JP2001194667A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further improve picture quality and further reduce power consumption of a liquid crystal display device of a dot matrix system. SOLUTION: The dot matrix system (M. Kimura, Proc. Euro Display' 99 Late-NewsPapers, 71, selected drawing) is provided with a phase plate optimized to minimize visual angle dependency or coloring to an off-state. In such a manner, it is possible to compensate for the visual angle dependency and coloring very completely, and realize a liquid crystal display device of high image quality. Moreover, ultra low power consumption can be realized by providing the device with a scanning pause period, arranging the device to be an internal reflection single polarizer type liquid crystal display device, using an ultra- twisted nematic liquid crystal, etc.

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, and more particularly to an active matrix liquid crystal display device realizing high image quality and ultra-low power consumption.

[0002]

BACKGROUND ART At present, liquid crystal display devices are personal computer monitors,
It has already been widely and widely used as display devices for notebook computers, portable information terminals, car navigation systems, portable televisions, wall-mounted televisions, video viewfinders, digital cameras, mobile phones, and the like. It is expected that the range of use will further expand in the future. from now on,
Especially for portable applications such as notebook computers, personal digital assistants, portable TVs, video viewfinders, digital cameras, and mobile phones, which are applications that take advantage of the low power consumption characteristics of liquid crystal display devices. Will be required. of course,
When reducing power consumption, it is required to maintain high image quality.

[0003] One of the driving methods of a liquid crystal display device is an area gray scale method (Japanese Patent Laid-Open No. 09-090345, Japanese Patent Laid-Open No. 10-068931,
JP 09-233107, M. Kimura, et al., Proc. Euro Dis
play '99 Late-News Papers, 71, M. Kimura, et al.,
Proc. IDW 99, 171; Japanese Patent Application No. 11-305740). In the area gray scale method, pixels are formed in a matrix, the pixels are composed of a plurality of sub-pixels, and the sub-pixels are controlled to one of two states, an on state in which a voltage is applied and an off state in which a voltage is not applied. In this method, a gray scale is obtained by changing an area ratio between an on state and an off state.

A feature of the area gradation method with respect to the liquid crystal display device is, firstly, the realization of a gradation display in a reflection type color liquid crystal display device (Japanese Patent Laid-Open No. 09-090345). Secondly, digitalization leads to lower costs, lower power consumption, and higher yields (Japanese Patent Laid-Open No. 10-068931). Third, wide viewing angle. A widely used twisted nematic liquid crystal or super-twisted nematic liquid crystal has a large viewing angle dependence when an intermediate voltage is applied. Therefore, a wide viewing angle is realized by using only the on-state and the off-state with little viewing angle dependence by the area gradation method. Fourth, lower power consumption by lowering the screen scanning frequency (Japanese Patent Application No. 11-305740), and
Ultra-low power consumption by scanning thinning.

[0005]

By applying the area gray scale method to the liquid crystal display device, various features including low power consumption are realized. In particular, ultra-low power consumption is realized by scanning thinning. Therefore, in the present invention,
It is an object of the present invention to achieve higher image quality and lower power consumption for an area gradation type liquid crystal display device.

[0006]

(1) According to the first aspect of the present invention, pixels are formed in a matrix, and the pixels are composed of a plurality of sub-pixels. A liquid crystal display device which is controlled to one of two states of an off state in which no voltage is applied and obtains a gray scale by changing an area ratio between an on state and an off state, and which has viewing angle dependency or coloring with respect to the off state. A liquid crystal display device comprising a phase plate optimized to be minimized.

According to this configuration, it is possible to minimize the viewing angle dependence or coloring of the area gradation type liquid crystal display device.

(2) The present invention according to claim 2 provides the invention according to claim 1.
In the liquid crystal display device described above, there are provided a scanning period in which writing of an ON state or an OFF state is performed on a sub-pixel and a scanning pause period in which writing is not performed while the ON state or the OFF state of the sub-pixel is held. A liquid crystal display device characterized in that:

According to this configuration, the power consumption of the driving circuit is reduced for the area gradation type liquid crystal display device, so that the power consumption can be further reduced.

(3) The present invention described in claim 3 provides the invention according to claim 1.
The liquid crystal display device described in the above, which is a reflection type liquid crystal display device.

According to this configuration, the power consumption of the backlight is reduced, so that the power consumption can be further reduced.

(4) The present invention described in claim 4 provides the present invention in claim 3
The liquid crystal display device according to claim 1, wherein the liquid crystal display device includes a reflection plate on an inner surface of a glass substrate, and includes only one polarizing plate on an outer surface of the glass substrate, which is a reflection type liquid crystal display device. .

According to this configuration, since only one polarizing plate is provided, the luminance can be improved.

(5) The present invention described in claim 5 is the first invention.
In the liquid crystal display device described, the twist angle is from 180 degrees to 2
A liquid crystal display device comprising a super-twisted nematic liquid crystal up to 70 degrees.

According to this configuration, the driving voltage of the super-twisted nematic liquid crystal is low, so that the power consumption can be further reduced.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described.

In this embodiment, a polycrystalline silicon thin-film transistor (low-temperature process) formed by a low-temperature process at 600 ° C. or lower is used.
p-Si TFT). The low-temperature p-Si TFT can be formed on a large and inexpensive glass substrate and can incorporate a driving circuit on a panel, and is a technology suitable for display device manufacturing. Also,
This technology is suitable for the area gray scale method because it has a high driving capability even in a small size and can incorporate complicated circuits even in one pixel. The concept of the present invention is also effective for a liquid crystal display device using an amorphous silicon thin film transistor (a-Si TFT) instead of the low-temperature p-Si TFT. In addition, low temperature p-
The idea of the present invention is also effective for a liquid crystal display device using a thin film diode (TFD) instead of the Si TFT. In these cases, the drive circuit is provided outside the panel.

FIG. 1 is a pixel equivalent circuit diagram of an embodiment of the present invention. Here, only one pixel is illustrated, but actually, there are a large number of pixels in a plurality of rows and a plurality of columns extending vertically and horizontally. Multiple scanning lines 11 and multiple signal lines 12
Are formed in a matrix. At the intersection of each scanning line 11 and each signal line 12, a thin film transistor 13, a storage capacitor 14, and a pixel 15 are formed. While the selection pulse is applied to the scanning line 11, an image signal is applied to the signal line 12, and is held in the storage capacitor 14 and the pixel 15 through the thin film transistor 13. The transmittance is controlled by the voltage applied to the pixel 15.
If the pixel 15 has a sufficient voltage holding capacity, the holding capacity
14 may not be required.

In this embodiment, since the gray scale is represented by 2 bits, the signal line 12 comprises a low bit signal line 121 and a high bit signal line 122. Correspondingly, the thin film transistor 13 also includes a low bit thin film transistor 131 and a high bit thin film transistor 132, and the storage capacitor 14 also includes a low bit storage capacitor 141 and a high bit storage capacitor 142. Further, the pixel 15 also has a low-bit sub-pixel 151 and a high-bit sub-pixel 1.
Consists of 52. Low bit sub-pixel 151 and high bit sub-pixel 1
The area ratio of 52 is 1: 2. Since the light transmission amount is proportional to the light transmission area, the light transmission amount is also 1: 2, and four gradations can be obtained. The concept of the present invention is also effective for gray scales of 3 bits or more.

FIG. 2 is a diagram showing a pixel control method according to an embodiment of the present invention. In this embodiment, a super-twisted nematic liquid crystal having a twist angle from 180 degrees to 270 degrees is used. The horizontal axis indicates the applied potential of the pixel 15, and the vertical axis indicates the transmittance of the pixel 15. The idea of the present invention is also effective for a liquid crystal display device using a twisted nematic liquid crystal having a twist angle of about 90 degrees.

The pixel 15 is in the ON state 21 or the OFF state 22.
Controlled in one of two states. Here, a normally white condition in which light is transmitted when no voltage is applied is used. In the on state 21, a voltage is applied to the pixel 15, no light is transmitted, and in the off state 22, the pixel 15 is not transmitted. No voltage is applied, and light is transmitted.

FIG. 3 is a sectional view of a liquid crystal panel according to an embodiment of the present invention. First, S on the thin film transistor-side substrate 311
Amorphous silicon is formed by PECVD using iH4 or LPCVD using Si2H6. Amorphous silicon is crystallized by laser irradiation such as excimer laser or solid phase growth, and polycrystalline silicon 312 is formed. After patterning the polycrystalline silicon 312, the gate insulating film 313
Is formed, and the gate electrode 314 is formed and patterned. Impurities such as phosphorus and boron are implanted into the polycrystalline silicon 312 in a self-aligned manner using the gate electrode, activated, and the source and drain regions 315 of the CMOS structure are formed.
Is formed. A first interlayer insulating film 316 is formed, a contact hole is opened, and a source electrode and a drain electrode are formed.
317 is formed and patterned, and a second interlayer insulating film 318 is formed. Thereafter, a thin film transistor-side alignment film 321 is formed, and an alignment process such as rubbing is performed. On the other hand, a counter electrode 324 is formed on the counter substrate 325,
An opposite substrate side alignment film 323 is formed, and an alignment process such as rubbing is performed. The thin film transistor-side substrate 311 and the opposing substrate 325 are bonded to each other after performing a process such as dispersing spacers as necessary. Thin film transistor side substrate
Between the 311 and the counter substrate 325, by an injection method or a dropping method,
Liquid crystal 322 is enclosed.

FIG. 4 shows the configuration of an optical element according to an embodiment of the present invention. 41 is a liquid crystal panel, 42 is a phase plate, and 43 is a polarizing plate. Note that although the number of the phase plate 42 is only one here, the idea of the present invention is also effective for a configuration using a plurality of phase plates 42.

In this embodiment, the technique described in claim 1 is used. That is, the pixels 15 are formed in a matrix, and the pixels 15 are composed of a plurality of sub-pixels, that is, a low-bit sub-pixel 151 and a high-bit sub-pixel 152, and the sub-pixel is in an ON state 21 for applying a voltage or an OFF state for not applying a voltage. Status
A liquid crystal display device which is controlled to one of two states and obtains a gray scale by changing an area ratio between the on state 21 and the off state 22, wherein viewing angle dependency or coloring is minimized with respect to the off state 22. A liquid crystal display device comprising a phase plate 42 optimized to optimize the structure. Since the a priori research (Shinichi Komura, IEICE technical report EID88, 9-16, Fukuda Ichiro, IEICE technical report EID88, 17-23) on the optimization method of the phase plate 42, Although many techniques have been devised, the idea of the present invention is effective no matter what technique is used.

It should be noted here that, for the off-state 22 only, the viewing angle dependence or coloring is completely compensated for by the optimized phase plate 42. In addition, for the ON state 21, the viewing angle dependency and the coloring are essentially nonexistent. Therefore, according to the area gradation method, only these two states are used, so that the viewing angle dependency and the coloring can be compensated for completely, and a high-quality liquid crystal display device is realized.

In this embodiment, the technique described in claim 2 is used. That is, the scanning period in which the ON state 21 or the OFF state 22 is written to the sub-pixel, that is, the low-bit sub-pixel 151 and the high-bit sub-picture 152, and the writing while the ON state or the OFF state of the sub-pixel is maintained A liquid crystal display device characterized in that a non-performing scanning pause period is provided. According to this configuration, the power consumption of the drive circuit is reduced with respect to the area gradation type liquid crystal display device,
A significant reduction in power consumption is possible. This is because the power consumption of the drive circuit becomes almost zero during the scan pause period.

In this embodiment, the technique described in claim 3 is used. That is, the liquid crystal display device is a reflection type liquid crystal display device. According to this configuration, the power consumption of the backlight is reduced, thereby further reducing the power consumption.

In this embodiment, the technique described in claim 4 is used. That is, provided with a reflection plate on the inner surface of the glass substrate, provided with only one polarizing plate 43 on the outer surface of the glass substrate, characterized in that it is an internal reflection type liquid crystal display device,
It is a liquid crystal display device. According to this configuration, since only one polarizing plate 43 is provided, the brightness can be improved as compared with the case where two polarizing plates are provided.

In this embodiment, the technique described in claim 5 is used. That is, the twist angle changes from 180 degrees to 270
A liquid crystal display device comprising a super-twisted nematic liquid crystal having a degree up to a degree. According to this configuration, since the driving voltage of the super-twisted nematic liquid crystal is low,
Further lower power consumption is possible. For internal reflection type liquid crystal display using super twisted nematic liquid crystal,
For an optimization method of the phase plate 42, see S. Fujita, J. SID
7, 135 (1999). In addition, display irregularities caused by gap irregularities in super-twisted nematic liquid crystal, which are often a problem in the passive matrix driving method, are ignored by providing a voltage margin for the ON state 21 and the OFF state 22 in the area gradation method. It can be reduced as much as possible. Note that, even when a nematic liquid crystal having a twist angle of about 90 degrees is used instead of the super-twisted nematic liquid crystal, the ideas of the present invention other than the claims are effective.

[Brief description of the drawings]

FIG. 1 is a pixel equivalent circuit diagram according to an embodiment of the present invention.

FIG. 2 is a diagram showing a pixel control method according to the embodiment of the present invention.

FIG. 3 is a cross-sectional view of the liquid crystal panel according to the embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of an optical element according to an example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Scan line 12 Signal line 121 Low bit signal line 122 High bit signal line 13 Thin film transistor 131 Low bit thin film transistor 132 High bit thin film transistor 14 Retention capacity 141 Low bit retention capacity 142 High bit retention capacity 15 Pixel 151 Low bit Sub-pixel 152 High bit sub-pixel 21 ON state 22 OFF state 311 Thin film transistor-side substrate 312 Polycrystalline silicon 313 Gate insulating film 314 Gate electrode 315 Source region and drain region 316 First interlayer insulating film 317 Source electrode and drain electrode 318 2 interlayer insulating film 321 Thin film transistor side alignment film 322 Liquid crystal 323 Counter electrode side alignment film 324 Counter electrode 325 Counter electrode side substrate 41 Liquid crystal panel 42 Phase plate 43 Polarizing plate

Claims (5)

[Claims] 1. A pixel is formed in a matrix, and the pixel is composed of a plurality of sub-pixels, and the sub-pixel is controlled to one of two states of an on state in which a voltage is applied and an off state in which a voltage is not applied. A liquid crystal display device that obtains a gray scale by changing an area ratio between the on state and the off state, wherein the phase plate is optimized to minimize viewing angle dependency or coloring with respect to the off state. A liquid crystal display device comprising: 2. The liquid crystal display device according to claim 1, wherein a scanning period in which the on-state or the off-state writing is performed on the sub-pixel and the on-state or the off-state of the sub-pixel are maintained. A liquid crystal display device having a scan pause period in which writing is not performed. 3. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is a reflection type liquid crystal display device. 4. The liquid crystal display device according to claim 3, further comprising: a reflection plate provided on an inner surface of the glass substrate;
A liquid crystal display device comprising an internal reflection type liquid crystal display device having only one polarizing plate. 5. The liquid crystal display device according to claim 1, further comprising a super-twisted nematic liquid crystal having a twist angle of 180 degrees to 270 degrees.