JPH05241126A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent influence to an image signal by canceling the dispersion of the threshold value of the TFT of a source follower and to hold well gradation property by sending a uniform image signal to each pixel always and to reduce power consumption, as well by providing an off-interval in an amplifier. CONSTITUTION:This device is a liquid crystal display device where an analog switch consisting of the TFT, the amplifier consisting of the source follower and a sample-hold circuit are provided in the driver of a display signal line 19 and the sample-hold circuit is provided with constitution where a clamp TFT 15, a sample TFT 17 are connected with a capacitor 13 in series respectively and the image signal is outputted from a horizontal shift register 1 at an interval when the amplifier is turned off and the image signal is transferred to each pixel 22 and the pixel is driven at the interval when the amplifier is turned on.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device provided with a switching element for each pixel, and more particularly to a liquid crystal display device characterized by a driver (driving circuit) of its display signal line. Regarding

[0002]

2. Description of the Related Art An active matrix liquid crystal display device using a thin film transistor (hereinafter referred to as "TFT") and having a driver portion for an image signal line and a scanning signal line built therein can be made small and lightweight. Is expected as a movable display device such as a portable TV. For that purpose, it is necessary to drive the battery or battery having a certain amount of capacity for as long as possible, and reduction of power consumption is an issue. On the other hand, in order to improve the quality of the displayed image,
There is a demand for a display device that has an increased number of pixels, an increased resolution, an improved contrast, and an enhanced gradation.

In order to satisfy the above requirements, for example, JP-A-61-292268 and JP-A-62-1.
As disclosed in Japanese Patent No. 43095, by devising a driver, particularly a driver circuit for a display line, a driving method capable of transferring a signal to each pixel at a higher speed and more uniformly has been developed. In these technologies, an analog switch, a capacitor, and an amplifier (source follower amplifier) are provided in a signal line driver circuit, and the amplifier is operated only for a predetermined period to improve writing speed and contrast and reduce power consumption. It is a thing.

[0004]

However, in the conventional driver circuit, the variation in the threshold value _Vth of the transistor used in the circuit is not taken into consideration.
Usually, this variation is about ± several hundred mV, and this variation is reflected as it is in the variation of the output signal level of the amplifier. Therefore, variation occurs in the image signal itself, which affects the gradation.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display device which solves the above problems and prevents the threshold value of the transistor in the driver circuit of the display line from affecting the image signal. It was done.

That is, the present invention is an active matrix liquid crystal display device with a built-in driver, in which the driver of the display signal line is an analog switch made up of thin film transistors, an amplifier made up of a source follower, and a capacitor in series with a clamp transistor and a sample transistor, respectively. A liquid crystal display device characterized by comprising a sample and hold circuit connected to the. In the liquid crystal display device of the present invention, the image signal is transferred to the display line through the sample hold circuit after the output from the amplifier. Further, the amplifier operates only during a predetermined selection period and does not operate during the ratio selection period.

[0007]

EXAMPLES AND ACTION The following will specifically describe the constitution and action of the present invention with reference to examples of the present invention.

FIG. 1 shows an equivalent circuit of an embodiment of the present invention. In the figure, 1 is a horizontal shift register, 2 is an image signal line, 3
Is a transfer TFT for transferring an image signal to a source follower amplifier, 4 is an output wiring of a horizontal shift register for turning on / off the transfer TFT, 5 is a terminal for holding an image signal, and 6 is a source follower. Waamp Driver TF
T and 7 are load TFTs of the source follower amplifier, 8 is an amplifier operating TFT for making a current flow in the source follower amplifier only when the amplifier is on, and 9 is for turning on the amplifier operating TFT. A pulse input line for inputting a gate voltage to the gate of the TFT, 10 is a positive voltage application terminal for applying a positive voltage to the amplifier, 11 is a negative voltage application terminal for applying a negative voltage to the amplifier, 12 Is an amplifier output terminal, 13 is a capacitor of the sample hold circuit,
14 is a terminal, 15 is a clamp TFT, 16 is a clamp T
A clamp pulse application terminal for applying a gate pulse to the gate of the FT 15 to turn on and off the clamp TFT 15, 17 is a sample TFT, and 18 is a sample TFT 1.
The sample TFT by applying a gate pulse to the gate of 7
A sample pulse applying terminal for turning on and off
Is a display line (source line), 20 is a vertical shift register, 2
Reference numeral 1 is a scanning line (gate line), 22 is a pixel TFT which is a switching element of a pixel, the gate is connected to the scanning line 21, the source is connected to the display line 19, and the drain is connected to the pixel electrode (not shown). However, since the source and drain are names depending on the characteristics of the transistor, they are inverted depending on the type and the potential. In the present invention, for convenience, the side connected to the pixel electrode is treated as the drain and the side connected to the display line is treated as the source. Reference numeral 23 is a liquid crystal cell, 24 is a counter electrode, 25 is a reset TFT, and 26 is a reset pulse application terminal for applying a gate pulse to the gate of the reset TFT 25 to turn the reset TFT on and off.

In the present invention, the above-mentioned driver and TFT of each pixel may be a conventional TFT using a polycrystalline Si thin film or an amorphous Si thin film, but by using a TFT made of a single crystal Si thin film, higher speed is achieved. It can be driven, and can cope with future high image quality. Here, a method for manufacturing this single crystal Si thin film will be described. The single crystal Si thin film that can be used in the present invention refers to a Si thin film obtained by epitaxially growing on a porous surface of a porous Si substrate, which is an ideal single crystal thin film having few defects.

According to observation with a transmission electron microscope, holes having an average diameter of about 600 Å are formed in this porous Si substrate, and the density thereof is less than half that of single crystal Si. Nevertheless, its single crystallinity is maintained and it is also possible to epitaxially grow a single crystal Si layer on top of the porous layer. However, at 1000 ° C. or higher, rearrangement of internal holes occurs and the characteristics of enhanced etching are impaired. Therefore, for the epitaxial growth of the Si layer, the molecular beam epitaxial growth method, plasma CV
Low temperature growth such as D method, thermal CVD method, photo CVD method, bias sputtering method, liquid crystal growth method, etc. is suitable.

Here, a method of epitaxially growing a single crystal layer after making P-type Si porous will be described.

First, a Si single crystal substrate is prepared, and H
It is made porous by the anodization method using the F solution. The density of single crystal Si is 2.33 g / cm ³ , but the density of the porous Si substrate changes to 0.6 to 1.1 g / cm ³ by changing the HF solution concentration to 20 to 50% by weight. Can be made. This porous layer is P because of the following reasons.
It is easily formed on the mold Si substrate.

Porous Si was discovered in the course of research on electrolytic polishing of semiconductors, and Si in anodization was used.
In the dissolution reaction of 1), holes are required for the anodic reaction of Si in the HF solution, and the reaction is shown as follows.

Si + 2HF + (2-n) e ⁺ → SiF ₂
+ 2H ⁺ + ne ^- SiF ₂ + 2HF → SiF ₄ + H ₂ SiF ₄ + 2HF → H ₂ SiF ₆ or Si + 4HF + (4-λ) e ⁺ → SiF ₄ + 4H ⁺ + λ
e ⁻ SiF ₄ + 2HF → H ₂ SiF ₆ Here, e ⁺ and e ⁻ represent a hole and an electron, respectively. Further, n and λ are the numbers of holes necessary for dissolving Si1 atoms, respectively, and porous Si is formed when the condition of n> 2 or λ> 4 is satisfied.

From the above, P-type Si in which holes are present
Can easily be said to be porous.

On the other hand, it has been reported that high-concentration N-type Si can also be made porous, so that it can be made porous regardless of whether it is P-type or N-type.

Further, since the porous layer has a large amount of voids formed therein, its density is reduced to less than half. As a result, the surface area increases dramatically compared to the volume,
Its chemical etching rate is significantly increased as compared with the etching rate of a normal single crystal layer.

The conditions for making single crystal Si porous by anodization are shown below. The starting material of porous Si formed by anodization is not limited to single crystal Si, and Si having another crystal structure may be used.

Applied voltage: 2.6 (V) Current density: 30 (mA · cm ⁻² ) Anodizing solution: HF: H ₂ O: C ₂ H ₅ OH = 1: 1
1: 1 time: 2.4 (hour) Thickness of porous Si: 300 (μm) Porosity: 56 (%) Single crystal Si thin film prepared by epitaxially growing Si on the porous Si substrate thus formed. To form.
The thickness of the single crystal Si thin film is preferably 50 μm or less, more preferably 20 μm or less.

Next, after oxidizing the surface of the above-mentioned single crystal Si thin film, a substrate which will eventually form a substrate is prepared,
The oxide film on the surface of the single crystal Si and the above substrate are bonded together. Alternatively, after the surface of a newly prepared single crystal Si substrate is oxidized, it is attached to the single crystal Si layer on the porous Si substrate. The reason for providing this oxide film between the substrate and the single crystal Si layer is that, for example, when glass is used as the substrate, the interface level generated by the underlying interface of the Si active layer is higher than that of the glass interface. This is because the level can be lowered and the characteristics of the electronic device can be significantly improved. Furthermore, only the single crystal Si thin film from which the porous Si gas has been removed by etching by selective etching described below may be attached to a new substrate. The bonding is such that the surfaces are sufficiently adhered so that they cannot be easily peeled off by Van der Waals force only by bringing them into contact with each other at room temperature, but this is further 200 to 900 ° C, preferably 600 to 900 ° C. Heat treatment under a nitrogen atmosphere at the temperature of and bond them completely.

Further, a Si ₃ N ₄ layer is deposited as an etching prevention film on the whole of the above-mentioned two bonded substrates, and only the Si ₃ N ₄ layer on the surface of the porous Si substrate is removed.
Apiezon wax may be used instead of the Si ₃ N ₄ layer. Then, the porous Si substrate is entirely removed by a method such as etching to obtain a semiconductor substrate having a thin film single crystal Si layer.

A selective etching method for electroless wet etching only this porous Si substrate will be described.

As an etching solution which does not have an etching effect on crystalline Si but can selectively etch only porous Si, a buffered material such as hydrofluoric acid, ammonium fluoride (NH ₄ F) or hydrogen fluoride (HF) is used. Hydrofluoric acid, mixed solution of hydrofluoric acid or buffered hydrofluoric acid with hydrogen peroxide solution, hydrofluoric acid with alcohol or buffered hydrofluoric acid, hydrofluoric acid with hydrogen peroxide solution and alcohol or buffer A mixed solution of dehydrofluoric acid is preferably used. Etching is performed by moistening the substrate bonded to these solutions. The etching rate depends on the solution concentration and temperature of hydrofluoric acid, buffered hydrofluoric acid, and hydrogen peroxide solution. By adding hydrogen peroxide solution, the oxidation of Si can be accelerated and the reaction rate can be increased as compared with that without addition. By further changing the ratio of hydrogen peroxide solution, the reaction rate can be increased. Can be controlled. Further, by adding alcohol, it is possible to instantaneously remove the bubbles of the reaction product gas due to etching from the etching surface without stirring, and it is possible to uniformly and efficiently etch the porous Si.

The HF concentration in the buffered hydrofluoric acid is set in the range of preferably 1 to 95% by weight, more preferably 1 to 85% by weight, further preferably 1 to 70% by weight, based on the etching solution. The NH ₄ F concentration in the buffered hydrofluoric acid is set in the range of preferably 1 to 95% by weight, more preferably 5 to 90% by weight, further preferably 5 to 80% by weight, based on the etching solution.

The HF concentration is preferably set in the range of 1 to 95% by weight, more preferably 5 to 90% by weight, further preferably 5 to 80% by weight, based on the etching solution.

The H ₂ O ₂ concentration depends on the etching solution.
It is preferably 1 to 95% by weight, more preferably 5 to 90% by weight, still more preferably 10 to 80% by weight, and is set within a range in which the effect of the hydrogen peroxide solution is exhibited.

The alcohol concentration is preferably 80% by weight, more preferably 60% by weight, based on the etching solution.
Hereafter, it is more preferably set to 40% by weight or less and within the range in which the effect of the alcohol is exhibited.

The temperature is preferably set in the range of 0 to 100 ° C, more preferably 5 to 80 ° C, further preferably 5 to 60 ° C.

As the alcohol used in this step, in addition to ethyl alcohol, isopropyl alcohol and the like which can practically be used in the production process and which can be expected to have the above-mentioned alcohol addition effect can be used.

In the semiconductor substrate thus obtained, a single crystal Si layer equivalent to that of an ordinary Si wafer is flatly and uniformly thinned to have a large area over the entire substrate.

The single crystal Si layer of this semiconductor substrate is separated by a partial oxidation method or etched into an island shape, and is doped with impurities to form a p- or n-channel transistor.

Next, FIG. 2 shows a timing chart of the liquid crystal display device shown in FIG.

In the figure, φ _a is a source follower pulse that turns on the amplifier during the horizontal blanking period. φ _r
Is a reset pulse applied from the reset pulse applying terminal 26, φ _c is a clamp pulse applied from the clamp pulse applying terminal 16, and φ _s is a sample pulse applying terminal 1
8 is a sample pulse applied from the vertical shift register, φ _g is a gate pulse applied from the vertical shift register to the gate line 21, and V ₅ , V ₁₉ and V ₂₃ are the terminal 5 and the display line 1, respectively.
9 shows the potential of the liquid crystal 23.

First, within the horizontal blanking period, that is, φ _a
When turned on, φ _{r is} turned on at time t ₁ and terminal 5 is G
It is reset to the ND potential.

Next, at time t ₂ , φ _c and φ _s are turned on at the same time, the terminal 14 and the display line 19 are reset to the GND potential, and at the same time, the GND potential is input to the amplifier as the potential of the amplifier output terminal 12. It will be in a state of being stuck. This establishes a corresponding relationship with the GND potential of the terminal 14 and the display line 19 in terms of AC across the capacitor 13.

At time t ₃ , φ _c and φ _s are turned off, and the clamp TFT 15 and the sample TFT are turned off.

Next, at time t ₄ , φ _r is turned off and the reset TFT 25 is turned off. After that, φ _a is turned off, the amplifier is turned off, and the horizontal blanking period ends.

At time t ₅ , the horizontal shift register 1
A pulse is output to the column selected by and the transfer TFT 3 is turned on, and in synchronization with this, the image signal is transferred from the image signal line 2 to the terminal 5.

At time t ₆ , the output of the horizontal shift register 1 is turned off, the transfer TFT 3 is turned off, and the image signal is held at the terminal 5.

The next horizontal blanking period starts at time t ₇ , φ _a turns on and the amplifier turns on.
Therefore, the potential output from the amplifier output terminal 12 becomes the potential when the image signal held in the terminal 5 is input to the amplifier. At subsequent time t ₈ , φ _s and φ _g
Is turned on, the sample TFT 17 and the pixel TFT are turned on, and the image signal output to the terminal 12 is AC-accepted by the sample TFT 17, the display line 19, and the liquid crystal cell 23 with the capacitor 13 interposed therebetween. At this time, the potential corresponding to the difference between the image signal held at the terminal 5 and the GND potential is taken into the terminal 14, the display line 19, the liquid crystal cell 23, and the amplifier output terminal 12 as a difference potential based on the GND potential. Therefore, the variation of the threshold V _th of the source follower is canceled (corrected) and does not affect the image signal. Therefore, a constant image signal is always transferred to the liquid crystal cell.

At time t ₉ , φ _s and φ _g are turned off, and the liquid crystal cell 23 holds the image signal potential.

At time t ₁₀ , the reset pulse φ _r is turned on, and the write operation of the liquid crystal cell in the next row is performed. That is, the above t _{1 to} t ₉ are repeated and writing is sequentially performed for each row.

[0043]

As described above, in the liquid crystal display device of the present invention, the variation of the threshold value of the source follower is canceled in the driver of the display line and does not affect the image signal, so that the image transferred to each pixel is Since the signal is always constant, it is possible to perform display with excellent gradation. Also,
Since the amplifier is turned on only during the horizontal blanking period, the power consumption can be reduced, and the liquid crystal display device of high quality suitable for portable use can be driven for a long time with a constant capacity.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram showing an embodiment of a liquid crystal display device of the present invention.

FIG. 2 is a timing chart of the liquid crystal display device shown in FIG.

[Explanation of symbols]

 1 horizontal shift register 2 image signal line 3 transfer TFT 4 horizontal shift register output wiring 5 terminal 6 driver TFT 7 load TFT 8 amplifier operation TFT 9 pulse input line 10 positive voltage application terminal 11 negative voltage application terminal 12 amplifier output terminal 13 capacitor 14 terminal 15 clamp TFT 16 clamp pulse application terminal 17 sample TFT 18 sample pulse application terminal 19 display line 20 vertical shift register 21 scanning line 22 pixel TFT 23 liquid crystal cell 24 counter electrode 25 reset TFT 26 reset pulse application terminal

Continuation of front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical location H01L 29/784 (72) Inventor Akira Ishizaki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (1)

[Claims] 1. An active matrix liquid crystal display device with a built-in driver, wherein a driver of a display signal line has an analog switch made of a thin film transistor, an amplifier made of a source follower, and a capacitor connected in series with a clamp transistor and a sample transistor, respectively. A liquid crystal display device comprising a sample and hold circuit comprising