JPH08160461A - Liquid crystal display device - Google Patents

Abstract

PURPOSE: To improve a voltage holding rate and also to obtain high reliability by restraining the conduction of an TFT making a light shielding layer as a gate while specifying the signal voltage of the light shielding layer. CONSTITUTION: The voltage Vbm of the light shielding layer is set to a level in which a constant width is pulled down from a common electrode voltage Vcom and the conduction of a parastic TFT is restrained with constitution in which the voltage difference Vbs between the Vbm and a source voltage Vs and the voltage difference Vbd between the Vbm and a drain voltage Vd become equal to or lower than the threshold value of the TFT even in their maximums. Moreover, the voltage difference of the light shielding layer with respect to source and drain electrodes is made equal to or lower than the threshold value of the TFT in all periods by applying a level shift so that the common electrode voltage Vcom become the same level as a voltage VgL while gating the center level of the common voltage Vcom and impressing it on the light shielding layer. Thus, since the OFF resistance of the TFT is not lowered by the field effect making the light shielding layer the gate, the change in the source voltage Vs due to a leakage current is prevented, the decrease in a holding voltage V1c is restrained to increase the voltage holding rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device in which a black matrix is formed on the array substrate side of a thin film transistor to improve the aperture ratio.

[0002]

2. Description of the Related Art Liquid crystal display devices have advantages such as small size, thin shape, and low power consumption, and are being put to practical use in fields such as OA equipment and AV equipment. In particular, the active matrix type, which uses a thin film field effect transistor (hereinafter abbreviated as TFT) as a switching element and is capable of line-sequential scanning drive, theoretically performs static drive with a duty ratio of 100% in a multiplexed manner. Therefore, it is possible to display a large screen and a moving image with a high contrast ratio.

The active matrix type liquid crystal display device is
Pixel electrodes for driving liquid crystal are arranged and formed in a matrix,
Further, an array substrate in which a TFT is connected to each pixel electrode is formed, and an opposite substrate on which a common electrode is formed over the entire surface are bonded together with a liquid crystal interposed therebetween. The facing portion of the pixel electrode and the common electrode serves as a pixel capacitor in which liquid crystal is used as a dielectric layer. The common electrode is set to a predetermined voltage, and a signal voltage selected while the TFT is ON is applied to each pixel electrode to drive each pixel capacitance. The voltage applied to the pixel capacitor is the O of the TFT until rewriting in the next field.
It is held by the FF resistor. The alignment state of the liquid crystal is finely adjusted by the electric field thus formed in each pixel capacitor,
The light is modulated, and by visually recognizing the combined light macroscopically, it is observed as a display image.

The structure of such a liquid crystal display device will be described with reference to FIGS. 3 and 4. 3 is a plan view of a unit pixel portion, and FIG. 4 is a cross-sectional view taken along the line AA of FIG. A gate line (11L) made of Cr and the like and an auxiliary capacitance electrode (12) are formed on a substrate (10) such as glass, and a part of the gate line (11L) is projected as a gate electrode (11G) of a TFT. There is. Gate line (1
1L) and SiN on the entire surface covering the auxiliary capacitance electrode (12)
A gate insulating layer (13) such as X is covered. At the position corresponding to the gate electrode (11G) on the gate insulating layer (13), an etching stopper (1) such as an a-Si layer (14) which is a channel layer of the TFT and SiNx is formed.
5) and an N + a-Si layer (16) interposed in the source and drain regions for making ohmic contact.
S, 16D) is formed. On the other hand, a pixel electrode (1) made of an indium-tin oxide alloy (ITO) is formed on the gate insulating layer (13).
7) has been formed. Furthermore, a gate insulating layer (13)
A source electrode (18), a drain electrode (19D), and a drain line (19L) are formed in Al by using Al or the like. The source electrode (18) is connected to the pixel electrode (17) and N +
The a-Si layer (16S) is connected, and the drain electrode (19
D) is connected to the N + a-Si layer (16D) integrally with the drain line (19L). The overlapping portion of the pixel electrode (17) and the auxiliary capacitance electrode (12) forms an auxiliary capacitance for holding charges, and the gate line (11L) and the drain line (19).
L) is located in the peripheral region of the pixel electrode (17) and is arranged to intersect. In addition, the entire surface that covers these is Si
NX is coated to form an interlayer insulating layer (20), and a light shielding layer (21) such as Cr is formed on the interlayer insulating layer (20) to cover the entire area between the pixel electrodes (17), It is a black matrix (BM).

Further, above the TFT array substrate having such a structure, there is arranged an opposing substrate in which a common electrode (31) of ITO is formed on the substrate (30) via the liquid crystal layer (32). Then, the common electrode (31) and the liquid crystal layer (32) are standardized by the pixel electrode (17) to form a pixel capacitance, and the liquid crystal display device is completed. The above is the BM on TFT array substrate structure in which the BM is formed on the TFT array substrate side, and realizes a high contrast ratio and a high aperture ratio. That is, the BM that blocks unmodulated light outside the area of the pixel capacitance
Is formed on the side of the TFT array substrate to allow for misalignment when the BM is formed on the side of the counter substrate.
A margin of -10 μm was eliminated, and a positional shift at the time of alignment between the pixel electrode (17) and the light shielding layer (21) was taken into consideration 2
The BM is formed with a margin of ˜3 μm, and as a result of the advantages and disadvantages of these effective display areas, the aperture ratio is improved.

FIG. 5 is a waveform diagram showing a driving method of a conventional liquid crystal display device. VgL and VgH are gate voltages, Vd
Is the drain voltage, Vs is the source voltage, and Vcom is the common electrode voltage and auxiliary capacitance electrode voltage. Gate voltage is Vg
When it rises to H, the TFT is turned on and the source voltage (Vs) becomes the same level as the drain voltage (Vd).
Then, when the gate voltage falls to VgL, the TFT is turned off and enters the holding state, but the source voltage (Vs) is the gate voltage (Vg) in the parasitic capacitance portion between the gate and the source.
Under the influence of the falling edge of, shifts downward by ΔVs. Therefore, the common electrode voltage (Vcom) shifts its center level (Vc) by ΔVs below the center level of positive / negative inversion of the drain voltage (Vd). In the line inversion drive described here, the common electrode voltage (Vco
In (m), the positive / negative is inverted every scanning period, and the gate voltage is Vg.
During the period of L, it becomes a holding period, and the source voltage (Vs) is
The holding voltage (Vlc), which is swung according to the inversion of the common electrode voltage (Vcom) and applied to the pixel capacitance, is kept constant. The positive / negative inversion of the drain voltage (Vd) is opposite to the positive / negative inversion of the common electrode voltage (Vcom) every scanning period, and the positive / negative relationship between the drain voltage (Vd) and the common electrode voltage (Vcom) is Reverse every time,
The positive and negative electric fields applied to the liquid crystal layer (32) are also inverted field by field, and deterioration of the liquid crystal is prevented.

[0007]

As shown in FIG. 4, the light-shielding layer (21) functions as a black matrix over the entire display screen area through the interlayer insulating layer (20) for all TFTs and drain lines (19L). Is commonly superimposed on. Since the light-shielding layer (21) is in a floating state, the voltage (Vbm) of the light-shielding layer (21) is 0 [average] due to the influence of the positive / negative inversion of the drain signal voltage due to the parasitic capacitance with the drain line (19L). V] is fixed. Therefore, the common electrode (V
com) is a negative period (A1, A2, ...),
The source voltage (Vs) greatly decreases, the voltage (Vbs) between the light shielding layer (21) and the source electrode (19D) exceeds the threshold value of the TFT, and a leak current occurs. That is, the light shielding layer (21)
The parasitic TFT having the gate as a gate is rendered conductive. As a result, the voltage holding ratio of the pixel capacitance decreases, and the period (A1, A2, ...
,), The source voltage (Vs) changes to the drain voltage (V
Since the voltage changes toward the level of d), the holding voltage (Vlc) of the pixel capacitance decreases, causing a problem that the contrast ratio decreases.

Further, even in the field where the drain voltage (Vd) is opposite in positive and negative to that in FIG. 5, when the negative level of the drain voltage (Vd) becomes large, the light shielding layer (21) and the drain electrode (1
9D) voltage (Vbd) exceeds the threshold, and again TF
This causes a leak of T. Since such a period in which a leak current can occur corresponds to 1/2 of the entire period, the source voltage (Vs) changes depending on the display image, the voltage holding ratio decreases, and the contrast ratio decreases significantly. It was.
Especially, in the central part of the display screen, a-
Light leaks into the Si layer (14) and the resistance of the TFT decreases, so that leak current is more likely to occur.
Black or white is not sufficiently obtained and becomes gray, so-called light leakage occurs, and the display quality is deteriorated.

[0009]

First, a plurality of pixel electrodes formed on a first substrate to constitute one of pixel capacitors for driving liquid crystal, a plurality of thin film transistors connected to the respective pixel electrodes, and interlayer insulation. A light-shielding layer covering the thin film transistor with a layer sandwiched between them, and a second substrate fixed at an opposite position with the liquid crystal layer sandwiched between the first substrate and entirely defined by the pixel electrodes. In a liquid crystal display device provided with a common electrode that constitutes the other of the pixel capacitors, the light-shielding layer is set to a voltage such that a voltage with respect to a drain electrode and a source electrode of the thin film transistor is a threshold value of the thin film transistor or less. And

Secondly, the light shielding layer is formed over the entire area between the regions of the pixel capacitor. Thirdly, the light shielding layer is applied with a voltage obtained by level-shifting the voltage of the common electrode.

[0011]

In the first structure, by applying a voltage to the light-shielding layer covering the thin film transistor so that the voltage difference between the drain electrode and the source electrode is less than or equal to the threshold value of the thin film transistor, the thin-film transistor by the field effect of the light-shielding layer is formed. Leakage current is suppressed, and a decrease in the voltage holding ratio of the pixel capacitance is prevented.

In the second structure, a voltage is applied to the light-shielding layer that covers the entire area between the pixel capacitors so that the voltage difference between the drain electrode and the source electrode is less than the threshold value of the thin film transistor. With regard to the above, the leak current of the thin film transistor due to the electric field effect of the light shielding layer is suppressed, and the reduction of the voltage holding ratio is prevented. In the third configuration, the common electrode signal is level-shifted and a voltage lower than the threshold value of the thin film transistor is applied to the light shielding layer, so that the leak current of the thin film transistor can be prevented and the increase in cost can be suppressed low. .

[0013]

Next, examples of the present invention will be described. FIG.
FIG. 5 is a waveform diagram showing the voltage to be applied to each electrode and the electrical behavior of each electrode in the liquid crystal display device having the structure shown in FIGS. 3 and 4. Similar to the one used in FIG. 3, Vg
L and VgH are gate voltages, Vd is a drain voltage, Vs is a source voltage, and Vcom is a common electrode voltage and a storage capacitor electrode voltage. Vbm is a voltage applied to the light shielding layer, which is a voltage level-shifted from the common electrode voltage (Vcom), and is set so as not to exceed the threshold value of the TFT even at the H level.

In terms of the circuit, a part of the common electrode signal is input to a well-known level shift circuit, and its output is a light shielding layer (21).
It is configured to be applied to. When the gate voltage is VgH, T
The FT is turned on, and the source voltage (Vs) is set to the same level as the drain voltage (Vd). After that, when the gate voltage falls to VgL, the TFT is turned off and enters the holding state, but the source voltage (Vs) is affected by the fall of the gate voltage (Vg) in the parasitic capacitance portion between the gate and the source. And shift down by ΔVs. Therefore, the common electrode voltage (Vcom) has a central level (Vc) of ΔV higher than the central level of positive / negative inversion of the drain voltage (Vd).
It shifts down by s. In the line inversion drive described above, the common electrode voltage (Vcom) is inverted in polarity for each scanning period, becomes a holding period during the gate voltage VgL period, and the source voltage (Vs) becomes the common electrode voltage (Vcom).
The pixel voltage is held up and down (V
lc) is kept constant. The positive / negative inversion of the drain voltage (Vd) is opposite to the positive / negative inversion of the common electrode voltage (Vcom) every scanning period, and the drain voltage (Vd) is reversed.
The positive / negative relationship between d) and the common electrode voltage (Vcom) is reversed for each field, and the electric field applied to the liquid crystal layer (30) is also reversed for each field to prevent deterioration of the liquid crystal.

In the present invention, the voltage (Vbm) of the light shielding layer is
The voltage is set to a level lower than the common electrode voltage (Vcom) by a certain width, and the voltage difference Vbs from the source voltage (Vs) and the voltage difference Vbd from the drain voltage (Vd).
Is less than the threshold of the TFT at the maximum, the parasitic TFT
The conduction of is suppressed. In this embodiment, in particular, the common electrode voltage (Vcom) is changed to the gate L voltage (Vg
L) is applied to the light-shielding layer (21) by level-shifting so that the light-shielding layer (2) can be applied to the source and drain electrodes during the entire period.
The voltage difference of 1) is set to be equal to or less than the threshold value of the TFT. As a result, TF is generated by the electric field effect using the light shielding layer (21) as a gate.
The OFF resistance of T does not decrease, the change of the source voltage (Vs) due to the leakage current is prevented, the reduction of the holding voltage (Vlc) is suppressed, and the voltage holding ratio is consequently improved.

FIG. 2 shows the experimental results of examining the relationship between the voltage and the transmitted light intensity of the liquid crystal display device thus constructed.
(A) is a characteristic diagram showing the results of experiments conducted on the liquid crystal display device of the present invention, and (b) is a characteristic diagram of a liquid crystal display device driven as in the past as a comparative example.
In the experiment, in the line inversion drive, the voltage (Vbm) of the light shielding layer (21) is set to floating and
The amplitude of the gate voltage (Vg) is set to 15 [V] while the level shift signal from the common electrode voltage (Vcom) is set and the amplitude of the drain voltage (Vd) is set to the maximum width of 10 [V] within the practical range. With the gate L voltage (VgL) changed from V = −25 [V] to 0 [V] while keeping the value at 0, the transmitted light intensity (T) was examined and a graph was prepared. Here, the graphs shown by white dots are the experimental results of the device in the initial state, and the graphs shown by black dots are the experiments for examining the reliability by applying a load for 30 minutes under the condition of 4,000,000 lx and 60 ° C. The result. The value of the transmitted light intensity (T) is arbitrarily set based on the change fraction. Further, the experiment was conducted in the normally white mode, and in particular, the purpose was to evaluate the OFF characteristic of the TFT by the transmitted light intensity (T).

From the graph of (a), the voltage (V) is -7.
At [V], the transmitted light intensity (T) changes rapidly,
From this, it can be seen that the threshold (Vth) begins to exist. That is, depending on the level of the drain voltage (Vd), the voltage between the light-shielding layer and the drain electrode (Vbd) or the voltage between the light-shielding layer and the source electrode (Vbs) exceeds the threshold value, and a leak current is generated to cause the transmitted light intensity (Tb). ) Has changed and the characteristics have become unstable. On the other hand, in the range below the threshold value (Vth), the transmitted light intensity (T) is kept constant at a sufficiently low level, and the characteristics are very stable. From this, the gate L voltage (V
It can be seen that by setting gL) below this threshold value (Vth), the OFF resistance of the TFT is sufficiently increased, the source voltage (Vs) is maintained at a constant level, and a high voltage holding ratio is obtained. In addition, there is almost no change in the characteristics after the reliability test.

On the other hand, in (b), the change of the transmitted light intensity (T) is gradual, and the transmitted light intensity (T) also changes as the voltage (V) changes. The low level is not obtained. That is, the light shielding layer (21)
Since the voltage holding ratio is reduced due to the TFT leakage due to the electric field effect, the transmitted light intensity (T) cannot be kept sufficiently low and constant. Further, in this case, in order to obtain the maximum contrast ratio, the gate L voltage (Vg
L) needs to be set to -25 [V] or less, and the amplitude of the gate voltage (Vg) becomes large as a whole, which causes problems such as an increase in power consumption and durability of the driver element. . In addition, the characteristics are greatly changed by the reliability test. That is, it is understood that the leakage current of the TFT is further increased under the light irradiation or the high temperature condition, so that the voltage holding ratio is lowered and the sufficiently low level cannot be maintained.

In the present invention, the electric field effect of the light-shielding layer is eliminated, the leak current of the TFT is suppressed, and the voltage holding ratio is improved, so that as shown in (a), the light is transmitted in the region below the threshold value (Vth). The characteristic that the light intensity (T) is stable was obtained, and the reliability was improved. That is, OF of TFT
It was possible to maintain the transmitted light intensity (T) at F constant and with high reliability. Further, since the transmitted light intensity (T) changes sharply at the threshold value (Vth), the gate L voltage can be set to a high value that does not exceed the threshold value (Vth). Therefore, the ON / OF required for the gate L voltage
By reducing the F voltage ratio and setting the gate signal amplitude to be small, power consumption is reduced and deterioration of the driver element is prevented.

[0020]

As is apparent from the above description, the light-shielding layer is used as a gate by setting the signal voltage of the light-shielding layer so that the voltage of the light-shielding layer with respect to the source electrode and the drain electrode becomes equal to or lower than the threshold value of the TFT. It was possible to suppress conduction of the TFT, improve the voltage holding ratio, and obtain high reliability. Further, since the voltage-transmitted light intensity characteristic having a steep threshold characteristic can be obtained, the ON / OFF voltage ratio of the gate voltage can be suppressed small, and the gate signal amplitude can be reduced to reduce the power consumption. Further, this prevents the characteristic deterioration of the driving element and extends the life thereof.

[Brief description of drawings]

FIG. 1 is a waveform diagram showing the electrical behavior of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a voltage-transmitted light intensity characteristic curve comparing the characteristics of the present invention and the conventional liquid crystal display device.

FIG. 3 is a plan view of a unit pixel portion of a liquid crystal display device.

FIG. 4 is a cross-sectional view taken along the line AA of FIG.

FIG. 5 is a waveform diagram showing the electrical behavior of a conventional liquid crystal display device.

[Explanation of symbols]

 10, 30 substrate 11 gate electrode wiring 12 auxiliary capacitance electrode 13 interlayer insulating layer 14 a-Si 15 etching stopper 16 N + a-Si 17 pixel electrode 18 source electrode 19 drain electrode wiring 20 interlayer insulating layer 21 light-shielding layer 31 common electrode 32 Liquid crystal layer Vg Gate voltage Vd Drain voltage Vs Source voltage Vcom Common electrode voltage Vbm Light-shielding layer voltage Vbd Light-shielding layer-drain electrode voltage Vbs Light-shielding layer-source electrode voltage Vlc Holding voltage

Claims (3)

[Claims] 1. A plurality of pixel electrodes formed on a first substrate to constitute one of pixel capacitors for driving a liquid crystal, a plurality of thin film transistors connected to the respective pixel electrodes, and a thin film transistor covered in the previous period with an interlayer insulating layer interposed therebetween. Light-shielding layer, and the first
A liquid crystal display provided with a common electrode which is formed over the entire surface of a second substrate fixed at opposing positions sandwiching the liquid crystal layer of the substrate, and which is standardized by the pixel electrodes and constitutes the other of the pixel capacitors. In the device, the liquid crystal display device is characterized in that the light shielding layer is set to a voltage such that a voltage with respect to a drain electrode and a source electrode of the thin film transistor is equal to or less than a threshold value of the thin film transistor. 2. The liquid crystal display device according to claim 1, wherein the light-shielding layer is formed over the entire area between the regions of the pixel capacitor. 3. The liquid crystal display device according to claim 1, wherein a voltage obtained by level-shifting the voltage of the common electrode is applied to the light shielding layer.