JPH04340932A - Liquid crystal display device and its driving method - Google Patents

Abstract

PURPOSE:To realize the large-area active matrix display device by eliminating the deficient writing of liquid crystal due to the delay of a scanning line signal. CONSTITUTION:The active matrix type liquid crystal display device is constituted by providing plural scanning lines 101 and picture element electrode arrays on a 1st substrate, providing nonlinear elements between the scanning lines and picture element electrodes, and charging liquid crystal in the gap between the 1st and 2nd substrates. A photoconductive film 104 is sandwiched between the picture element electrodes and scanning lines, photoconductive layers 102 and 102' are provided nearby the photoconductive film 104, and light beams LZ1 and LZ2 are propagated to the photoconductive films 102 and 102' to irradiate the photoconductive film 104 from the photoconductive layers 012 and 102'; and thus the resistance of the photoconductive film 104 is varied and this variation is utilized to apply a voltage to the picture element electrodes, thereby writing data in picture elements. Consequently, rate determination based upon the delay of the scanning lines of a conventional active matrix type liquid crystal display device is eliminated and the writing to the liquid crystal is performed at a high speed.

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.

0002

2. Description of the Related Art In conventional liquid crystal display devices, each pixel is electrically selected in order to perform writing on the liquid crystal. In an active matrix type liquid crystal display device, thin film elements that control voltage applied to liquid crystal are arranged in a matrix. Some control elements use thin film transistors or thin film diodes. This conventional example is described in "Color Liquid Crystal Display, edited by Shunsuke Kobayashi, published by Sangyo Tosho, pp. 137-181."

Liquid crystal display devices using thin film transistor elements are used in display devices that require high image quality and high definition, such as viewfinders for video cameras and light valves for liquid crystal projectors. However, the manufacturing process of the liquid crystal display device using the thin film transistor element is complicated;
Yield is poor and therefore the price must be high.

On the other hand, although the image quality of liquid crystal display devices using thin film diode elements is somewhat inferior to that of liquid crystal display devices using thin film transistor elements, the manufacturing process is simple, the manufacturing process is simple, the price is low, and the screen size is large. can provide a liquid crystal display device.

The structure of a liquid crystal display device using thin film diode elements will be explained below with reference to FIG. FIG. 2 is an equivalent circuit diagram of an active matrix type liquid crystal display device using the thin film diode element. thin film diode 203,
Scanning lines 201 and 201' are connected to the first electrode 203'. Moreover, the thin film diodes 203 and 20
A pixel electrode is connected to the second electrode 3'. Liquid crystal is sealed in the gap between the signal lines 202, 202' on the opposing substrate side and the pixel electrode. 204 and 20 in FIG.
4' is an equivalent representation of the capacity of the liquid crystal.

An example of a method for driving a liquid crystal display device having the above structure will be explained with reference to FIGS. 2 and 3. FIG. 3 is a diagram illustrating an example of a method for driving a liquid crystal display device using thin film diode elements.

First, a scanning line signal 304 in the form of an alternating current waveform whose polarity is changed at regular intervals is input to the scanning line 201 . On the other hand, an AC waveform signal line signal 305 having a video signal to be written to the pixel is input to the signal line 202. At this time, the voltage applied to the thin film diode 203 is the difference between the scanning line signal 304 and the signal line signal 305. It is.

Next, during a pixel selection period 307, a pulse waveform is input to the scanning line 304 such that the potential difference between the signal line signal 305 and the scanning signal 304 becomes equal to or higher than the voltage at which the thin film diode becomes conductive. be done. At this time, the thin film diode becomes conductive, thereby writing the video signal into the pixel.

The pixels to which the video signal has been written during the selection period retain the written voltage until the selection period begins again.

By performing the above operations all at once in the horizontal scanning direction, the video signal can be written to pixels in the horizontal scanning direction.

Furthermore, by repeating the above operations in the vertical scanning direction, it is possible to write to all pixels and display an image.

Although the driving method shown in FIG. 3 uses gradation display using a pulse width modulation method, there is also a method that uses gradation display using a pulse height modulation method.

[0013]

[Problems to be Solved by the Invention] Generally, in an active matrix type liquid crystal display device using nonlinear elements,
The parasitic capacitance of the thin film diode is added in parallel to the wiring capacitance of the scanning line, and the length of the scanning line is longer than the signal line, so the time constant of the scanning line is larger than the time constant of the signal line. When the time constant of this scanning line is so large that it cannot be ignored with respect to one horizontal scanning period, the following problem occurs due to the delay of the scanning line input signal. That is, the farther a pixel is from the scanning line signal input end of the scanning line, the higher the resistance of the thin film diode in a conductive state becomes, and the voltage of the signal line cannot be applied to the pixel, resulting in insufficient writing. Particularly in a large-area active matrix type liquid crystal display device, the scanning lines are inevitably long, so the resistance of the scanning lines becomes large, and pixels are insufficiently written and images cannot be displayed. For this reason, it is extremely difficult to increase the area of an active matrix type liquid crystal display device using conventional techniques. Therefore, in order to increase the area, it is necessary to eliminate the limitation of pixel writing due to the delay of the scanning line input signal.

In addition, in a liquid crystal display device using a thin film diode element, the parasitic capacitance of the thin film diode itself is
When the capacitance is not negligible compared to the liquid crystal capacitance, there is a problem that the voltage written to the liquid crystal capacitor is shifted due to capacitance division with the parasitic capacitance, and a sufficient voltage cannot be applied to the liquid crystal. There are two possible ways to reduce this shift amount. One method is to reduce the parasitic capacitance of the thin film diode element so that it is negligible compared to the liquid crystal capacitance. As a specific method, using a lateral type thin film diode is known. The second method is
The idea is to lower the voltage applied to the thin film diode during the selection period. However, the thin film diode element is an element that utilizes the Poole-Frenkel current that flows when a high voltage is applied as a conductive state. Therefore, in a liquid crystal display device using a thin film diode element, it is difficult to lower the applied voltage during the selection period using the second method. Therefore, a control element is required to replace the thin film diode element.

[0015]

[Means for Solving the Problem] A plurality of scanning lines and a pixel electrode array are provided on a first substrate, a nonlinear element is provided between the scanning line and the pixel electrode, and a plurality of signal lines are provided on a second substrate. In an active matrix liquid crystal display device comprising a liquid crystal sandwiched between the first substrate and the second substrate, a photoconductive film is sandwiched between the pixel electrode and the scanning line;
By providing a photoconductive layer close to the photoconductive film, using the photoconductive film as the nonlinear element, propagating light to the photoconductive layer, and irradiating the light from the photoconductive layer to the photoconductive film. The above problem was solved by applying a voltage to the pixel electrode using the resulting change in resistance of the photoconductive film and writing to the pixel.

Further, in the liquid crystal display device, the pixel group is composed of a plurality of blocks divided in the vertical scanning direction, and the scanning line is electrically independent for each block. Selection is made to a group of pixels for each block, and the selection period does not overlap for each block,
The above problem was solved by using a driving method in which the polarity of the input signal of the scanning line is inverted for each block and the polarity of the input signal is inverted for each field.

Furthermore, in the liquid crystal display device, a portion of the photoconductive layer also serves as a scanning line electrode, thereby making it possible to provide a high-speed, large-screen liquid crystal display device without lowering the aperture ratio.

[0018]

[Function] The present invention uses the photoconductive film whose resistance changes upon irradiation with light as a control element for the voltage applied to the pixel. Therefore, there is almost no delay in the scanning line, and voltage can be applied to the pixels at a very high speed. This eliminates the limitation on increasing the area due to the delay of the scanning line described above, and it is possible to realize an active matrix type large area liquid crystal display device.

Further, in the liquid crystal display device in which the scanning lines are electrically independent for each block, the retention period of each block is lengthened by ensuring that the writing of the pixel groups in each block does not overlap between blocks. It is possible,
It is possible to drive an active matrix type large area liquid crystal display device at extremely high speed.

Further, when the liquid crystal display device and the driving method of the present invention are used, the potential difference between the scanning line signal and the signal line signal is made smaller than that of a conventional liquid crystal display device using a thin film diode, and the potential difference between the scanning line signal and the signal line signal is reduced. A liquid crystal display device can be driven. This makes it possible to reduce the shift amount of the write voltage due to the capacitance division.

Further, in the liquid crystal display device, since a part of the photoconductive layer also serves as a scanning line, there is no need to provide the scanning line separately from the photoconductive layer, so that the photoconductive layer can be used without lowering the aperture ratio. Layers and scan lines can be provided.

[0022]

EXAMPLE An example of a method for manufacturing a liquid crystal display device of the present invention will be described below with reference to FIG. First, IT is deposited on a first insulating substrate 501 made of quartz, glass, etc. using sputtering or the like.
A transparent conductive thin film such as O is deposited and photoetched to form transparent conductive thin films 502 and 502'. The transparent conductive thin film 502 is used as a pixel electrode, and the transparent conductive thin film 502' is used as a core of a photoconductive layer and a scanning line. Here, the transparent conductive thin film 502' has both the function of the core of the photoconductive layer and the function of the scanning line, and there is no need to newly provide the core of the photoconductive layer or the scanning line.

Next, Si is grown using a low pressure chemical vapor deposition method or the like.
An O2 film is deposited and photoetched to form a SiO2 film 503 used as a cladding of the photoconductive layer. The SiO2 film 503 is the transparent conductive thin film 502'
A substance with a refractive index smaller than that may be used instead.

Next, after successively depositing a p-type amorphous Si film, an amorphous Si film, and an n-type amorphous Si film using plasma chemical vapor deposition, etc., photoetching is performed to form the amorphous silicon film. Quality Si
A film 504 is formed. The amorphous Si film 504 is used as a photoconductive film. Instead of the amorphous Si film 504, an amorphous metal such as amorphous Se, a compound semiconductor such as CdSe, or a photoconductive material such as an organic metal may be used.

Next, after depositing a Ti film using sputtering or the like, photoetching is performed to form the transparent conductive thin film 50.
2 and the Ti film 505 adjacent to the amorphous Si film 504.
form. The Ti film contains Al, Cr, T instead.
Metals such as a, Cu, etc. may be used.

After that, the SiO2 film 5 is formed using sputtering or the like.
06 is deposited and photo-etched. This Si
The O2 film 506 is used as a passivation film, and the S
An insulating film such as iNX or polyimide may also be used.

FIG. 4 is a view of the liquid crystal display device viewed from above the first insulating substrate.

On the other hand, a transparent conductive film such as ITO is deposited on the second insulating substrate by sputtering or the like, and then photo-etched. This ITO is used as a signal line.

A liquid crystal display device is obtained by placing the second insulating substrate and the first insulating substrate facing each other with a gap of several μm between them, and filling the gap with liquid crystal.

Next, a method for driving the liquid crystal display device of the present invention having the above structure will be explained with reference to FIGS. 1 and 6. FIG. 1 is an equivalent circuit diagram of the liquid crystal display device of the present invention. FIG. 6 is a diagram illustrating a method of driving the liquid crystal display device. Note that 103 in FIG. 1 equivalently represents the liquid crystal capacity of the liquid crystal.

Generally, liquid crystals need to be driven with alternating current. For this reason, a video signal 607 whose polarity is inverted at regular intervals is input to the signal line 105. On the other hand, scanning line 101
A scanning line signal 608 whose amplitude is at least larger than the sum of the voltage value indicating the optical threshold of the liquid crystal and the voltage value of 1/2 of the maximum amplitude of the video signal is input. The period of the scanning line signal is twice that of one field. By inverting the polarity of the scanning line signal every field, leakage of DC components from the scanning line to the liquid crystal can be eliminated.

At this time, the laser beams LZ1 and LZ2 are propagated to the photoconductive layers 102 and 102', and the photoconductive film 104
is irradiated. The photoconductive film 104, whose resistance has been lowered by the laser beam, applies the voltage of the scanning line 101 to the liquid crystal. As a result, the video signal is written into the liquid crystal. In addition, as long as the laser beam can propagate and give optical energy to the photoconductive film,
Other coherent light or incoherent light may be used instead.

Next, after the video signal to be displayed is written into the pixel, the irradiation of the laser beam LZ1 is stopped. At this time, since the photoconductive film 104 has a high resistance, the voltage of the scanning line 101 is not applied to the liquid crystal. This starts holding the pixel.

Next, by repeating the above operations for pixels close to the photoconductive layer 102', it is possible to write the video signal to the pixel group of the first block and start holding it. The pixel group continues to hold the video signal until it is irradiated with the laser beams LZ1 and LZ2 again.

Next, operations similar to those described above are performed on the pixel group of the second block, and a video signal can be written to the pixel group and retention can be started. Until the pixel group is irradiated with laser beams LZ1 and LZ2 again,
Continue to hold the video signal. However, the polarity of the potential of the scanning line is opposite to that of the scanning line of the first block.

As described above, the video signal is sequentially written into each block, the video signal is held, and an image is displayed.

[0037]

[Effect of the invention] By using the liquid crystal display device of the present invention,
Since the structure is not particularly complicated compared to a liquid crystal display device using thin film diode elements, a low-cost liquid crystal display device can be provided. Furthermore, by using the driving method of the present invention, it is possible to provide a large-area, high-speed active matrix liquid crystal display device that could not be realized conventionally.

Furthermore, in a conventional electrically scanned active matrix liquid crystal display device, when a scanning line is broken, the potentials on the left and right sides of the broken line are different due to the delay in the scanning line, resulting in display unevenness. By simultaneously inputting laser light from both sides of the photoconductive layer using the liquid crystal display device of the present invention, display unevenness as described above will not occur if there is only one disconnection for one photoconductive layer. .

[0039] Furthermore, the transparent conductive thin film of the liquid crystal display device has both the function of the core of the photoconductive layer and the function of the scanning line, so there is no need to newly provide the core of the photoconductive layer or the scanning line. . Therefore, the process is simplified, and a large-screen, high-speed liquid crystal display device can be realized without reducing the aperture ratio.

[Brief explanation of the drawing]

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

FIG. 2 is an equivalent circuit diagram of a liquid crystal display device using a conventional thin film diode element.

FIG. 3 is a diagram illustrating a method of driving a liquid crystal display device using a conventional thin film diode element.

FIG. 4 is a diagram of the liquid crystal display device of the present invention viewed from above the first insulator substrate.

FIG. 5 is a sectional view taken along line A-A' in FIG. 4.

FIG. 6 is a diagram illustrating a method for driving a liquid crystal display device of the present invention.

[Explanation of symbols]

101 Scanning line 102, 102' Photoconductive layer 103 Liquid crystal capacitor 104 Photoconductive film 105 Signal line 106 Scanning line 107 Photoconductive layer 108, 108' Scanning line voltage source 201, 201' Scanning line 202, 202' Signal line 203, 203 'Thin film diodes 204, 204' Liquid crystal capacitor 301 Axis representing time 302 Axis representing voltage 303 Ground potential 304 Scanning line signal 305 Signal line signal 306 Signal applied to pixel 307 Selection period 401 ITO film 402 ITO film 403 SiO2 Film 404 Photoconductive film 405 Ti film 501 Insulator substrate 502, 502' ITO film 503 SiO2 film 504 Photoconductive film 505 Ti film 506 SiO2 film 601 Axis 602 representing time Axis 603 representing signal line voltage Voltage of scanning line Axis 604 representing the voltage applied to the pixel 605 Axis representing the light intensity of the laser beam 606 Ground potential 607 Signal line signals 608, 608' Scanning line signal 609 Laser light 610 Signal applied to the pixel

Claims (5)

[Claims] 1. A plurality of scanning lines and a pixel electrode array are provided on a first substrate, a nonlinear element is provided between the scanning line and the pixel electrode, and a plurality of signal lines are provided on a second substrate,
In an active matrix liquid crystal display device in which a liquid crystal is sandwiched between the first substrate and the second substrate, a photoconductive film is sandwiched between a pixel electrode and a scanning line, and a photoconductive film is disposed adjacent to the photoconductive film. The resistance of the photoconductive film is generated by providing a photoconductive layer, using the photoconductive film as the nonlinear element, propagating light to the photoconductive layer, and irradiating the photoconductive film with the light from the photoconductive layer. 1. A liquid crystal display device characterized in that a voltage is applied to the pixel electrode using changes in the pixel electrode to write data to the pixel. 2. The liquid crystal display device according to claim 1, wherein the pixel group is composed of a plurality of blocks divided in a vertical scanning direction, and the scanning line is electrically independent for each block. A liquid crystal display device featuring: 3. The method for driving a liquid crystal display device according to claim 2, wherein a pixel group is selected for each block, and the selection period does not overlap for each block. Driving method. 4. The method for driving a liquid crystal display device according to claim 3, further comprising inverting the polarity of the input signal of the scanning line for each block and inverting the polarity of the input signal for each field. A method for driving a liquid crystal display device. 5. The liquid crystal display device according to claim 1, wherein a part of the photoconductive layer also serves as a scanning line.