JP3319561B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a liquid crystal display device.

[0002]

2. Description of the Related Art Liquid crystal displays are thin and have low power consumption, and are widely used in notebook computers and the like.
In particular, low power consumption is an excellent feature compared to other displays such as CRTs and plasma displays, and is expected to be applied to portable information devices in the future.

In the case of a portable device, it is desirable that the power consumption of the display is 500 mW or less, preferably as small as several mW. In response to this requirement, a reflection type of a simple matrix type of TN type liquid crystal which does not require a backlight and has low power consumption has been conventionally used. However, the TN type requires a polarizing plate and has a reflectance as low as about 30%, and the simple matrix type has a problem that the contrast is reduced when the number of pixels is increased, making it more difficult to see. Therefore, an attempt has been made to obtain a display having a high reflectance and a high contrast by performing driving using an active matrix using a PCGH (phase change guest host type) mode without using a polarizing plate for a liquid crystal display.

FIG. 7 shows the configuration of such a conventional example. The circuit configuration shown in the figure is equivalent to the active matrix of the conventional transmission type TN liquid crystal, and includes a signal line 71 and a gate line 72.
And a thin film transistor TFT 73 at the intersection thereof applies a charge to the liquid crystal 74 of each pixel and the storage capacitor 76 connected to the storage capacitor line 75. It is necessary to apply an alternating current to the liquid crystal 74, and this is realized by applying a signal line voltage so as to be a positive voltage and a negative voltage around the voltage of the counter electrode 77 of the counter substrate.

In such a liquid crystal display, an AC voltage needs to be applied even when the display does not change at all. Therefore, the pixel potential is rewritten every time a selection is made in a frame cycle. The power consumption when applying an alternating current to the capacitance is: P = f × V ² × C (frequency f; voltage V; capacitance C), so the higher the frequency, the higher the voltage, and the larger the capacitance, the more the power consumption Is big.

In the case of AC driving with a liquid crystal display, the driving frequency of each pixel is the frame frequency, the driving frequency of the signal line is the product of the frame frequency and the number of scanning lines, and the driving frequency of the signal line driver IC is the total pixel of the screen. The value is the product of the number and the frame frequency, or the value obtained by further dividing by the number of divisions if divided driving is performed. Currently, 10.4 inch diagonal color VG
In A (640 × RGB × 480 pixels), the power consumption of the signal line IC is about 1 W. Therefore, the number of pixels of an A4 size high-definition LCD equivalent to 150 dpi is 1600 which is 6.25 times that of VGA.
It is expected to be about × 1200 pixels, which will be as large as 2 to 3 W or more. In this case, the use time of the battery is short in a portable information device, which is problematic.

It is known that the use of a bistable ferroelectric liquid crystal (SSFLC) for this problem has a memory property and that the supply of voltage can be stopped as long as the display does not change. Can be reduced.

However, a bistable ferroelectric liquid crystal has a problem that the orientation is disturbed by an impact and a display failure occurs, and thus cannot be adopted as a portable display device. In addition, display quality (contrast, reflectance, etc.) is often limited in a liquid crystal having memory properties. For example, in SSFLC, the use of a polarizing plate is an indispensable display mode.
There was also a problem that only a dark screen of about 0% could be obtained.

[0009]

As described above, on the screen of a personal computer or the screen of a portable information device, there are many still images, and alternating current is supplied to the signal lines even if the screen is not rewritten. You are consuming.

Therefore, an object of the present invention is to solve the above-mentioned problems and reduce power consumption.

Another object of the present invention is to realize high definition by simplifying the pixel circuit.

[0012]

In order to solve such a problem, the liquid crystal display device according to the first aspect has first and second liquid crystal display devices .
Liquid crystal layer sandwiched between two electrodes
First to third signal supply lines for supplying signals, and a selection signal
And a gate unit for holding the selection signal.
A ferroelectric substance, and a source electrode connected to the first signal supply line.
Connected and the drain electrode is connected to the first electrode
A first threshold voltage and a first polarity
Having a first field-effect transistor and a gate
It has a ferroelectric that holds the selection signal, and the source electrode is
Wherein the first signal supply line is connected to the second signal supply line.
A second threshold voltage having an absolute value larger than the first threshold voltage;
And a second field effect transistor having the first polarity
And a ferroelectric that holds the selection signal in the gate section.
Having a source electrode of the second field effect transistor
And the drain electrode is connected to the first electrode.
Said first threshold voltage provided to be connected to an electrode;
Field-effect transistor having pressure and second polarity
And a ferroelectric that holds the selection signal in the gate section.
A source electrode is connected to the third signal supply line;
The drain electrode is the source of the third field-effect transistor.
The second threshold provided to be connected to a source electrode.
Value field voltage and a fourth field effect type having the second polarity
A transistor. In the present invention, the
Since there are three types of voltages to be applied, gradation can be obtained.
You.

Here, the first electrode is, for example, a pixel electrode,
The second electrode is, for example, a counter electrode, and the reference potential is, for example, a potential of the counter electrode, for example, a ground potential.

[0014] The first display signal and the second display signal are image signals.
Element state, that is, sandwiched between the first electrode and the second electrode.
This is a signal for controlling the state of the held liquid crystal layer. First table
One of the indication signal and the second indication signal is DC, the other is AC,
Alternatively, both may be DC or AC. For example
An alternating current is applied as the first display signal, and the second display signal and
Then, a constant potential may be applied. Also, the first display signal is
The means for supplying and the means for supplying the second display signal are, for example,
This is a form like a feed line. Supply the first display signal
Means for supplying the second display signal.
Means for supplying a third display signal;
Providing means for supplying, etc., and indications supplied to those means
A signal may also be selectively applied to the liquid crystal .

A selection signal is used to select a state of a pixel.
Signal from the drive circuit via a selection signal line, for example.
Be added. A switch is placed between the selection signal line and the ferroelectric.
Via a switching element, for example, an FET whose gate is connected to a scanning line.
Insert. Scan lines, select signal lines and switching elements
Thus, the state of an arbitrary pixel can be controlled. I mean
A display signal applied to an arbitrary pixel is changed according to a selection signal.
It is possible to select between the first display signal and the second display signal.
Wear. The selection signal is held as the ferroelectric polarization state.
You.

According to the selection signal held in the ferroelectric,
Applying the first or second display signal to the first electrode
The `` selection means '' is, for example, in the form of a switching element.
However, it may be in the form of a control element. S
A pair of p-channel and n-channel switching elements
Field effect transistor. Field effect transistor
There is a thin film transistor. Thin film transistor
Configuration is top gate type, bottom gate type, etc.
What is necessary is just to design. And, for example, p-cha
And the gate of the n-channel field-effect transistor.
One signal is turned on and the other is turned on simply by applying the same signal.
Can be turned off and select the voltage to be applied to the first electrode.
You can choose. As a mode of selection, ferroelectric
Complementarily turns on and off according to the selection means held by the body
The first switching element and the second switching element
You. The first display when the first switching element is on
A signal is applied to a first electrode and a second switching element
Is turned on, a second display signal is applied to the first electrode.
Will be.

And a ferroelectric material and a selection unit.
The first switching element and the second switching
On / off according to the polarization state of the ferroelectric using the element
What should I do? The first switch constituting the selecting means
The switching element and the second switching element
Turn on and off complementarily according to the polarization state of the body
If the first display signal is applied to the liquid crystal layer,
And the state where the second display signal is applied to the liquid crystal layer.
It can be switched automatically. Complementary on / off
First switching element and second switching
As a form of the element, for example, there is a complementary MOSFET. this
Ferroelectric as a holding means in the gate of the complementary MOSFET
Or connected to the gate of a complementary MOSFET
A dielectric may be provided. Ferroelectric is the first switching
The element and the second switching element may be separated from each other.
Alternatively, they may be formed integrally and shared.

[0018]

[0019]

[0020]

[0021]

[0022]

[0023]

FIG. 1 is an equivalent circuit diagram of one pixel of a liquid crystal display device according to the present invention.

As shown in FIG. 1, a unit pixel has a pixel selection transistor 1, a scanning line 2 for selecting the pixel selection transistor, and a signal line 3 for supplying a signal for determining the state of the pixel. You.

The gates of an n-channel transistor 4 and a p-channel transistor 5 are connected to the drain of the transistor 1 for pixel selection. The gates of the transistors 4 and 5 are provided with a ferroelectric substance, and the transistors can be kept on and off depending on the state of internal polarization. The ferroelectric is provided on part of the gate insulating film of the transistor. The sources of the n-channel and p-channel transistors 4 and 5 are connected to the two wirings 7 and 8, respectively, and the drains are connected to each other to apply a voltage to the liquid crystal 6. The other potential of the liquid crystal 6 is a counter electrode 9.

The potential of the counter electrode 9 is set to 60
When an alternating current of about z is applied to set the wiring 7 to the same potential as the counter electrode 9, the gate signal is turned on (FIG. 2A), and the transistor 4 is turned on (FIG. 2D) when the scanning signal pulse is applied (FIG. 2B). When a predetermined AC voltage is applied to the liquid crystal 6 (FIG. 2E), the gate signal is OFF (FIG. 2A), the scanning signal pulse is OFF (FIG. 2B), and the transistor 5 is ON (FIG. 2C). A state where no voltage is applied to the liquid crystal 6 is obtained (FIG. 2E). Since these two states are stored by the transistors 4 and 5, if the image does not change, it is not necessary to apply a high-frequency alternating current to the signal line 3 and low power consumption is realized. Since the transistors 4 and 5 operate reciprocally even when the same gate potential is applied, the pixel circuit is greatly simplified. Although it is possible to control two transistors independently by using two signal lines, an external drive circuit is simplified and higher integration of pixels is achieved.

The wirings 7 and 8 are not limited to the above-described signal application. Both may be DC potentials.

FIG. 3 is a plan view of a pixel which realizes the circuit of FIG. 1, and FIG. 4 is a sectional view taken along the line AA 'of FIG. 30X (X =
1 to 9) correspond to the number X in FIG. In addition, FIG.
Reference numeral 309 denotes a pixel electrode.

The main process of the present invention will be described with reference to FIG. First, an amorphous silicon film is deposited on the insulating substrate 401, polycrystallized by excimer laser annealing, and patterned into an island shape. n-channel transistor
407 and channel region of p-channel transistor 408
402 is polycrystalline silicon (70 nm thick). In this example, both n-ch and p-ch are non-doped polycrystalline silicon, but n-ch and p-ch may be slightly doped independently for controlling the threshold voltage of the TFT.

Subsequently, a gate insulating film 405 containing a ferroelectric is deposited. In this example, a silicon oxide film (50 nm thick) deposited by ECR-CVD and a barium titanate (BaTiO ₃ ) ferroelectric film (20 to 400 nm thick) were laminated. A ferroelectric single layer may be used, an insulating film provided on a ferroelectric, a three-layered one, an intermediate electrode provided between a ferroelectric and a paraelectric, and the like may be used. PZT ([Pb (Zr, Ti) O
₃ ]), layered oxides (such as Bi ₄ Ti ₃ O ₁₂ ) may be used as well as organic materials (vinylidene fluoride (VDF; vinylidene fluoride) and trifluoroethylene (TrFE)). a mixture of trifluoroethylene)
May be. As a film formation method, a sputtering method, a sol-gel method, a laser ablation method, a CVD method, or the like can be used.

A gate electrode 406 is formed thereon, and source and drain regions are formed by ion doping. n-chT
In the FT 407, phosphorus is added to the source and drain regions 403, and p-chT
In the FT408, the source and drain regions 404 are doped with boron. In addition, each has an LDD structure, and a medium-concentration impurity region is provided between the source / drain region and the channel. The whole is covered with an interlayer insulating film 409 such as a silicon oxide film or a silicon nitride film, and after opening a through hole, the source and drain electrodes and the wiring connected thereto are formed.
410,411,412 were formed. Acrylic resin, BC on it
Second interlayer insulating film 413 such as B (benzocyclobutene)
After forming a through hole, a pixel electrode 414 was formed.

Thus, the TFT array is completed, and the counter substrate 417 and the counter electrode 416 made of glass or plastic are used.
The display device is completed by arranging the liquid crystal between them.

As the liquid crystal, a guest-host type liquid crystal was used.
A liquid crystal in which the host liquid crystal is rotated by 90 to 360 degrees, an amorphous guest host type in which the orientation is random, and the like are effective in increasing the contrast and the reflectance. Further, a TN type, a selective reflection-transmission mode using a cholesteric liquid crystal may be used, and a ferroelectric liquid crystal, an antiferroelectric liquid crystal, a polymer dispersed liquid crystal, an OCB mode liquid crystal, or the like may be used.
The display method is also free, and speaking of the optical change classification,
Transmission-absorption, transmission-scattering, scattering-
Any of those that obtain absorption may be used. Since the number of pixel elements is large, a reflection type in which an insulating film is provided on the elements to form pixel electrodes is desirable, but a transmission type is also possible depending on the pixel size. Naturally, it can be monochrome or color display. The liquid crystal layer may be a single layer or a multilayer.

In the structure shown in FIG. 4, the pixel electrode covers the TFT portion and the wiring, but a predetermined signal is always supplied to the liquid crystal (a period during which no pixel is selected) as in the present invention. Therefore, unlike the conventional example, there is no problem of noise due to the parasitic capacitance between the pixel electrode and the signal line, the scanning line, or other wiring, and the design becomes easy. That is, conventionally, when the TFT portion and the like are covered with the pixel electrode, the pixel electrode
A parasitic capacitance is generated between the FT portion and the like, which has an adverse effect on electrical characteristics of the TFT portion and the like, and has a bad effect on display quality and the like. On the other hand, in the present invention, since a transistor including a ferroelectric substance is arranged below the pixel electrode, deterioration in display quality and the like due to parasitic capacitance is eliminated.

The selection transistor in FIG. 1 is not limited to one, and a circuit of two transistors which take an AND operation such that a signal line voltage is applied only to the intersection of scanning lines provided vertically and horizontally can also be used. Alternatively, a circuit that reads information of one selection line, and performs pixel selection and signal reception may be used.

In the above embodiment, the transistor is a polycrystalline silicon TFT. However, the transistor may be an amorphous silicon TFT, microcrystalline silicon or a silicon-germanium alloy. The transistor structure is not limited to a planar type, and may be a staggered type or an inverted staggered type, and is not limited to a self-aligned type, and may be a non-self-aligned type.

FIG. 5 is a circuit diagram of a pixel portion according to another example of the present invention. In this example, the n-channel transistor 505 and p
The channel transistor 503 is a normal field-effect transistor, and its channel portion is made of polycrystalline silicon or amorphous silicon. Whether the n-channel or the p-channel is determined depending on whether the impurity carriers in the source and drain regions are electrons or holes. The ferroelectric is constituted by ferroelectric capacitors 504 and 506 connected to the gate electrode.

To apply a voltage to the gates of this pair of transistors, there are two selection transistors 501 and 502, and both one scanning line 510 and the other scanning line 511 are at a high level (when the transistors 501 and 502 are n-channel). ), The voltage from the signal line 512 is applied. Thereby, a signal can be given only to an arbitrary pixel.

In this embodiment, the ferroelectric material and the transistor are separated from each other, and have a feature that they can be easily formed in a process.

It should be noted that the ferroelectric capacitor and the transistor may or may not be planarly separated. That is, a gate insulating film made of a ferroelectric substance and a paraelectric substance can be arranged between the gate electrode and the channel, and an intermediate electrode can be provided between them. In addition, an oxide electrode such as ITO can be used as the electrode in contact with the ferroelectric.

The structure of the transistor is not limited to a planar type, and may be a staggered type, an inverted staggered type, or the like. In addition to polycrystalline silicon, semiconductors such as monocrystalline silicon, microcrystalline silicon, silicon-germanium alloy, Te, C
A semiconductor such as dSe may be used. In FIG. 5, the ferroelectric capacitors 504 and 506 are connected to the gates of the transistors 507 and 508, respectively. However, as shown in FIG.
02, 603 may be connected via one ferroelectric capacitor 604.

FIG. 8 shows still another example. In this example,
More than two types of voltages to be applied to the liquid crystal, for example, four types can be applied. In the figure, a pair of p-channel and n-channel transistors 802 and 803 are connected to a liquid crystal 812.
In this method, a ferroelectric material is used for a gate insulating film, and it is rough. When the polarization of the ferroelectric is inverted, the gate voltage at which the transistor 803 is turned on is set to Vth1, and at the same time, the transistor 802 is turned from on to off.

The transistor 803 is connected to a pair of p-channel and n-channel transistors 804 and 805, and includes a ferroelectric material.
It has Vth2 different from 802,803.

A pair of p-channel and n-channel transistors 806 and 807 are connected to the transistor 805.
3 has a threshold voltage of Vth3 '. These gates
Are connected and driven by the pixel selection transistor 801.

When the signal from the pixel selecting transistor 801 is sufficiently negative, the transistor 802 is turned on. Therefore, the signal of the signal line 808 is applied to the liquid crystal 812. Assuming that Vth1 <Vth2 <Vth3, when the voltage 816 exceeds Vth1, the transistor 802 is turned off and the transistor 803 is turned on, so that the signal on the signal line 809 is applied to the liquid crystal 812. When the voltage 816 exceeds Vth2, the transistor 804 turns off and the transistor 805 turns on. Since the transistor 806 remains on, the signal line 81
A signal of 0 is applied to the liquid crystal 812. The voltage of code 816 is V
When it exceeds th3, the transistor 806 is turned off and the transistor 807 is turned on, so that the signal on the signal line 811 is applied to the liquid crystal 812.

In this way, four types of signals can be switched and a memory can be provided. As a method of changing Vth, besides changing the thickness of the ferroelectric, there is also a method of changing the thickness of the paraelectric gate insulating film. Alternatively, the channel may be doped. A method of connecting ferroelectric capacitors as shown in FIGS. Thereby, a gradation can be obtained.

The signals on the signal lines 808, 809, 810, 811 are:
What is necessary is just to apply externally to each pixel. For example, only the signals of the signal lines 808 and 811 are externally applied, and
The 9,810 signals can be created by dividing resistors or dividing capacitors in a pixel.

In addition, various modifications can be made without departing from the spirit of the present invention.

[0049]

As described in detail above, according to the present invention,
Power consumption of the liquid crystal display can be reduced. Since a predetermined potential is constantly applied to the liquid crystal, good image quality can be obtained. Further, the circuit is simplified and high definition is realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a pixel portion according to the present invention.

FIG. 2 is a driving timing chart of the liquid crystal display device according to the present invention.

FIG. 3 is a plan view of a pixel portion according to the present invention.

FIG. 4 is a cross-sectional view of a pixel portion according to the present invention.

FIG. 5 is a circuit diagram of a pixel portion according to another example of the present invention.

FIG. 6 is a circuit diagram of a pixel portion according to another example of the present invention.

FIG. 7 is a circuit diagram of a conventional pixel portion.

FIG. 8 is a circuit diagram of a pixel portion according to another example of the present invention.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 pixel selection thin film transistor 2 scanning line 3 signal line 4 n-channel thin film transistor including ferroelectric in gate 5 p-channel thin film transistor including ferroelectric in gate 6 liquid crystal 7, 8 signal supply line 9 counter electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-118912 (JP, A) JP-A-5-119298 (JP, A) JP-A-49-131646 (JP, A) (58) Investigation Field (Int.Cl. ⁷ , DB name) G02F 1/1362 G02F 1/1343

Claims (1)

(57) [Claims] (1) Liquid crystal sandwiched between first and second electrodes
Layers and First to third signals for supplying different display signals
No. supply line, Means for providing a selection signal; A gate portion having a ferroelectric material for holding the selection signal;
A source electrode connected to the first signal supply line;
An electrode is provided to be connected to the first electrode;
A first field effect having a first threshold voltage and a first polarity
Fruit transistor, A gate portion having a ferroelectric material for holding the selection signal;
So that the source electrode is connected to the second signal supply line.
A second threshold voltage having an absolute value larger than the first threshold voltage;
Threshold voltage and a second field effect having the first polarity
Fruit transistor, A gate portion having a ferroelectric material for holding the selection signal;
The source electrode is a drain of the second field effect transistor.
Drain electrode is connected to the first electrode.
The first threshold voltage, and
A third field-effect transistor having a second polarity; A gate portion having a ferroelectric material for holding the selection signal;
A source electrode is connected to the third signal supply line,
The electrode is a source electrode of the third field effect transistor
The second threshold voltage and the second threshold voltage
And a fourth field effect transistor having the second polarity
A liquid crystal display device comprising: