JP3103161B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thin film transistor (TF)
The present invention relates to an active matrix type liquid crystal display device using a switching element comprising T).

[0002]

2. Description of the Related Art Liquid crystal display devices are widely used as display elements for televisions and graphic displays.

[0003] Among them, an active matrix type liquid crystal display device is particularly excellent in high-speed response and suitable for increasing the number of pixels, and realizes high image quality, large size, and color screen of a display screen. Some are expected, research and development has been advanced, and some are already in practical use.

[0004] The display element portion of this active matrix type liquid crystal display device includes an active element substrate on which a switching element composed of an active element such as a thin film transistor and a pixel electrode are disposed, and a counter electrode disposed opposite thereto. The main part is composed of a counter substrate on which is disposed, a liquid crystal composition sandwiched between these two substrates, and a polarizing plate attached to each of the two substrates.

An active element substrate is provided on a transparent insulating substrate such as a glass plate or a plastic film so that scanning lines and signal lines are orthogonal to each other, and a switching element and a pixel electrode are provided at each intersection of the scanning lines and signal lines. Is arranged.

[0006] Since the driving control of each pixel is performed in a decentralized manner by the switching element, and the variation in the voltage applied to the liquid crystal is greatly reduced, the pixel can be driven at high speed, and the number of pixels and the area can be increased. It becomes possible.

As a switching element, a thin film transistor (Thin Film Transistor) having a steep on / off characteristic suitable for the intended use is usually used.
r, hereinafter abbreviated as TFT). TFT is thin film I
This is a kind of an insulated gate field effect transistor formed by using the C technology.

FIG. 5 is an equivalent circuit diagram of a display element portion (for one pixel) of a conventional active matrix type liquid crystal display device. This is because the gate (G) is on the scanning line 501, the drain (D) is on the signal line 502, and the source (S) is on the pixel electrode 5.
03 n-type TFT switching element 504 connected to
Is used.

Further, each of the pixel electrodes 503 forms a pixel composed of an individual liquid crystal cell 506 with a counter electrode 505 via a liquid crystal. The liquid crystal cell 506 can be represented by an electrostatic capacitance _CLC as an equivalent circuit.

An auxiliary capacitance C _S 509 for holding a charge is connected in parallel with the liquid crystal cell 506. That one end 507 pixel electrode 503 of the liquid crystal cell 506 of the auxiliary capacitance C _S 509, the other end 508 is an auxiliary capacitor voltage line 513
It is connected to the. This auxiliary capacitance C _s 509 is
After the source output voltage pulse of the FT switching element 504 is applied, the voltage is maintained, and a constant voltage is applied to the connected liquid crystal cell 506 to continuously excite the liquid crystal until the next frame scanning.

FIG. 6 is a waveform diagram showing various pulses applied to a conventional liquid crystal display device.

[0012] Figure 6 when the video signal pulse V _X is applied to the drain shown in (b), about 63.5 in one horizontal line scan period (e.g., NTSC standard for television display image as shown in FIG. 6 (a) When the scanning pulse _VY is applied to the gate at a period of (μs), a current flows between the drain and the source, a voltage is applied to the pixel electrode 503 connected to the source, and the potential of the liquid crystal cell 506 becomes The potential V _S becomes as shown in FIG. At this time the auxiliary capacitor C _S 509 pixel electrode side potential same as the potential V _S at one end 507 of the
Becomes

Then, the voltage V _S applied to the pixel electrode 503 of the liquid crystal cell and the voltage applied from the counter electrode voltage application section 512 to the counter electrode 505 are superimposed and applied to the liquid crystal to drive the liquid crystal. You.

However, the stray capacitance C _GS 510 exists between the gate and the source of the TFT switching element 504 as described above. Due to this, when the waveform suddenly falls as shown in FIG. _6A for each scanning cycle T _F of the scanning pulse V _Y , the source output voltage of the TFT switching element 504 changes to the level shown in FIG. ) Occurs as shown in FIG.

[0015] The level shift ΔV1 is shifted to the steeply varying the source output voltage, also always lower despite a central value of the source output voltage to the polarity of the video signal pulse V _X (low potential direction).

The ΔV1 is such that the amplitude of the scanning pulse voltage V _Y is dV _Y , the auxiliary capacitance C _S is C _S [F], the capacitance of the liquid crystal cell is C _LC [F], and the floating capacitance C _GS is C _GS [ F], it can be represented by the following equation. That is, ΔV 1 = dV _Y × [C _GS / (C _GS + C _LC + C _S )] According to this equation, in order to set the level shift ΔV 1 to 0 and suppress flicker on the display screen, it is necessary to erase C _GS or
Alternatively, dV _Y must be set to 0. However, this _CGS is inevitably present in the structure of the TFT switching element, and it is virtually impossible to eliminate itself. In addition, the amplitude dV _Y of the scanning pulse voltage is always 0.
Obviously you can't.

Therefore, in the conventional liquid crystal display device, a DC bias voltage is applied to the counter electrode 505 of the liquid crystal display element to compensate for the voltage corresponding to the level shift, and the voltage of the liquid crystal applied voltage reduced by the decrease in the source output voltage is reduced. The method of raising the median is already known.

However, since the capacitance C _LC of the liquid crystal cell changes depending on the voltage applied to the liquid crystal, ΔV 1 changes with the change of the capacitance C _LC as is apparent from the above equation. In a liquid crystal display device in which a different voltage is applied to each pixel, the voltage value to be raised is different for each pixel. Impossible. Therefore, in the conventional liquid crystal display device, the voltage of the level shift can be compensated only approximately by taking the average value of the liquid crystal applied voltage and raising the median value. As a result, there is a problem that flicker and display unevenness remain depending on a pixel, and a problem that a DC voltage is applied to the liquid crystal, which causes deterioration of the liquid crystal and burn-in of the display.

Further, as a means to try to reduce the above ΔV1 itself, by disposing the like delay circuit to the output line of the scanning line driver circuit, when the fall of the scanning pulse V _Y, i.e. the gate input of TFT504 When turning off the voltage pulse, there is already known means for suppressing ΔV1 by turning off the voltage pulse, instead of sharply dropping it into a square pulse, and turning it off.

However, in the means for disposing the delay circuit on the output line of the scanning line driver circuit as described above, it is necessary to add a problem that the switching speed itself is reduced and a complicated delay circuit is added to the device. However, there are problems that the apparatus and its manufacturing process are complicated, the manufacturing yield is reduced, and the manufacturing cost is increased accordingly.

[0021]

As described above, in the conventional liquid crystal display device, the level shift of the source output voltage caused by the stray capacitance of the TFT switching element causes a problem.
There has been a problem that flicker, display unevenness, and display burn-in occur on the display screen. Moreover, even in the known solution to this, there is a problem that the level shift .DELTA.V1 is only approximately compensated, and the deterioration of the liquid crystal is accelerated.

Further, in the means for reducing the level shift ΔV1 itself, there is a problem that the switching speed itself is reduced, and a complicated delay circuit must be added to the device, which complicates the device and its manufacturing process. However, there is a problem that the production yield is reduced and the production cost is increased accordingly.

The present invention has been made to solve such a problem, and an object of the present invention is to solve the problem of flicker on a display screen caused by a level shift of an output voltage of a TFT switching element. Another object of the present invention is to provide a liquid crystal display device which can realize stable and high-quality image display, has a simple structure, and has a low manufacturing cost.

[0024]

In order to solve the above-mentioned problems, a liquid crystal display device according to the present invention comprises a scanning line, a signal line disposed so as to intersect the scanning line, the scanning line and the signal line. An active matrix type liquid crystal display element provided with a switching element composed of a transistor connected to a line, and a scanning line having an output buffer and applying a scanning pulse to the switching element via the output buffer and the scanning line In a liquid crystal display device having a drive circuit and a signal line drive circuit for applying a signal pulse to the signal line, the output buffer is provided at a final output stage to cause a trailing edge of the scan pulse to fall slowly. It is characterized by having a load type inverter.

The scanning line driving circuit may be provided on the same substrate in the liquid crystal display device together with the scanning lines, signal lines, and switching elements.

[0026]

[Function] When the gate voltage sharply falls in a square wave shape,
The resistance between the drain and the source of the TFT instantaneously becomes high.
Then, no electric current flows into the TFT from the previous stage circuit, so that no charge flows into the TFT, and the TFT and the capacitors C _GS ,
The charge moves and is redistributed between C _LC and C _S. This means that the charges stored in the capacitance C _LC and the auxiliary capacitance C _S of the liquid crystal cell move to the floating capacitance C _GS .
As a result, a level shift ΔV1 of a value as shown in the above-mentioned relational expression occurs.

Therefore, in the liquid crystal display device of the present invention, the final output stage of the output buffer used in the scanning line driving circuit is a constant current source load type inverter. In this constant current source load type inverter, the leading edge of the scanning pulse has a steep rising, and the trailing edge of the scanning pulse has a slow falling.
That is, the voltage at the rise of the scan pulse applied to the switching element of the liquid crystal display device rises sharply, and the voltage at the fall gradually falls slowly.

As described above, the gate voltage of the TFT gradually decreases by slowly lowering the scanning pulse over time. Therefore, while the gate voltage is equal to or higher than the threshold voltage of the TFT, the capacitance of the signal line itself is reduced. Alternatively, the electric charge accumulated in the capacitance added to this is supplied to the floating capacitance _CGS or the like through this TFT, and the gate voltage becomes equal to or less than the threshold voltage of the TFT, so that the voltage between the drain and the source of the TFT becomes high. After the resistance, the charge moves between the capacitances C _GS , C _LC and C _S and is redistributed. At this time, the amount of charge transferred between the capacitors C _LC and C _S and C _GS is reduced, and the level shift ΔV1 of the liquid crystal applied voltage is reduced. Thus, the level shift ΔV1 can be suppressed.

Also, since the above-mentioned constant current source load type inverter is provided as the final output stage of the output buffer,
It is only necessary to partially change the structure of the final output stage of the conventional output buffer, and it is not necessary to newly add a complicated circuit such as a delay circuit outside the conventionally used scanning line drive circuit. Therefore, the level shift of the liquid crystal applied voltage as described above can be suppressed while the structure is simple and the manufacturing cost is low.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the liquid crystal display device of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a simplified diagram showing the configuration of the liquid crystal display device of the present invention, and FIG. 2 is a circuit diagram showing the configuration of the scanning line driving circuit.

This liquid crystal display device, as shown in FIG.
It includes a liquid crystal display element 1, a signal line drive circuit 2, and a scan line drive circuit 3.

The liquid crystal display element 1 includes an active element substrate 1
01, an opposing substrate (not shown) provided so as to face the active element substrate, and a liquid crystal composition (not shown) sandwiched between these two substrates.

The active element substrate 101 has the scanning lines 10
3 and the signal line 102 intersect at right angles, and a poly-Si (polysilicon) n-MOS disposed so as to be connected to the scanning line 103 and the signal line 102 at each intersection.
Switching element 105 composed of a TFT of a type (thin film transistor), and a pixel electrode 111 and an auxiliary capacitor 107 of a liquid crystal cell 106 connected to the switching element 105 so that a voltage applied to the liquid crystal cell 106 is controlled by the switching element 105. And this auxiliary capacity 10
And a storage capacitor common line 108 connected to the switching element 105 and the liquid crystal cell 10.
A portion 109 for displaying an image such as 6 has the same structure as an existing general liquid crystal display panel.

A scanning line 10 is provided on an outer peripheral portion 110 of a portion 109 of the active element substrate 101 for displaying an image.
A scanning line driving circuit 3 for outputting a scanning pulse to the scanning line 3 and a signal line 1
02 and a signal line driving circuit 2 for outputting a signal pulse are provided.

An outline of the structure of the active substrate 101 will be described briefly following the manufacturing process. A poly-Si film is formed on a quartz substrate, and is etched to leave a portion serving as a TFT and a storage capacitor in an island shape. Then, thermal oxidation is performed to form a gate insulating film of the TFT and an insulating film of the storage capacitor. After a second poly-Si film is formed thereon, it is etched to form a gate electrode of a TFT, a scanning line, and a common line of a storage capacitor. Next, As (arsenic) or P (phosphorus) is used for a portion to be an n-MOS-TFT, and B (phosphorus) is used for a portion to be a p-MOS-TFT.
(Boron) is ion-implanted. Then, after forming a SiO ₂ film thereon, a contact hole is opened,
A signal line and other wiring made of (aluminum) are formed. The pixel electrode is formed by forming an ITO film and then patterning by etching to form a pixel electrode formed of a transparent electrode.

The signal line drive circuit 2 and the scan line drive circuit 3 were also formed as TFT circuits together with the switching elements 105 during such a manufacturing process.

An opposing substrate is provided so as to oppose the active substrate 101. A common counter electrode 112 facing the pixel electrode 111 is provided on the counter substrate.

The signal line drive circuit 2 is a general signal line drive circuit that outputs a signal pulse to a signal line when video signal information is input from outside.

As shown in FIG. 2, the scanning line driving circuit 3
It has a shift register 201 provided for each scanning line, and an output buffer 204 including a first-stage output buffer 202 and a second-stage output buffer 203 connected to the shift register 201. .

The shift register 201 and the first-stage output buffer 202 are constituted by C-MOSs generally used conventionally. Thus, the shift register 201 and the first-stage output buffer 202
The switching speed is fast enough and low power consumption to realize high quality image display.

The output buffer 203 of the second stage is
This is a constant current source load type inverter composed of two p-MOSs 211 and 212.

The output pulse P ₀ from the shift register 201
Are output to the first-stage output buffer 202, the first-stage output buffer 202 outputs the CM
Because of the OS configuration, the first-stage pulse P ₁ having the same waveform as the input pulse and inverted in phase is output to the second-stage output buffer 203.

When the pulse P ₁ of the first stage is applied to the output buffer 203 of the second stage, first, the output buffer 2
03, the p-MOS 211 in the previous stage is turned on, and the gate of the TFT switching element 105 of the liquid crystal display element and the floating capacitance C _GS thereof are connected via the low-resistance p-MOS 211.
Scan pulse V _Y is applied to. And this stray capacitance C
_GS , the capacitance of the pixel and the capacitance of the conductor of the scanning line 103 are charged. The p-MOS2 at this time
11 is in an ON state between the drain and the source,
The resistance is small enough. Therefore, in this case, the scan pulse voltage rises sharply because the time constant is small.

When the output pulse P ₀ is no longer output from the shift register 201 to the first-stage output buffer 202 and the output of the first-stage pulse P ₁ from the first-stage output buffer 202 is turned off. First, it takes time until the p-MOS 211 in the preceding stage of the second stage is completely turned off. Then, when the p-MOS 211 is completely turned off and a state in which it can be considered that the scanning line is substantially open, the T-line of the liquid crystal display element is transmitted through the scanning line.
FT switching elements, and are you substantially connected to the floating capacitance C _GS is the subsequent stage as a constant current source load p-
Since only the MOS 212 is provided, the p-MOS 212 has a large resistance due to its constant current operation. Therefore, the time constant is large, and as a result, the scanning pulse voltage gradually falls.

Since the fall of the scan pulse is slow, charges can be supplied or discharged to the floating capacitance _CGS through the scan line within this time. This allows
A part of the electric charge to be applied to the liquid crystal is not deprived by the floating capacitance _CGS, and the level shift of the liquid crystal application voltage is suppressed.
Eliminate flicker and display unevenness.

FIG. 3 shows waveforms of various pulses applied to one pixel of the liquid crystal display device of the present invention, that is, a scanning pulse (a), a signal pulse (b), and a liquid crystal applied voltage (c) during one frame period. It is a waveform diagram.

As shown in FIG. 3A, the scanning pulse has a sharp rising edge at the leading edge of the waveform and a gentle falling edge at the trailing edge of the waveform, so that the potential gradually decreases. I have.

As a result, as shown in FIG. 3C, the liquid crystal applied voltage had a waveform in which the level shift was almost eliminated.

At this time, the value of the level shift ΔV 1 was measured and compared with the conventional case. As a result, in the conventional case, the ΔV 1 was about 0.5 V, but in the liquid crystal display device of the present invention, it was about 0.05 V. It was confirmed that it was reduced to about 1/10 compared to the conventional one. Further, when the quality of the display screen at this time was visually inspected, flicker and display unevenness were not found.

Further, as described in the above description of the structure, since the same manufacturing process can be used as in the conventional case, the yield and the manufacturing cost are the same as those in the conventional case, and a complicated delay circuit is separately provided. Such problems as a decrease in yield and an increase in manufacturing cost do not occur.

In this embodiment, the final output stage of the output buffer of the scanning line driving circuit is a constant current load type inverter composed of a p-MOS type TFT, but the present invention is not limited to this. For example, an inverter as shown in FIG. 4 may be used for this final output stage. That is, in the case of FIG. 4A, the former stage is a p-MOS and the latter stage is an n-MOS. At this time, the switching element on the liquid crystal display element side is desirably an n-MOS.

In the case of FIG. 4B, the former stage is an n-MO
S and the subsequent stage are also n-MOS. At this time, the switching element on the liquid crystal display element side is preferably a p-MOS.

In the case of FIG. 4C, the former stage is the n-MO
S, the subsequent stage is a p-MOS. At this time, the switching element on the liquid crystal display element side is preferably a p-MOS.

[0054]

As described in detail above, the liquid crystal display device of the present invention eliminates the problem of flicker on the display screen caused by the level shift of the output voltage caused by the stray capacitance of the TFT switching element and is stable. This is a liquid crystal display device which can realize a high quality image display, and has a simple structure and a low manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing a simplified configuration of a liquid crystal display device of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a scanning line driving circuit of the liquid crystal display device of the present invention.

FIG. 3 is a waveform chart showing various pulses applied to one pixel of the liquid crystal display device of the present invention.

FIG. 4 is a diagram showing a configuration of an inverter that can be used in a final output stage of the liquid crystal display device of the present invention.

FIG. 5 is an equivalent circuit diagram of a display element portion of a conventional active matrix type liquid crystal display device.

FIG. 6 is a waveform diagram showing various pulses applied to a conventional active matrix type liquid crystal display device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Liquid crystal display element 2 ... Signal line drive circuit 3 ... Scan line drive circuit 201 ... Shift register 202 ... First stage output buffer 203 ... Second stage output buffer 204 ... Output buffer 211 ... Second stage P-MOS- at the previous stage of the output buffer
TFT 212: p-MOS- in the second stage of the output buffer of the second stage
TFT

Claims (1)

(57) [Claims] 1. An active matrix type liquid crystal display comprising a scanning line, a signal line disposed to cross the scanning line, and a switching element including a transistor connected to the scanning line and the signal line. A liquid crystal having an element, a scanning line driving circuit having an output buffer and applying a scanning pulse to the switching element via the output buffer and the scanning line, and a signal line driving circuit applying a signal pulse to the signal line A liquid crystal display device according to claim 1, wherein said output buffer includes a constant-current-source-load-type inverter that makes a trailing edge of said scan pulse slowly fall at a final output stage.