodiment.

As shown in FIG. 9, the characteristic of the drain current Ids of the MFS-FET with respect to the gate voltage Vg has a memory property. Therefore, when switching from the moving image mode to the still image mode, as shown in FIG. 10, a pulse of voltage Vswoff is applied to the switch line to turn off the parallel thin film transistor, and the parallel circuit including the ferroelectric capacitor and the thin film transistor is formed. , Substantially only the ferroelectric capacitors are connected. When switching from the still image mode to the moving image mode, as shown in FIG.
A pulse of voltage Vswon is applied to the switch line to turn on the parallel thin film transistor, and in the parallel circuit including the ferroelectric capacitor and the thin film transistor, the ferroelectric capacitor is substantially short-circuited.

As in the second embodiment, the thin film transistor may be a polysilicon TFT whose channel layer is made of polycrystalline silicon, in addition to the amorphous silicon TFT whose channel layer is made of amorphous silicon. When the TFT is used, it is possible to simultaneously form the scanning line drive circuit, the signal line drive circuit and the switch line drive circuit around the display area when forming the TFT in the display area on the array substrate. .

In addition to the MFS-FET described above, the switching element connected in parallel with the ferroelectric capacitor has a field mobility transistor (metal-ferroelectric-metal-insulator-semiconductor gate structure). Metal-Ferroelect
ric-Metal-Insulator-Semiconductor Field Effect Tra
A transistor having a memory characteristic for switching characteristics such as nsistor (MFMIS-FET) may be used.

In the present embodiment as well, as in the first embodiment, since no voltage is applied to the capacitor formed of the ferroelectric thin film when a moving image is displayed, the input signal voltage can be made smaller and lower than that of the conventional liquid crystal display device. It becomes possible to convert to voltage,
A liquid crystal display device with low power consumption can be obtained.

(Embodiment 4) This embodiment relates to another driving method of the liquid crystal display device shown in the first embodiment or the like. This driving method will be described with reference to the timing charts shown in FIGS. 12 is a timing chart in the still image mode, and FIG. 13 is a timing chart in the moving image mode.

Regarding the driving method in the still image mode,
This will be described with reference to the timing chart shown in FIG.

First, a line-sequential operation is performed by sequentially supplying the selection signal Vgon to each scanning line (gate line) to sequentially turn on the main thin film transistors connected to each gate line. The gate selection period is the reset period T in the first half.
r and the latter half writing period Tw, and the reset period Tr
Then, the signal Vr for resetting the polarization of the ferroelectric substance is supplied from the signal line (reset), and the image signal Vw is written (write) in the writing period Tw. The write frame Tfw for refreshing such a display signal may be at least once for each refresh. In the write frame period Tfw, the parallel switching element connected to the switch line may be turned off (Vswoff).

In the holding frame period Tfh following the writing frame period Tfw, a signal is given to the gate line in the still image mode to turn on the main thin film transistor, and the holding signal voltage Vh for holding the pixel state is applied ( Retention). Such a hold frame typically lasts for two or more predetermined frame periods after the write frame. Also in the holding frame period Tfh, the parallel switching element connected to the switch line may be turned off.

Next, the driving method in the moving image mode will be described with reference to the timing chart shown in FIG.

First, the selection signal Vgon is applied to each gate line.
Is sequentially supplied to sequentially turn on the main thin film transistors connected to each gate line, and the image signal Vw is written (written) in the period in which the gate is selected. Such writing is performed once for each pixel for each frame period Tf. Further, the parallel switching element connected to the switch line may be turned on (Vswon). The reset may be performed before writing.

By using the driving method of this embodiment, it is possible to display a still image and a moving image at the same time within one screen. That is, since ON / OFF control of the main thin film transistor and the parallel thin film transistor can be performed in a unit of a plurality of pixels connected to the same scanning line, a still image and a moving image can be switched in a unit of scanning line. The parallel thin film transistor is selectively turned off in the portion displaying the still image to drive the still image mode as shown in FIG. 12, and the parallel thin film transistor is selectively turned on in the portion displaying the moving image. Driving in the moving image mode as shown in FIG. Note that the determination as to whether to display a still image or a moving image is performed if the display signal supplied to a certain pixel is different between consecutive display frames, the display of the pixel is regarded as a moving image, and the pixel is displayed. All pixels connected to the same scanning line as are the moving image display portion.

In this embodiment as well, as in the first embodiment, since no voltage is applied to the capacitor composed of the ferroelectric thin film when a moving image is displayed, the input signal voltage can be made smaller and lower than that of the conventional liquid crystal display device. It becomes possible to convert to voltage,
A liquid crystal display device with low power consumption can be obtained.

(Fifth Embodiment) This embodiment relates to the overall system configuration of the liquid crystal display device shown in the first embodiment and the like. Hereinafter, the liquid crystal display device according to the present embodiment will be described with reference to the block diagram shown in FIG.

In the device shown in FIG. 14, the display signal (image signal) and the synchronization signal are displayed by the display timing controller 61.
The signal generated by the timing controller 61 is sent to the frame memory 62, the scanning line drive circuit 63, and the switch line drive circuit 65 (reset line drive circuit). A display signal for one frame or several frames is recorded in the frame memory 62, and it is compared and judged whether the display signal is the same as the display signal for the immediately preceding one frame or several frames. . If the display signals are the same, it is determined to be a still image, and if they are not the same, it is determined to be a moving image, and the scanning line drive circuit 63 and the signal line drive circuit 64.
Signals corresponding to the still image mode or the moving image mode are sent to the switch line driving circuit 65 and the switch line driving circuit 65, respectively. A signal corresponding to the still image mode or the moving image mode is supplied to the display area 66 from each of the drive circuits 63, 64, and 65. The display area 66 has a configuration such as that shown in each of the above-described embodiments.

It should be noted that the determination as to the identity of the display signal for switching between the moving image mode and the still image mode may be made for the entire screen for each frame, or may be made for each scanning line unit for each frame. May be.

(Sixth Embodiment) FIG. 15 is a diagram showing a configuration example of an active matrix type liquid crystal display device according to the present embodiment, and FIG. 16 is a diagram showing an equivalent circuit of the unit pixel 12 shown in FIG. Is. The basic configuration is similar to that of the first embodiment shown in FIGS. 1 and 2, and corresponding components are designated by the same reference numerals.

In the configuration shown in FIGS. 1 and 2, the switch line 17 is arranged in the horizontal direction, that is, in the direction parallel to the scanning line 13. However, in the present embodiment, the switch line 17 is arranged in the vertical direction, that is, the signal line 14. It is arranged in a direction parallel to. With respect to such a configuration, the display operation is realized by sending a predetermined signal as shown in FIGS. In this case, the determination as to the identity of the display signal for switching between the moving image mode and the still image mode may be made for each frame on the entire screen.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention. Furthermore, the embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining the disclosed constituent features. For example, even if some constituents are deleted from the disclosed constituents, any invention can be extracted as an invention as long as a predetermined effect can be obtained.

[0085]

According to the present invention, in the liquid crystal display device utilizing the memory property of the ferroelectric substance, the switching element is provided in parallel with the ferroelectric capacitor, so that the still image display and the moving image display can be performed. Since the connection state of the body volume can be changed, the driving voltage in moving image display can be reduced, and a liquid crystal display device with low power consumption can be obtained.

[Brief description of drawings]

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

FIG. 2 is a diagram showing an equivalent circuit of a unit pixel of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 3 is a timing chart showing a driving method in the still image mode of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 4 is a timing chart showing a driving method in a moving image mode of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 5 is a plan view showing a configuration example of a unit pixel of a liquid crystal display device according to a second embodiment of the present invention.

FIG. 6 is a cross-sectional view showing a configuration example of a unit pixel of a liquid crystal display device according to a second embodiment of the present invention.

FIG. 7 is a plan view showing a configuration example of a unit pixel of a liquid crystal display device according to a third embodiment of the present invention.

FIG. 8 is a sectional view showing a configuration example of a unit pixel of a liquid crystal display device according to a third embodiment of the present invention.

FIG. 9 relates to a third embodiment of the present invention and relates to MFS-FE.
The figure which showed the characteristic with respect to the gate voltage of the drain current of T.

FIG. 10 is a timing chart showing a driving method in a still image mode of the liquid crystal display device according to the third embodiment of the present invention.

FIG. 11 is a timing chart showing a driving method in a moving image mode of the liquid crystal display device according to the third embodiment of the present invention.

FIG. 12 is a timing chart showing a driving method in a still image mode of the liquid crystal display device according to the fourth embodiment of the present invention.

FIG. 13 is a timing chart showing a driving method in a moving image mode of the liquid crystal display device according to the fourth embodiment of the present invention.

FIG. 14 is a block diagram showing a system configuration of a liquid crystal display device according to a fifth embodiment of the present invention.

FIG. 15 is a diagram showing a configuration example of a liquid crystal display device according to a sixth embodiment of the present invention.

FIG. 16 is a diagram showing an equivalent circuit of a unit pixel of a liquid crystal display device according to a sixth embodiment of the present invention.

FIG. 17 is a diagram showing a basic configuration example of a liquid crystal display device according to a conventional technique.

FIG. 18 is a diagram showing an equivalent circuit of a unit pixel of a liquid crystal display device according to a conventional technique.

FIG. 19 is a diagram showing an equivalent circuit of a unit pixel of a liquid crystal display device according to a conventional technique.

FIG. 20 is a timing chart showing a driving method of a liquid crystal display device according to a conventional technique.

FIG. 21 is a diagram showing a hysteresis characteristic and a load curve of a ferroelectric capacitor in a liquid crystal display device according to a conventional technique.

FIG. 22 is a diagram showing, as a model, a hysteresis characteristic and a load curve of a ferroelectric capacitor in a liquid crystal display device according to a conventional technique.

[Explanation of symbols]

11, 66 ... Display area 12 ... Unit pixel 13 ... Scan line 14 ... Signal line 15, 63 ... Scan line drive circuit 16, 64 ... Signal line drive circuit 17 ... Switch line 18, 65 ... Switch line drive circuit 31 ... Thin film transistor ( Main transistor) 32 ... Ferroelectric capacitor 33 ... Liquid crystal layer 34 ... Pixel electrode 35 ... Counter electrode 36 ... Thin film transistor (parallel transistor) 61 ... Timing controller 62 ... Frame memories 101a, 101b, 201a, 201b ... Glass substrates 102, 202 ... Undercoat layers 103, 203b ... Base electrode patterns 104, 204a, 204b ... Ferroelectric thin films 105, 205 ... Ferroelectric capacitor upper electrodes 106a, 106b, 206a ... Gate electrodes 107, 207 ... Gate insulating films 108a, 108b, 208a , 208b ... Channel 109a, 109b, 209a, 209b ... Channel stopper layers 110a-110d, 210a-210d ... Source / drain electrodes 111, 211 ... Photosensitive resin layers 112, 212 ... Concavo-convex pattern layers 113, 213 ... Pixel electrodes 114a, 114b, 214a, 214b ... Alignment film 115, 215 ... Liquid crystal layer 116, 216 ... Counter electrode 117, 217 ... Color filter layer 118, 218 ... Black matrix layer 119, 219 ... Spacer 203a ... Gate electrode of MFS-FET

Continuation of front page (56) Reference JP-A-9-236824 (JP, A) JP-A-11-30789 (JP, A) JP-A-7-72511 (JP, A) JP-A-2-120790 (JP , A) JP 5-289114 (JP, A) JP 9-138428 (JP, A) JP 49-3649 (JP, A) JP 56-21193 (JP, A) JP 2001 -195028 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/1362 G02F 1/1343 G02F 1/1333 G02F 1/133

Claims (4)

(57) [Claims] 1. A liquid crystal layer which is provided corresponding to an intersection of a plurality of scanning lines provided in a horizontal direction and a plurality of signal lines provided in a vertical direction and which is sandwiched between a pixel electrode and a counter electrode. A liquid crystal display device having a plurality of pixel portions, wherein the pixel portion has a ferroelectric capacitor having a first end connected to the pixel electrode, a first end connected to the signal line, and a second end connected to the signal line. A first switching element connected to the second end of the ferroelectric capacitor and controlled by a scanning signal from the scanning line; and a second switching element connected in parallel to the ferroelectric capacitor. A liquid crystal display device comprising: 2. The second switching element is turned on when the display signal is different between successive display frames, based on a comparison result of the display signals supplied to the pixel section between successive display frames. The liquid crystal display device according to claim 1, further comprising a control unit that controls the liquid crystal display device. 3. The control unit can control ON / OFF of a second switching element in units of a plurality of pixel units connected to the same scanning line, and a display signal supplied to a certain pixel unit is continuous. The second switching element included in the pixel unit and another pixel unit connected to the same scanning line as the pixel unit is selectively turned on when the display frames differ from each other. The liquid crystal display device according to claim 2. 4. The pixel section is supplied with a display signal having a smaller amplitude when the second switching element is in an on state than when the second switching element is in an off state.
The liquid crystal display device according to item 1.