JPH0766419A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To reduce the generation of parasitic capacitance, and prevent the generation of flicker and the sticking of a fixed picture image, by forming a semiconductor layer on the scanning signal wiring of a thin film transistor, and forming the channel part of the thin film transistor in an L-shape. CONSTITUTION:A scanning signal wiring 1 is formed from aluminum or the like, and serves as the gate electrode of a thin film transistor. An insulating layer is formed on the scanning signal wiring 1, and acts as an interlayer insulating film between the scanning signal wiring 1 and a picture image signal wiring 2, and as the gate insulating film. On the scanning signal wiring 1, a semiconductor layer 4 is formed on which a source electrode 2a protruding from the picture image signal wiring 2 and a drain electrode 3a protruding from a picture element electrode 3 are formed. The channel part of the thin-film transistor is L-shaped as a whole. Thereby the formation area of a capacitance between the drain and the gate of the thin film transistor which area is shown by full oblique lines is reduced.

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 using thin film transistors as switching elements.

[0002]

2. Description of the Related Art An active matrix type liquid crystal display device has a higher contrast and a better multi-gradation display characteristic than a simple matrix type liquid crystal display device. . In particular, an active matrix type liquid crystal display device using a thin film transistor as a switching element has a CR
Image quality equivalent to T can now be obtained.

A conventional liquid crystal display device will be described below with reference to the drawings. FIG. 3 is an enlarged view of one pixel of a conventional liquid crystal display device, and FIG. 4 is an equivalent circuit diagram of one pixel. 3 and 4, 8 is a scanning signal wiring, 9 is an image signal wiring, 10 is a pixel electrode, 11 is a thin film transistor, and 12 is a counter electrode. Further, C _S is an additional capacitance, C _LC is a liquid crystal capacitance between the pixel electrode 10 and the counter electrode 12, C _gd is a parasitic capacitance between the gate and drain of the thin film transistor 11, and V _{g (n)}
And V _{g (n + 1)} are signal voltages on the nth and n + 1th scanning signal wirings 8, and VS _(n-1) and VS _(n) are n−1th and nth image signal wirings. 9 is a signal voltage on the counter electrode 12, V _t is a signal voltage on the counter electrode 12, and V _p is a signal voltage on the pixel electrode 10.

In this active matrix type liquid crystal display device, a thin film transistor 11 is provided via a scanning signal wiring 8.
The scanning signal V _g is supplied to the gate of the thin film transistor 11 to control ON / OFF of the thin film transistor 11. The image signal V _S is supplied to the pixel electrode 10 via the image signal wiring 9 and the source / drain of the thin film transistor 11. This image signal V
_S and the counter electrode signal V _t supplied to the counter electrode 12,
A pixel is displayed by applying the liquid crystal material held between the counter electrode 12 and the pixel electrode 10.

[0005]

FIG. 5 shows the waveforms of the scanning signals V _{g (n)} , V _{g (n + 1)} , the image signal V _{S (n)} and the pixel electrode signal V _p . The scanning signal V _{g (n)} is a signal supplied to the gate of the thin film transistor 11, and the voltage V _GH at which the thin film transistor 11 turns on and the thin film transistor 11
Is turned off and V _GL .

The image signal V _S is a signal supplied to the pixel electrode 10 via the thin film transistor 11, and is composed of V _S ⁺ and V _S ⁻ whose polarities are inverted every scanning period (1H). V _p is a voltage actually applied to the pixel electrode 10. The voltage actually applied to the pixel electrode 10 is determined by the parasitic capacitance C _gd between the gate and drain of the thin film transistor 11 when the scanning signal V _g changes from V _GH to V _GL .
The voltage fluctuates. This voltage fluctuation ΔV _p is expressed by the following equation.

ΔV _p = C _gd · (V _GH −V _GL ) / (C _gd + C _LC + C _S ) Generally, in order to correct this voltage fluctuation ΔV _p , the voltage applied to the counter electrode 12 is varied. . However, the value of ΔV _p varies due to the dielectric anisotropy of the liquid crystal material (the property that the dielectric constant of the liquid crystal material changes depending on the applied voltage).
The application of an effective DC voltage component to the liquid crystal material is not compensated, and therefore, there is a problem that flicker or image sticking occurs immediately after displaying a fixed image.

In order to solve this problem, efforts are being made to minimize the parasitic capacitance C _gd between the gate and drain of the thin film transistor 11 which is the main cause of ΔV _p . Further, since the liquid crystal display device has a lower brightness than that of a CRT and there is a strong demand for portable use, a liquid crystal panel with a bright screen and low power consumption is desired. In order to solve this problem, efforts have been made to develop a liquid crystal panel having a large aperture ratio.

In order to increase the aperture ratio, it is conceivable to form the thin film transistor 16 on the scanning signal wiring as shown in FIG. An enlarged view of the peripheral portion of the thin film transistor 16 when the thin film transistor 16 is formed on the scanning signal line 13 is shown in FIG. 7, and a cross-sectional view taken along the line AA ′ of FIG. 7 is shown in FIG. Further, another example is shown in FIG. 9 and a sectional view taken along the line AA ′ of FIG. 9 is shown in FIG. 7 to 10, 1
Reference numerals 7 and 24 denote scanning signal wiring / gate electrode of thin film transistor, 18 and 25 are image signal wiring / source electrode of thin film transistor, 19 and 26 are pixel electrode / drain electrode of thin film transistor, 20 and 27 are semiconductor layers of thin film transistor, and 21, 28. Is an insulating layer, 22 and 29 are glass substrates, 2
Reference numerals 3 and 30 are channel portions of the thin film transistors.

It is generally known that the gate-drain parasitic capacitance C _gd of a thin film transistor is generated in the region of the semiconductor layer from the drain side to a distance of ½ of the channel width. That is, in the thin film transistors of FIGS. 7 and 9, the area where the parasitic capacitance C _gd between the gate and the drain is generated is the area shown by the solid diagonal lines, which is a large area. Therefore, if the thin film transistor is simply formed on the scanning signal wirings 17 and 24, the parasitic capacitance of the thin film transistor becomes large, causing flicker and sticking of a fixed image.

[0011]

The liquid crystal display device according to the present invention has been made in view of the above drawbacks, and is characterized in that a plurality of image signal wirings and scanning signal wirings are crossed. In a liquid crystal display device in which a pixel electrode and a thin film transistor that supplies an image signal to the pixel electrode are provided in a matrix at each intersection of the image signal wiring and the scanning signal wiring, the thin film transistor is formed on the scanning signal wiring. In addition, the channel portion of this thin film transistor is formed in an L shape.

[0012]

As described above, the aperture ratio is improved by forming the switching transistor on the scanning signal wiring. Further, by forming the channel portion of the thin film transistor into an L-shape, it is possible to reduce the parasitic capacitance C _gd between the gate and drain of the thin film transistor, and as a result, a liquid crystal display device which is bright and does not cause flicker or image burn-in. Becomes

[0013]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 1 is an enlarged view of a thin film transistor portion showing an embodiment of the liquid crystal display device according to the present invention.
FIG. 2 is a sectional view taken along the line AA ′ of FIG. Here, 1 is a scanning signal wiring, 2 is an image signal wiring, 3 is a pixel electrode, 4 is a semiconductor layer of a thin film transistor, 5 is an insulating layer, 6 is a glass substrate, and 7 is a channel portion of the thin film transistor.

The scanning signal wiring 1 is made of aluminum (Al).
And is formed by a vacuum deposition method or the like. The scanning signal line 1 also serves as the gate electrode of the thin film transistor. An insulating layer 5 is formed on the scanning signal wiring 1. The insulating layer 5 includes a scanning signal wiring 1 and an image signal wiring 2
And acts as a gate insulating film of a thin film transistor. The insulating layer 5 is made of silicon nitride (SiN _x ) or silicon dioxide (SiO ₂ ) and is formed by a plasma CVD method or the like. The semiconductor layer 4 of the thin film transistor is formed on the insulating layer 5. The semiconductor layer 4 is made of amorphous silicon or the like and is formed by, for example, a plasma CVD method or the like. A drain electrode 3a connected to the pixel electrode 3 and a source electrode 2a connected to the image signal wiring 2 are formed on the semiconductor layer 4. The pixel electrode 3 is made of a transparent conductive film such as ITO and is formed by a sputtering method or the like.
The image signal wiring 2, the source electrode 2a, and the drain electrode 3a are made of aluminum or the like and are formed by a vacuum vapor deposition method or the like.

In the liquid crystal display device according to the present invention, the semiconductor layer 4 is formed on the scanning signal wiring 1, and the source electrode 2 a protruding from the image signal wiring 2 and the drain electrode protruding from the pixel electrode 3 are formed on the semiconductor layer 4. 3a is formed. Therefore, the channel portion 7 of the thin film transistor is L
It is shaped like a letter. Therefore, the generation area of the gate-drain capacitance C _gd of the thin film transistor is the area shown by the solid diagonal lines in FIG. 1, which is smaller than that of the conventional liquid crystal display device of FIGS. 7 and 9. The gate-drain capacitance C _gd of a thin film transistor when a thin film transistor is designed is compared with a minimum line width of 8 μm and a minimum line interval of 6 μm.

The thin film transistor channel shape vertical I-shaped conventional thin film Trang Njisuta present invention (FIG. 7) the horizontal I-shaped (FIG. 9) L-shaped C _gd generating region 114μm ² 95μm ² 54μm ² C _gd capacitance value 0. 015pF 0.013pF 0.007pF The above values have a relative dielectric constant of the gate insulating layer of 6 and a film thickness of 0.4.
μm, the channel width W of the thin film transistor is 6 μm, and the channel length L is 18 μm.

As described above, according to the present invention, since the channel portion of the thin film transistor is L-shaped, the channel portion is I-shaped when the W / L ratios that determine the capability of the transistor are the same. As compared with the conventional thin film transistor of FIGS. 7 and 9, the parasitic capacitance between the gate and the drain can be reduced to about 1/2.

[0018]

As described above, according to the liquid crystal display device of the present invention, since the switching thin film transistor is formed on the scanning signal wiring and the channel portion of this thin film transistor is formed in the L-shape, The parasitic capacitance C _gd between the gate and the drain of the thin film transistor can be reduced, so that the fluctuation of the pixel electrode voltage due to the scanning signal can be reduced, and the flicker of the displayed image and the image sticking that occurs immediately after the fixed image is displayed can be prevented. In addition, since the channel portion of the thin film transistor is L-shaped, even when the thin film transistor is formed on the scanning signal wiring, the parasitic capacitance between the gate and drain of the thin film transistor can be reduced, the reliability is high and the aperture ratio is large. A bright liquid crystal display device can be realized.

[Brief description of drawings]

FIG. 1 is an enlarged view of a pixel switching thin film transistor according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line A-A ′ in FIG.

FIG. 3 is an enlarged view of one pixel of a conventional liquid crystal display device.

FIG. 4 is an equivalent circuit diagram of one pixel of the liquid crystal display device.

5 is a signal waveform of each terminal in FIG.

FIG. 6 is an enlarged view of one pixel when a pixel switching thin film transistor is formed on a scanning signal line.

FIG. 7 is an enlarged view of a thin film transistor when a pixel switching thin film transistor is formed on a scanning signal line.

8 is a cross-sectional view taken along the line A-A ′ of FIG.

FIG. 9 is an enlarged view of a thin film transistor when a pixel switching thin film transistor is formed on a scanning signal line.

10 is a cross-sectional view taken along the line A-A ′ of FIG.

[Explanation of symbols]

1 ... Scan signal wiring, 2 ... Image signal wiring, 3 ...
・ Pixel electrode, 4 ... Semiconductor layer, 5 Insulation layer, 6
... Glass substrate, 7 ... Channel section

Claims (1)

[Claims] 1. A matrix comprising a plurality of image signal wirings and scanning signal wirings intersecting each other, and a pixel electrode and a thin film transistor for supplying an image signal to the pixel electrode at each intersection of the image signal wirings and the scanning signal wirings. In a liquid crystal display device provided in a strip shape, the thin film transistor is formed on the scanning signal line, and a channel portion of the thin film transistor is formed in an L shape.