JPH11337963A - Vertical alignment type liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress unevenness of residual DC component in AC driving and prevent a display from getting burnt by letting liquid crystal capacitance and auxiliary capacitance satisfy a specific relation. SOLUTION: Liquid crystal capacitance CLC in which initial alignment of liquid crystal is controlled vertically to a substrate for each picture element and auxiliary capacitance connected with the liquid crystal capacitance CLC in parallel are formed. The auxiliary capacitance CSC consisting of a p-Si film 14, an auxiliary capacitance electrode 12, and a gate insulating film 13 is set so as to satisfy a relation, 0.61×CLC<=CSC<=2.0×CLC, with the liquid crystal capacitance CLC consisting of picture element electrode 20, common electrode 33, and liquid crystal 40. Thus, in a vertical alignment type, resultant capacitance of the liquid crystal capacitance CLC and the auxiliary capacitance CSC becomes sufficiently large by making the auxiliary capacitance CSC 0.61 times as large as the liquid crystal capacitance CLC or more. Therefore, burning phenomenon is prevented from occurring even if ON-OFF ratio of the liquid crystal capacitance becomes large.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a vertical alignment type liquid crystal display (LCD).

[0002]

2. Description of the Related Art In recent years, flat panel displays such as LCDs, organic electroluminescence (EL) displays, and plasma displays have been actively developed and put to practical use. Among them, LCD is thin,
Excellent in terms of low power consumption, etc.
It has become mainstream in the field of equipment. In particular, an active matrix type LCD in which a TFT is arranged in each pixel as a switching element for controlling the timing of rewriting pixel information,
Since a large screen and high-definition moving image can be displayed, it is widely used for various televisions, personal computers, and monitors of portable computers, digital still cameras, video cameras, and the like.

FIG. 2 is an equivalent circuit diagram of such an LCD. The gate line (1) and the drain line (2) are arranged so as to intersect. At the intersection, a thin film transistor (TFT) (3) as a switching element and one of the capacitance electrodes are connected to the TFT (3). It has a liquid crystal capacitor (4) and an auxiliary capacitor (5), and a counter electrode line (6) connected to the other of the capacitor electrodes of the liquid crystal capacitor (4) and the auxiliary capacitor (5). The counter electrode line (6) is common to all the liquid crystal capacitors (4) and the auxiliary capacitors (5). The TFT (3), one of the liquid crystal capacitors (4), and the auxiliary capacitor (5) are formed on the same substrate, and the other of the liquid crystal capacitors (4) is formed on another substrate. An LCD is formed by sandwiching a liquid crystal between these substrates.

A scanning signal voltage is applied to the gate line (1), and a high level for turning on the TFT (3) for a predetermined period arrives for each field. During this period, the display signal voltage applied to the drain line (2) is given to the liquid crystal capacitance (4) and the auxiliary capacitance (5). On the other hand, counter electrode line (6)
Is applied with a counter voltage, and a difference between the display signal voltage and the counter voltage is applied as a driving voltage of the liquid crystal, and is controlled to a desired transmittance to form a display screen. In this display state, the drive voltage applied to the liquid crystal capacitance (4) and the auxiliary capacitance (5) is held by the off-resistance of the TFT, and is continued for one field period.

The auxiliary capacitance (5) is connected in parallel with the liquid crystal capacitance (4). By increasing the combined capacitance in this way, the voltage holding ratio is increased and a constant display can be performed over one field period. It is provided in.

Further, at the moment when the gate voltage falls from the high level to the low level, the voltage VLC once held in the liquid crystal capacitance (4) is reduced by the following equation due to the fall of the gate voltage due to the parasitic capacitance inside the TFT. Phenomena of falling by the amount represented by.

[0007]

(Equation 2) Here, VG is a gate voltage, CGS is a parasitic capacitance, CSC is an auxiliary capacitance, and CLC is a liquid crystal capacitance. Thus, by providing the auxiliary capacitance (5), the change width ΔVG of the gate voltage can be obtained.
Of the liquid crystal capacitance (3) with respect to. Further, in the LCD, so-called AC driving is generally performed in which the polarity of the voltage applied to the liquid crystal capacitor (4) is inverted every predetermined period in order to suppress the deterioration of the liquid crystal. However,
ΔVLC in the equation (2) is generated in the direction of decreasing the voltage applied to the liquid crystal during the positive polarity period and in the direction of increasing the voltage applied to the liquid crystal during the negative polarity period. As a component, it is always applied to the liquid crystal, which causes so-called burn-in. For this reason, by setting the counter electrode voltage to a level lowered by ΔVLC in advance, the voltage applied to the liquid crystal is made equal between the positive polarity period and the negative polarity period, thereby preventing the burn-in phenomenon. .

[0008]

Conventionally, in a TN type LCD using a liquid crystal having a positive dielectric anisotropy, the auxiliary capacitance CSC is set to be about 0.2 times the liquid crystal capacitance CLC. However, a vertical alignment type LCD using a liquid crystal having the same design and a negative dielectric anisotropy is used.
Then, there was a problem that a seizure phenomenon was observed.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve this problem, and a liquid crystal is sealed between a pair of substrates having electrodes for driving a liquid crystal formed on opposing surfaces, and an initial liquid crystal is provided for each pixel. A liquid crystal capacitor whose orientation is controlled in a direction perpendicular to the substrate,
In a vertical alignment type liquid crystal display device having an auxiliary capacitance connected in parallel with the liquid crystal capacitance, the liquid crystal capacitance CLC
And the auxiliary capacity CSC

[0010]

(Equation 3) It satisfies the relationship of

[0011]

FIG. 1 is a sectional view of a display pixel portion of an LCD according to an embodiment of the present invention. Substrate (10)
A gate electrode (11) of a TFT made of Cr, Ti, Ta or the like, and an auxiliary capacitance electrode (1
2) is formed, and a gate insulating film (13) is formed to cover this. On the gate insulating film (13), p-S
The i film (14) is formed in an island shape of the TFT so as to pass above the gate electrode (11). In the p-Si film (14), a region immediately above the gate electrode (11) is a non-doped channel region (CH), and both sides of the channel region (CH) are lightly doped with N-type impurities such as phosphorus. A low concentration region (LD), and further outside, a source region (NS) doped with the same impurity at a high concentration.
And the drain region (ND), and the LDD (ligh
tly doped drain) structure. p-Si film (1
The source region (NS) of (4) extends not only in the TFT region but also on the gate insulating film (13) to form the auxiliary capacitance electrode (1).
It overlaps above 2) to form an auxiliary capacitance.

On the channel region (CH), an implantation stopper (15) used as a mask for ion implantation when forming the low concentration region (LD) is left. An interlayer insulating film (16) such as a silicon nitride film or a silicon oxide film is formed so as to cover the p-Si film (14), and a drain electrode (1) such as Al / Mo is formed on the interlayer insulating film (16).
7) and a source electrode (18) are formed. Each of the source electrodes (18) is formed through a contact hole opened in the interlayer insulating film (16).
-Connected to the drain region (ND) and the source region (NS) of the Si film (14). These drain electrodes (1
7) and over the source electrode (18), SOG, BP
A flattening insulating film (19) made of SG, acrylic resin or the like is formed, and ITO (indium tint) is formed on the flattening insulating film (19).
in oxide) or pixel electrode made of Al (20)
Is formed and connected to the source electrode (18) via a contact hole opened in the planarization insulating film (19). On this, a vertical alignment film (45) such as polyimide is formed. Thus, the TFT substrate is configured.

At a position facing the TFT substrate (10), a substrate (30) serving as a counter substrate is arranged. On the substrate (30), R, G, and B made of a film resist are provided.
Are formed and provided at positions corresponding to the respective pixel electrodes (20). In the gap region of the color filter (31), a black matrix (32) made of an opaque film resist is provided. A common electrode (33) such as ITO is formed on the layers of the color filter (31) and the black matrix (32). In the common electrode (33), there is provided an orientation control window (34) formed as a region where no ITO exists. On the common electrode (33),
The same vertical alignment film (46) as the substrate (10) side is provided. Thus, the counter substrate is configured.

The TFT substrate (10) and the counter substrate (3
Between 0), a liquid crystal (40) having a negative dielectric anisotropy is loaded.

In this example, the alignment control window (34) provided in the common electrode (33) has a direction in which the liquid crystal molecules (41) which are vertically aligned in the initial state, ie, when no voltage is applied, are inclined by the voltage application. Has the function of dividing That is, at the edge of the pixel electrode (20), the boundary of the liquid crystal molecules (41) continuously inclined in a direction against the electric field (42) generated in the oblique direction is formed by the weak electric field and the oblique electric field in the alignment control window (34). This is fixed by (41). Further, the flattening insulating film (19) on the TFT substrate side serves as a base for the pixel electrode (20) to enhance flatness,
Due to the unevenness of the interface with the liquid crystal (40), the liquid crystal molecules (41)
Is prevented from being disturbed, and the controllability by the oblique electric field (41) and the orientation control window (34) is made effective.

In the present invention, the auxiliary capacitance CSC comprising the p-Si film (14), the auxiliary capacitance electrode (12), and the gate insulating film (13) comprises the pixel electrode (20), the common electrode (33) and the liquid crystal (40). ) Are set so as to satisfy the following relationship.

First, in the TN method, the dielectric constant (ε⊥,
When a liquid crystal having ε // of (4.0, 8.0) is used (dielectric anisotropy is Δε = 4.0> 0), when no voltage is applied (O
N) and the liquid crystal capacitance CLC // when voltage is applied (OFF)
Has a relative dielectric constant (ε (OFF), ε (ON)) of (4.0, 5.5)
Becomes At this time, | ε (ON) −ε (OFF) | = 1.5. As described above, the auxiliary capacitance CSC is 0.2% of the liquid crystal capacitance CLC.
Therefore, the ON / OFF ratio of the combined capacitance CCOM // of the liquid crystal capacitance CLC // and the auxiliary capacitance CSC is:

[0018]

(Equation 4) Becomes

If the liquid crystal capacitance CLC is different between ON and OFF, ΔVLC in equation (2) also differs. For example, when the counter electrode voltage level is set based on ΔVLC when CLC (ON) is adopted as the liquid crystal capacitance CLC in the equation (2), ΔVLC becomes different during the OFF period. Even if the voltage is set, the DC component cannot be completely eliminated. However, the auxiliary capacitance CSC is sufficiently large, and the ON / OFF ratio C
If COM (ON) / CCOM (OFF) is set to a value small enough to be obtained by the expression (3), a noticeable burn-in phenomenon can be prevented from actually occurring.

On the other hand, the vertical alignment type LCD of the present invention
, The dielectric constant (ε⊥, ε //) as the liquid crystal (40)
Is (8.0, 4.0) and the dielectric anisotropy (Δε = −4.
When 0) is negative, the relative dielectric constant (ε (ON), ε (OFF)) of the liquid crystal capacitance CLC⊥ when a voltage is applied (ON) and when no voltage is applied (OFF) is (6.0). , 4.0). At this time, | ε (ON) −ε (OFF) | = 2.0, and T
It is larger than N. For this reason, as in the case of TN,
If the auxiliary capacitance CSC is set to be 0.2 times the liquid crystal capacitance CLC, the ON / OFF ratio of the combined capacitance CCOM # becomes

[0021]

(Equation 5) And becomes larger than in the case of TN. As a result,
In the equation (2), the liquid crystal capacitance CLC between ON and OFF is calculated.
Is out of the permissible range, the variation in ΔVLC is large, and even if the counter electrode voltage level is adjusted, a large residual DC component is generated, and the burn-in phenomenon occurs.

As described above, although the dielectric anisotropy Δε of the liquid crystal is the same, the difference between the TN type and the vertical alignment type is caused by the | ε (ON) −ε (OFF) | value. Is different between the TN type and the vertical alignment type. However, even in the vertical alignment type, the ON / OFF ratio of the combined capacitor CCOM is preferably 1.31 or less as in the TN type. Therefore, the ratio between the auxiliary capacitance CSC and the liquid crystal capacitance CLC is calculated as follows:
Can be reset.

[0023]

(Equation 6) Is required. CLC⊥ (OFF): Assuming that CSC = 1: X
By transforming equation (5),

[0024]

(Equation 7) And solve this,

[0025]

(Equation 8) Is obtained.

As described above, in the vertical alignment type, by setting the auxiliary capacitance CSC to be 0.61 times or more of the liquid crystal capacitance CLC⊥ (OFF) or more, the combined capacitance of the liquid crystal capacitance CLC⊥ and the auxiliary capacitance CSC is obtained. Because CCOM is big enough,
Even if the ON / OFF ratio of the liquid crystal capacitor CLC⊥ increases, Δ
Since the fluctuation range of VLC falls within the allowable range, the occurrence of the burn-in phenomenon is prevented. However, when the auxiliary capacitance CSC increases, the combined capacitance CCOM increases, and as a result, the time constant increases and the time required for writing increases.
From experience, it is appropriate that the upper limit of the auxiliary capacitance CSC is about twice the liquid crystal capacitance CLC #. Therefore, the relationship between the auxiliary capacitance CSC and the liquid crystal capacitance CLC # in the vertical alignment type LCD is as follows.

[0027]

(Equation 9)

[0028]

As is apparent from the above description, according to the present invention, by setting the auxiliary capacitance of the vertical alignment type liquid crystal display device to be 0.61 times or more of the liquid crystal capacitance when no voltage is applied, it is possible to reduce the AC driving. Reduces the variation of the residual DC component,
Seizure could be prevented.

[Brief description of the drawings]

FIG. 1 is a sectional view of a vertical alignment type liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gate line 2 Drain line 3 TFT 4 Liquid crystal capacitance 5 Auxiliary capacitance 10, 30 Substrate 11 Gate electrode 13 p-Si 17 Drain electrode 18 Source electrode 19 Flattening insulating film 20 Pixel electrode 31 Color filter 33 Common electrode 34 Alignment control electrode 40 Liquid crystal layer 41 Liquid crystal molecules 42 Electric field 45, 46 Vertical alignment film

Claims (1)

[Claims] A liquid crystal is sealed between a pair of substrates having liquid crystal driving electrodes formed on opposing surfaces, and a liquid crystal capacitor in which initial alignment of liquid crystal is controlled in a direction perpendicular to the substrate for each pixel; In a vertical alignment type liquid crystal display device in which an auxiliary capacitance connected in parallel with a capacitance is formed, the liquid crystal capacitance CLC and the auxiliary capacitance CSC are expressed as follows. A vertical alignment type liquid crystal display device characterized by satisfying the following relationship: