JPH11183932A - Active matrix type liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active matrix type liquid crystal display device with a high picture quality and a high numerical aperture by suppressing increase in a punch-through voltage, a time constant of scanning lines, and occurrence of reverse. SOLUTION: A lot of scanning lines 34a, 34b and signal lines 32 formed on an insulating substrate and auxiliary capacity lines 35 extending in parallel to scanning lines are formed on an array substrate 20 of a liquid crystal display panel. In a region surrounded by the scanning lines and the signal lines, a picture element electrode 36 is formed and connected with the scanning lines and the signal lines via a TFT 38. A part of the picture element electrode is formed overlapping the scanning line 34b and composes a 1st auxiliary capacity Cs1. Moreover, the picture element electrode is formed overlapping the auxiliary capacity line electrically separated from the picture element electrode and the scanning line, and composes a 2nd auxiliary capacity Cs2 across the auxiliary capacity line.

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 having a TFT array substrate using thin film transistors (hereinafter, referred to as TFTs).

[0002]

2. Description of the Related Art Since an active matrix type liquid crystal display device can display a high contrast ratio without crosstalk, a large screen and a high definition display are being developed and commercialized. In particular, active matrix type liquid crystal display devices have been actively developed into direct-view transmission type displays in which TFTs and MIMs are provided as switching elements on a transparent insulating substrate, and can be easily formed on a large-area substrate. For this reason, many TFTs use amorphous silicon (a-Si) as the semiconductor layer.

At present, a diagonal of 10 using an a-Si TFT is used.
Direct-view transmissive liquid crystal display devices of inch class or higher have already been commercialized, and developments for larger screens and higher definition have been actively pursued. At the same time, the development of high aperture ratio devices aiming at higher luminance and lower power consumption is also being actively pursued.

In general, in an active matrix type liquid crystal display device, a large number of scanning lines are formed in a row direction and a large number of signal lines are formed in a column direction on a TFT array substrate. Are disposed. The gate electrode of the TFT is connected to a scanning line, the drain electrode is connected to a signal line, and the source electrode is connected to a pixel electrode. Part of the pixel electrode is formed so as to overlap on an adjacent scanning line, and forms a storage capacitor (Cs).

In the TFT array substrate, a scanning line pattern integral with a gate electrode made of metal such as Mo is formed on a transparent insulating substrate, and a gate insulating film made of an insulator such as SiO is superposed on the scanning line. Are formed. An active layer made of a semiconductor such as a-Si on the gate electrode;
A channel protective film made of an insulator such as SiN and a contact layer made of a semiconductor such as a-Si doped with impurities are formed to constitute a TFT.

On the gate insulating film other than the TFT portion, an IT
A pixel electrode made of a transparent conductive film such as O is formed. Further, a signal wiring pattern integral with a drain electrode made of a metal such as Al and a source electrode are formed, and the source electrode is connected to the pixel electrode. Then, a passivation film made of an insulator such as SiN is formed in a region excluding these upper pixel electrodes to constitute a TFT array substrate.

On the other hand, on a counter substrate of a liquid crystal display device, a black matrix (BM) pattern made of a light-shielding material such as Cr is formed on a transparent insulating substrate, and red, green, and blue colored layers are formed thereon. In addition, a counter electrode made of a transparent conductive film such as ITO is formed on the colored layer.

[0008] Then, a TFT array substrate and a counter electrode substrate are bonded to face each other, and a liquid crystal composition is sealed in a gap therebetween to form a liquid crystal display device. In the equivalent circuit of the liquid crystal display device having such a configuration, the gate electrode of the TFT is connected to the scanning wiring, and the drain electrode is connected to the signal wiring. The source electrode is connected to a liquid crystal capacitance (Clc) formed for the counter electrode and an auxiliary capacitance (Cs) formed for the previous gate wiring. As the parasitic capacitance, the capacitance between the gate and the pixel electrode (Cgs) and the capacitance between the signal wiring and the pixel electrode (Cp)
s) also exists.

[0009]

In the liquid crystal display device of the active matrix type configured as described above, when the potential of the gate wiring of the preceding stage is switched from the off level to the on level, the potential of the pixel electrode of the next stage is reduced by capacitive coupling. fluctuate.
The amount of change between pixel potentials (hereinafter referred to as a push-up voltage) (ΔVr) is obtained by calculating the amount of change in the gate potential at the preceding stage by ΔVg
Then, it is expressed by the following equation. ΔVr = ΔVg × Cs / (Clc · + Cs + Cgs + Cps) (1) ΔVr is a DC voltage application component to the liquid crystal layer,
An increase in Vr causes poor image quality such as flicker and a shift in voltage transmittance (VT) characteristics.

On the other hand, in a liquid crystal display device, a black matrix (BM) pattern is generally provided on a counter substrate in order to block unmodulated light from other than the pixel electrodes. The BM pattern and the pixel electrode are designed to overlap by about 4 to 8 μm in consideration of the alignment accuracy between the TFT array substrate and the counter substrate. That is, since the pixel electrode overlaps the BM pattern, there is an ineffective portion that does not contribute to the aperture ratio around the pixel electrode.

Therefore, in order to form an auxiliary capacitance using such an ineffective portion, a pattern extending from the scanning line is provided in a gap between the signal wiring and the pixel electrode, and the extended pattern and the pixel electrode are connected to each other. There has been proposed a liquid crystal display device formed by laminating the above. Such a liquid crystal display device can improve the aperture ratio by the following two points.

One is that the auxiliary capacitance formed on the scanning line can be reduced and the scanning line width can be reduced. The other is B in which an extended pattern is formed on a TFT array substrate.
Utilizing as an M pattern, the amount of overlap between the extended pattern and the pixel electrode can be reduced.

However, also in the liquid crystal display device having such a configuration, the equivalent circuit is the same as the above-described equivalent circuit of the liquid crystal display device, and there is a problem of the thrust voltage (ΔVr). Further, since the width of the scanning line is reduced without changing the load capacitance of the scanning line, the time constant of the scanning line increases. The increase in the number of scanning lines slows down the gate potential waveform for turning the TFT on and off, causing a reduction in the on-time of the TFT and a delay in the off-timing. A reduction in the on-time of the TFT results in a decrease in contrast ratio, and a delay in the off-timing results in a reduction in resolution, resulting in poor image quality.

Another problem is that an area where reverse is likely to occur occurs near the overlapping portion between the extended pattern and the pixel electrode. The reverse width decreases as the potential difference between the extended pattern and the pixel electrode decreases, but the extended pattern has the same potential as the scanning line, and the degree of freedom in setting the potential is low. When the overlap between the extended pattern and the pixel electrode is reduced in order to improve the aperture ratio, reverse occurs inside the opening from the overlapping portion between the extended pattern and the pixel electrode, causing image quality defects such as screen roughness. Cheap.

In order to avoid the problem related to ΔVr with respect to the above problems, the amount of Cs may be reduced.
In this case, the holding capacity (CIc + Cs) is reduced, and the holding performance as an active matrix liquid crystal display device cannot be satisfied.

In order to avoid a problem associated with an increase in wiring capacitance, there is a method of increasing the scanning line width and lowering the wiring resistance, but in this case, the aperture ratio is reduced. As another method, power can be supplied to the scanning line from both left and right ends of the scanning line. However, in this case, the number of driver-ICs for driving the scanning line is doubled and the cost is increased.

In order to solve the problem of reverse, the amount of overlap between the extended pattern and the pixel electrode may be increased, but the aperture ratio is reduced. The present invention has been made in view of the above points, and its object is to increase a push-up voltage ΔVr, which is a problem when an auxiliary capacitance is formed between a scanning line and a pixel electrode, to increase a time constant of a scanning line, An object of the present invention is to provide an active matrix type liquid crystal display device which suppresses reverse occurrence and has high image quality and a high aperture ratio.

[0018]

In order to achieve the above object, according to the active matrix type liquid crystal display device of the present invention, a necessary auxiliary capacitance is formed between the pixel electrode and the scanning line, and The electrode and the scanning line are formed separately from the portion formed between the storage capacitor line and the electrically separated storage capacitor wiring.

According to this structure, a part of the necessary auxiliary capacitance can be formed separately for the scanning line and the auxiliary capacitance wiring electrically separated from the scanning line, so that the thrust voltage (ΔVr) Can be controlled independently of the entire auxiliary capacity. As a result, the push-up voltage can be easily reduced, and an active matrix liquid crystal display device free from flicker and a shift in VT characteristics can be provided.

Further, according to the active matrix type liquid crystal display device of the present invention, a necessary auxiliary capacitance is electrically separated from a portion formed between the pixel electrode and the scanning line and from the pixel electrode and the scanning line. And an extension portion branched from the auxiliary capacitance line is formed in a gap between the signal line and the pixel electrode.

According to the above configuration, in addition to the effect on the push-up voltage (ΔVr) similar to that of the liquid crystal display device, an extension is formed in the gap between the signal wiring and the pixel electrode in order to improve the aperture ratio. Even in this case, since the load capacity of the scanning line is not increased, it is possible to suppress an increase in the time constant of the scanning line. Further, since the degree of freedom in setting the potential of the auxiliary capacitance line electrically separated from the scanning line is high, reverse suppression can be suppressed by setting the potential even when the extension portion and the pixel electrode overlap little.

As a result, it is possible to provide an active matrix type liquid crystal display device having a high aperture ratio, with reduced image quality deterioration such as flicker, shift in VT characteristics, reduction in contrast ratio, and screen roughness.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an active matrix type liquid crystal display device according to an embodiment of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, an active matrix type liquid crystal display device 10 includes a liquid crystal display panel 12, a signal line driving circuit board 14 for driving the liquid crystal display panel, a scanning line driving circuit board 16, each driving circuit board and a liquid crystal display. A plurality of tape carrier packages (referred to as TCP) 18 electrically connected to the panel are provided.

As shown in FIGS. 1 and 3, the liquid crystal display panel 12 includes an array substrate 20 and a counter substrate 22,
These substrates are opposed to each other with a predetermined gap by bonding their peripheral parts with a sealing agent (not shown). Then, the array substrate 20 and the opposing substrate 2
2, a liquid crystal composition 26 is sealed as a light modulation layer. Polarizing plates 28 and 30 are arranged on the outer surfaces of the array substrate 20 and the opposing substrate 22, respectively, so that their polarization axes are orthogonal to each other.

As shown in FIGS. 2 and 3, the array substrate 20 has a transparent insulating substrate 31 made of glass, on which a large number of signal lines 32 and scanning lines 34 (34a, 34a, 34b) are provided in a matrix so as to be substantially orthogonal to each other. Pixel electrodes 36 are provided in regions surrounded by the signal lines 32 and the scanning lines 34, respectively.
Each pixel electrode is connected to a signal line 32 through a thin film transistor (hereinafter, referred to as a TFT) 38 as a switching element.
And the scanning line 34 are connected to each other. An auxiliary capacitance line 35 is formed below each pixel electrode 36, and extends in parallel with the scanning line 34.

The signal line 32 is drawn out to the long side of the array substrate 20, and the signal line driving circuit substrate 15
It is connected to the. Further, the scanning lines 34 are
And is connected to the scanning line drive circuit 16 via the TCP 18.

Hereinafter, the configuration of the array substrate 20 will be described in detail. As shown in FIGS. 2 and 3, first, the insulating substrate 3
For example, Mo having a thickness of about 2,000 Å
After forming the metal film, the scanning lines 34a and 34b integrated with the gate electrode 40 and the auxiliary capacitance wiring 35 are formed by a photolithography process.

Subsequently, a gate insulating film 42 made of an insulating film of SiO or the like having a thickness of about 4000 Å is formed on the entire surface. Each TFT 38 has a semiconductor film 43 made of an amorphous silicon film on a gate insulating film 42 on a gate electrode 40, and a scanning line 34 a on a semiconductor film 43.
And a silicon nitride film as a channel protective film 44 which is self-aligned.

The semiconductor film 43 is formed on the contact layer 4
It is electrically connected to the pixel electrode 36 via the n + type a-Si film arranged as 6 and the source electrode 48.
Further, the semiconductor film 43 is electrically connected to the signal line 32 via an n + type a-Si film disposed as a contact layer 46 and a drain electrode 50. The drain electrode 50 is formed integrally with the signal line 32 using aluminum or the like.

Each pixel electrode 36 is formed by a photolithography process after a transparent conductive film such as ITO having a thickness of about 1000 Å is formed on the gate insulating film 42. In this case, the pixel electrode 36 is formed so as to overlap the auxiliary capacitance line 35 and partially overlap the upper scanning line 34b. As a result, the first storage capacitor Cs1 between the pixel electrode 36 and the scanning line 34b, and the storage capacitor line 3
5, the second storage capacitors Cs2 are formed.

Lastly, the signal line 32, the scanning line 34, and the TFT 38 are overlapped with each other so as to cover the gap between the pixel electrodes 36.
A matrix-shaped protective insulating film 54 made of silicon nitride or the like having a thickness of about 000 Å is formed. The protective insulating film 54 is formed of, for example, silicon nitride to a thickness of about 3000 Å.

On the other hand, as shown in FIG. 3, the counter substrate 22 includes a transparent insulating substrate 60 made of transparent glass, and a light shielding layer (BM pattern) made of a chromium (Cr) oxide film is provided on the glass substrate. ) 62 are formed. Light shielding layer 6
2 denotes a TFT 38 on the array substrate 20, a gap between the signal line 32 and the pixel electrode 36, and a scanning line 34 and the pixel electrode 36.
Are formed in a matrix so as to shield each of the gaps from light. Further, on the glass substrate 60, red (R), green (G), and blue (B) coloring layers 64 are formed at positions facing the pixel electrodes 36 on the array substrate 20 side. Then, on the coloring layer 64 and the light shielding layer 62,
A counter electrode 68 made of a transparent conductive film such as ITO is formed.

The array substrate 20 and the opposing substrate 22 configured as described above are adhered to each other with a sealant (not shown), and the liquid crystal display panel 14 is formed by sealing the liquid crystal composition 26 between these substrates. I have.

As shown in FIG. 4, the equivalent circuit of the liquid crystal display panel 14 having the above configuration is such that the gate electrode 40 of the TFT 38 is connected to the scanning line 34a and the drain electrode 50 is connected to the signal line 32. The source electrode 48 of the TFT 38 is
The liquid crystal capacitance (Clc), the first auxiliary capacitance (Cs1) formed for the preceding scanning line 34b, and the second auxiliary capacitance formed for the auxiliary capacitance wiring 35 electrically separated from the scanning line 34. And a capacitor (Cs2).
As the parasitic capacitance, the capacitance between the gate and the pixel electrode (Cg
s) and the capacitance between the signal line and the pixel electrode (Cps) also exist.

In the case of the liquid crystal display device configured as described above, the push-up voltage ΔVr is expressed by the following equation. ΔVr = ΔVg × Cs1 / (Clc + Cs1 + Cs2 + Cgs + Cps) (2) In the above equation (2), considering that Cs1 + Cs2 in the denominator is the total storage capacity required, ΔVr in the conventional liquid crystal display device is considered. In comparison with the equation (1), ΔVr of the present embodiment / conventional ΔVr = Cs1 / (Cs
1 + Cs2). That is, the thrust voltage ΔVr is changed to Cs
It can be reduced by 1 / (Cs1 + Cs2). As a result, according to the liquid crystal display device of the present embodiment, flicker or V
Poor image quality such as -T characteristic shift can be suppressed.

FIGS. 5 and 6 show a liquid crystal display panel of a liquid crystal display according to another embodiment of the present invention. According to another embodiment, each auxiliary capacitance line 35 is formed to have an extending portion 39 extending in a direction parallel to the signal line 32 from both ends in the longitudinal direction. These extending portions 39 are formed in the gaps between the signal lines 32 and the pixel electrodes 36, and the light shielding layers 62 on the counter substrate 22 side.
And is formed in an area which becomes an invalid portion by overlapping.
Other configurations are the same as those of the above-described embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

In the liquid crystal display device configured as described above, the storage capacitor formed for the storage capacitor line 35 does not change depending on the presence or absence of the extension 39. The wiring width of 35 is smaller than that of the storage capacitance line in the above-described embodiment. Therefore, since only the wiring resistance increases without changing the load capacitance of the auxiliary capacitance line, the time constant of the auxiliary capacitance line increases.

However, unlike the scanning line, the auxiliary capacitance line only needs to apply the same potential to all the rows, so that both ends can be easily supplied with power. That is, the time constant limitation of the auxiliary capacitance line 35 electrically separated from the scanning line is relaxed as compared with the case where the auxiliary capacitance Cs1 is formed only on the scanning line 34. Therefore, forming the extension 39 on the auxiliary capacitance line 35 is easier than forming the extension from the scanning line.

Regarding the reverse of the portion where the extension 39 of the auxiliary capacitance line 35 and the pixel electrode 36 overlap, it is more advantageous to branch the extension from the auxiliary capacitance line for the following reason. That is, since the potential of the auxiliary capacitance line 35 can be set independently of the switching operation of the TFT 38, the potential may be set to an intermediate value of the potential that the pixel electrode 36 can take. For example, when driving the liquid crystal composition 26 at about 4V, the peak-to-peak value of the pixel electrode potential is 8V.
If the storage capacitor line potential is set in the middle, the potential difference between the pixel electrode 36 and the extension 39 branched from the storage capacitor line becomes ± 4V.

On the other hand, the potential of the scanning line 34 is set according to the switching operation of the TFT 38. As for the reverse, the setting of the off-potential of the scanning line 34 is mainly related,
The off-potential is set about 5 V lower than the minimum value of the pixel electrode potential. Therefore, when driving the liquid crystal composition 26 at 4 V, the potential difference between the pixel electrode and the extension branched from the scanning line 34 is 5 to 13 V. That is, the auxiliary capacitance line 3
When the extension 39 is branched from 5, the potential difference can be reduced to 1/3 or less as compared with the case where the extension is branched from the scanning line, and there is an effect of suppressing reverse.

As described above, according to the liquid crystal display device according to another embodiment, while avoiding the problem of the thrust voltage (ΔVr), there is no image quality deterioration such as screen roughness due to reverse, and the aperture ratio is high. An active matrix liquid crystal display device can be provided.

[0042]

As described above in detail, according to the present invention, a portion (Cs1) in which a necessary storage capacitor is formed between a pixel electrode and a scanning line, and a pixel electrode and a scanning line are electrically connected. (Cs) formed between the storage capacitor line and the storage capacitor line separated into
2), the active matrix type liquid crystal display device can control the thrust voltage (ΔVr) independently of the entire auxiliary capacitance, and has no flicker and no shift in VT characteristics. Can be provided.

Further, by providing the extension part branched from the auxiliary capacitance line in the gap between the signal line and the pixel electrode, the time constant of the scanning line is not increased, and the extension part and the pixel electrode are not connected. Can easily be suppressed by setting the potential of the auxiliary capacitance line, it is possible to provide an active matrix type liquid crystal display device having a high aperture ratio without a decrease in contrast ratio or screen roughness.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an active matrix type liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a plan view schematically showing a part of an array substrate of the liquid crystal display device.

FIG. 3 is a sectional view taken along line AA of FIG. 2;

FIG. 4 is a diagram showing an equivalent circuit of one pixel in the liquid crystal display panel.

FIG. 5 is a plan view schematically showing a part of an array substrate of an active matrix liquid crystal display device according to another embodiment of the present invention.

FIG. 6 is a sectional view taken along line BB in FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Liquid crystal display device 12 ... Liquid crystal display panel 20 ... Array substrate 22 ... Counter substrate 26 ... Liquid crystal composition 32 ... Signal line 34, 34a, 34b ... Scanning line 35 ... Auxiliary capacitance line 36 ... Pixel electrode 38 ... TFT 39 ... Extension Protruding part 54 ... Protective insulating layer 56 ... Contact area 58 ... Concave part Cs1, Cs2 ... Auxiliary capacitance 62 ... Light shielding layer 64 ... Coloring layer 68 ... Counter electrode

Claims (4)

[Claims] A plurality of scanning lines provided on an insulating substrate and extending in parallel with each other; a plurality of signal lines provided intersecting the scanning lines; and a region surrounded by the scanning lines and the signal lines, respectively. A plurality of pixel electrodes connected to the scanning line and the signal line via a switching element, and a plurality of auxiliary capacitance lines electrically separated from the scanning line and the signal line and provided on the insulating substrate And a liquid crystal composition sealed between the array substrate and the opposing substrate, the liquid crystal composition being sealed between the array substrate and the opposing substrate. An active matrix type liquid crystal display device, wherein a first auxiliary capacitance is formed between adjacent scanning lines and a second auxiliary capacitance is formed between the first auxiliary capacitance and the auxiliary capacitance line. 2. The storage device according to claim 1, wherein each of the auxiliary capacitance lines has an extension that branches off from the auxiliary capacitance line and extends to a gap between the signal line and each pixel electrode. 1. The active matrix type liquid crystal display device according to 1. 3. The active matrix liquid crystal according to claim 1, wherein the auxiliary capacitance line is provided between two adjacent scanning lines, and extends in parallel with the scanning lines. Display device. 4. The array substrate has a matrix-shaped insulating layer provided on the scanning line, the signal line, and the switching element so as to cover a gap between the pixel electrodes. The liquid crystal display device according to claim 2, wherein the protrusion is provided so as to overlap the insulating layer.