JPH10333646A - Liquid crystal display device and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the yield for forming scanning wirings and signal wirings on separate substrates and make applicable different reference potentials at positions in one panel by bundling reference potential lines into multiple units. SOLUTION: Scanning wirings 3 and reference potential lines 5 are formed on an insulating substrate, a gate electrode of a thin film transistor 1 formed on the insulating substrate is connected to the scanning wirings 3, the other of two electrodes is connected to picture element electrodes 6, signal wirings 4 are formed to cross the scanning wirings 3 on a substrate facing the insulating substrate, and the signal wirings 4 are capacitively coupled with the picture element electrodes 6 via a liquid crystal. The reference potential lines 5 are bundled into multiple units, an optional potential is applied to each unit, the yield is improved for forming the scanning wirings 3 and signal wirings 4 on separate substrates, and a liquid crystal display device can be realized without deteriorating display quality.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an active matrix type liquid crystal display device using a thin film transistor as a switching element and a driving method thereof.

[0002]

2. Description of the Related Art In an active matrix type liquid crystal display device, when scanning wiring and signal wiring are formed on the same glass substrate, disconnection due to a step, insulation failure between wirings, or short circuit occurs at the intersection. And the wiring becomes defective.

Therefore, an active matrix type liquid crystal display device as shown in an equivalent circuit of FIG.
No. 3478 proposes this. That is, a thin film transistor 21, a scanning line 23, and a pixel electrode (not shown) are formed on one of the glass substrates facing each other, the gate electrode of the thin film transistor 21 is connected to the scanning line 23, and the source electrode is grounded. The signal wiring 24 is formed on the other glass substrate as a common electrode of the liquid crystal element, and the liquid crystal 22 is sandwiched between the one and the other glass substrates.

Accordingly, since the scanning wiring 23 and the signal wiring 24 are formed on different glass substrates, wiring, insulation failure or short circuit due to a step does not occur.

In such a structure, of the two electrodes other than the gate electrode of each thin film transistor,
One is connected to each pixel electrode, while the other is generally set to the same potential in the panel by grounding or the like.

[0006]

In recent years, in particular, liquid crystal display devices have been increasing in size and definition, and the display quality required for the display devices has been increasing with the progress of multimedia. By the way, one of the factors hindering the realization of the high display quality is a flickering phenomenon of the screen called flicker. This cannot be solved by forming the scanning wiring and the signal wiring on separate substrates.

As the size of the active matrix type liquid crystal display device increases, the size of the glass substrate on which the active matrix type liquid crystal display device tends to increase also tends to increase.

In the process of manufacturing a thin film transistor, the thickness of a gate insulating film deposited on a glass substrate varies considerably due to unevenness of an inflow gas when the gate insulating film is formed. It is not difficult to imagine that the difference in film thickness will become even larger when the substrate becomes large.

Here, for example, if a large difference occurs in the thickness of the gate insulating film in one panel serving as a display device, even if the reference voltage is adjusted to suppress flicker, the thickness of the gate insulating film may be reduced. In some cases, it may not be possible to set a reference voltage suitable for both two places (pixels) having a large difference. That is, there is a problem that flicker occurs in any part of the screen and high display quality cannot be obtained.

In such a case, at least the display quality level that can be shipped as a product is not attained, and the yield is eventually reduced. In the future, as glass substrates or panels per sheet become larger, this will become an even bigger problem.

[0011]

According to a first aspect of the present invention, a scanning line and a reference potential line are formed on an insulating substrate, and a gate electrode of a thin film transistor formed on the insulating substrate is connected to the scanning electrode. Connected to the wiring, one of the other two electrodes is connected to the reference potential line, the other of the other two electrodes is connected to the pixel electrode, and intersects with the scanning wiring on the substrate facing the insulating substrate. In a liquid crystal display device in which a signal wiring is formed and the signal wiring is capacitively coupled to the pixel electrode via a liquid crystal, the reference potential line is bundled in a plurality of units.

Thus, the yield can be improved due to the fact that the scanning wiring and the signal wiring are formed on separate substrates, and it is possible to apply different reference potentials depending on the position in one panel.

According to a second aspect of the present invention, there is provided a driving method of a liquid crystal display device, wherein a scanning line and a reference potential line are formed on an insulating substrate.
A gate electrode of the thin film transistor formed over the insulating substrate is connected to the scan wiring, one of the other two electrodes is connected to a reference potential line, and the other of the other two electrodes is connected to a pixel electrode. In a method for driving a liquid crystal display device, a signal wiring is formed on a substrate facing an insulating substrate so as to intersect with a scanning wiring, and the signal wiring is capacitively coupled to the pixel electrode via a liquid crystal. An arbitrary potential is given to each of the plurality of units in which is bundled.

Thus, a variation in characteristics of the thin film transistor, which occurs particularly in a large screen, can be suppressed, and a liquid crystal display device with high display quality can be realized.

[0015]

Embodiments of the present invention will be described below.

(Embodiment 1) As shown in FIG. 3, in a glass substrate 7, there is a tendency that the thickness of a gate insulating film is the thickest in a peripheral portion 8 and conversely, it is thin in a central portion 9. .

FIG. 4 shows an example in which four panels are formed from one glass substrate. Looking at one of the liquid crystal panels (upper right side of the glass substrate in FIG. 4), the lower left side has a tendency to be thinner as shown in FIG.

Therefore, consideration is given to a structure in which the reference potential at the lower left of this panel is different from the reference potential at other portions. FIG. 2 schematically shows how two types of reference potentials are applied in the panel. FIG. 1 shows the state of the pixels 6 in each region. Here, the signal wiring 4 is formed by two dotted lines because it is formed on the opposite substrate.

Here, within one liquid crystal panel, the pixels A, which have different transistor characteristics depending on the thickness of the gate insulating film,
B will be described based on an equivalent circuit.

FIGS. 6A and 6B show simplified equivalent circuits when the gates are on and off in pixels A and B, respectively. Since the scanning wiring 3 and the signal wiring 4 are formed on separate substrates, the pixel electrode and the signal wiring 4 are capacitively coupled. Since the potential Vr from the reference potential line 5 is determined depending on whether the transistor is on or off, the thin film transistor 1 is shown here as a switch.

As shown in FIG. 6, when viewed from the gate side of the thin film transistor 1, the capacitance between the gate of the thin film transistor 1 and the pixel and the capacitance Clc of the liquid crystal layer are connected in series. Although the equivalent circuit diagrams of the pixels A and B are the same in the configuration, the capacitance between the gate and the pixel differs due to the variation in the thickness of the gate insulating film. To compensate for this, the reference potential is changed in the present invention. To apply.

Therefore, the capacitance between the gate and the pixel is Cgpa and Cgpb, respectively, and the reference potential is Vra and Cra, respectively.
It is represented by Vrb.

From FIG. 6, when the gate is turned on, the following equation holds for the charge in each capacitor for the pixel A.
The voltage when the gate is on is Vgh, the data voltage is Vd1,
The liquid crystal layer capacitance is Clc, the charge in the liquid crystal layer is Qlca,
Assuming that the charge between the gate and the pixel is Qgpa, equations (1) and (2) are obtained.

[0024]

(Equation 1)

[0025]

(Equation 2)

When the gate is turned off, each charge changes. Assuming that these are Qgpa 'and Qlca', the following equation is established. The voltage when the gate is off is Vgl, and the data voltage is Vd2. When the gate is on, the reference voltage Vra
Assuming that the voltage at the position where has been input is Va ′, equations (3) and (4) are obtained.

[0027]

(Equation 3)

[0028]

(Equation 4)

At this time, equation (5) holds.

[0030]

(Equation 5)

Therefore, the off-state potential Va ′ at the point where the reference potential Vra was input when the gate of the transistor is on is obtained from each of the above equations, as shown in Equation (6).

[0032]

(Equation 6)

When the pixel B is obtained in the same manner, the result is expressed by the following equation (7).

[0034]

(Equation 7)

Therefore, the voltage applied to the liquid crystal layer shifts in the negative direction by switching the gate of the transistor from on to off. As a result, asymmetry of the AC drive occurs, which causes flicker.

Conventionally, the shift amount also varies depending on the position of the pixel in the panel due to the variation in the thickness of the gate insulating film particularly in a large screen. In the present invention, this is compensated for by providing a plurality of reference voltages.

Here, for the sake of simplicity, a case where the same pattern signal is displayed in one-frame inversion driving will be considered. Therefore, the term of Clc (Vd2-Vd1) is omitted.

Therefore, from equations (6) and (7), V
a ′ is represented as in equation (8).

[0039]

(Equation 8)

Similarly, for pixel B, Vb 'is expressed as in equation (9).

[0041]

(Equation 9)

In the above equation, the voltage V applied to the liquid crystal
To make a ′ and Vb ′ equal, the equation (1)
0).

[0043]

(Equation 10)

By setting the reference potentials Vra and Vrb in such a relationship, the difference between the characteristics of the thin film transistors is compensated.

In particular, one pixel of the thickest portion 8 in this panel is denoted by A, and one pixel of the thinnest portion 9 is denoted by B. Assuming that the thickness of the pixel A is 7% greater than a predetermined value (for example, 3000 angstroms) and that of the pixel B is 7% smaller than the predetermined value, the capacitance has a relationship of Cgpb ≒ 1.15Cgpa. Become.

Cgpb = 1.15 Cgpa is calculated by the formula (1)
0) to obtain the relationship between Vra and Vrb, the result is as shown in Expression (11).

[0047]

[Equation 11]

Thus, the reference voltages Vra and Vr for compensating for the difference between the DC voltage level shifts of A and B
b was set. When the reference voltage was made uniform, flicker was observed. However, when the reference voltage was set in this manner, no flicker was observed.

When the reference voltage is divided into a plurality of units, different reference voltages are applied to the regions having greatly different characteristics. The variation in the characteristics of the thin film transistor within the region where the same reference voltage is applied is within a range where the display quality is not deteriorated.

The setting of the reference voltage can be performed after confirming the display quality at the stage of modularization, so that the inspection process time does not increase. In the case where the reference potential line is divided, it can be formed simultaneously with the scanning wiring, so that the number of steps does not increase.

In this embodiment, the same pattern is displayed in frame inversion for simplicity. However, the present invention is not limited to this, and is applicable to line inversion driving and general display pattern driving. This is a possible compensation method.

[0052]

As described above, in the liquid crystal display device of the present invention, the reference potential lines are bundled into a plurality of units, and each unit is provided with an arbitrary potential, so that the scanning lines and the scanning lines are connected. In addition to improving the yield due to the signal wiring and being provided on separate substrates, even in a large-screen liquid crystal display device, the DC voltage level shift of the liquid crystal layer due to the gate on / off switching of the thin film transistor is applied to the entire screen. It is possible to realize a liquid crystal display device that does not greatly vary and does not lower display quality.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a liquid crystal display device of the present invention.

FIG. 2 is a diagram showing input of two different reference voltages in the present invention.

FIG. 3 is a diagram showing a variation in thickness of a gate insulating film in a glass substrate.

FIG. 4 is a diagram showing a state where four panels are taken from one glass substrate.

FIG. 5 is a diagram showing one of the four panels.

FIG. 6 is an equivalent circuit diagram showing a pixel of the present invention.

FIG. 7 is an equivalent circuit diagram of a conventional active matrix type liquid crystal display device.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 thin film transistor 2 liquid crystal layer 3 scanning wiring 4 signal wiring 5 reference potential line 6 pixel electrode 7 glass substrate 8 thick gate insulating film (peripheral part in glass substrate) 9 thin gate insulating film (glass) (The center of the board)

Claims (2)

[Claims] 1. A scanning wiring and a reference potential line are formed on an insulating substrate, a gate electrode of a thin film transistor formed on the insulating substrate is connected to the scanning wiring, and one of the other two electrodes is a reference potential line. And the other of the other two electrodes is connected to a pixel electrode, and a signal wiring is formed on a substrate facing the insulating substrate so as to intersect with the scanning wiring. The signal wiring is connected to the pixel via a liquid crystal. A liquid crystal display device capacitively coupled to an electrode, wherein the reference potential lines are bundled in a plurality of units. 2. A scanning wiring and a reference potential line are formed on an insulating substrate, a gate electrode of a thin film transistor formed on the insulating substrate is connected to the scanning wiring, and one of the other two electrodes is a reference potential line. And the other of the other two electrodes is connected to a pixel electrode, and a signal wiring is formed on a substrate facing the insulating substrate so as to intersect with the scanning wiring. The signal wiring is connected to the pixel via a liquid crystal. A method for driving a liquid crystal display device capacitively coupled to an electrode, wherein an arbitrary potential is applied to each of the plurality of units in which the reference potential lines are bundled.