JP2005175248A - Liquid crystal display of field sequential system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the liquid crystal display of a field sequential system by which an ON current is increased without enlarging a size of a TFT of each pixel, and a speed of response is high. <P>SOLUTION: The liquid crystal display of the field sequential system includes the TFT 10 as an driving device of each pixel. Shapes of faces in which a source electrode S and a drain electrode D of the TFT 10 are confronted are arranged so that an end 16 of the source electrode S may be semicircular arc while the drain electrode may be semicircular. In this case, it is preferable that a distance between the semicircular arc source electrode S and the semicircular drain electrode D, namely a channel length L, is fixed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a field sequential liquid crystal display device that is optimal for driving.

  In general, liquid crystal display devices are characterized by thinness, light weight, and low power consumption. In particular, thin film transistor (“TFT”) type active matrix liquid crystal display devices are widely used from portable terminals to large-sized TVs. It's being used.

  In general, an active matrix type liquid crystal display device widely used has a predetermined distance between a pixel electrode arranged in a matrix and a substrate provided with a TFT element connected to each pixel electrode and a substrate provided with a common electrode. And a liquid crystal display device in which liquid crystal is sealed between both substrates. A sub-pixel provided with a red (R), green (G), or blue (B) filter for each pixel is provided on the common electrode side of the liquid crystal display device, and three R, G, and B filters are provided. One set of sub-pixels is used as one pixel. Further, a polarizing filter is disposed on both surfaces of the liquid crystal display device, and a backlight is disposed behind one of the polarizing filters to form a transmissive liquid crystal display device.

  This type of liquid crystal display device is referred to as a spatial mixing method, and obtains light of a desired color by changing the intensity of transmitted light of each RGB pixel and mixing them. However, there is a problem that the light use efficiency is low. That is, in the case of a normal color liquid crystal display device, the light transmittance by the polarizing plate is about ½ or less, and the light transmittance by the color filter is about ３ or less. The utilization efficiency of all light emitted from the light is substantially 10% or less.

  In recent years, field-sequential liquid crystal display devices have been developed as transmission-type liquid crystal display devices that do not use such color filters (see Patent Documents 1 and 2 below). This field-sequential liquid crystal display device is called a time difference mixing method, and uses a light source capable of emitting R, G, and B light as a backlight, and R, G, and R in each field. A color display is obtained by quickly switching each light of B and irradiating the liquid crystal display device. Since this field sequential type liquid crystal display device does not have a color filter, there is no need to consider light absorption by the color filter as in the conventional example, and a light source with lower power consumption than the conventional one is used. obtain. As a light source used in a field sequential type liquid crystal display device, a light emitting diode capable of switching light of R, G, and B at high speed and an inorganic or organic EL (Electro Luminescence) element are used.

  As described above, the field sequential type liquid crystal display device has low power consumption and can perform color display by time-dividing and transmitting light of each color of RGB by one pixel. Although high definition can be achieved as compared with a device, a liquid crystal display device is required to have a higher response than a conventional liquid crystal display device. That is, in order to prevent flicker (flickering of an image) from occurring at the time of color switching, one field is 1/60 second in the current common standard, and therefore, 1 field per field as shown in FIG. In order to display the color, it is necessary to switch within 1/180 seconds or less, that is, about 5.6 milliseconds or less. Therefore, a field-sequential liquid crystal display device is required to have a response speed much faster than that of a conventional one, and further improvements are still required.

For example, the following Patent Document 2 discloses a liquid crystal display device capable of high-speed response and multistage display by changing the pattern of the voltage applied to the liquid crystal display device within the unit light emission period of the light emitting unit. No. 3 discloses a technique for increasing the response speed of the liquid crystal display device by bend transition of liquid crystal molecules in an OCB (Optical Controlled Birefringence) mode liquid crystal display device.
Japanese Patent Laid-Open No. 2001-1000064 (Claims, paragraphs [0004] to [1007], [0042] to [0072], FIGS. 1 to 3) Japanese Patent No. 3338438 (Claims, paragraphs [0038] to [0048], FIG. 3)

  The invention disclosed in Patent Document 2 is a response of the liquid crystal display device from the viewpoint of the voltage applied to the liquid crystal display device, and the invention disclosed in Patent Document 3 is a response of the liquid crystal display device from the viewpoint of the alignment of liquid crystal molecules. Although it is intended to improve the speed, it has not yet achieved sufficient high-speed response as a field sequential type liquid crystal display device. Therefore, the present inventors have examined the possibility of improving the response speed of the liquid crystal display device by improving the TFT characteristics of the active matrix liquid crystal display device.

  A TFT is a switching element that selects a data signal input to a pixel electrode, and is a field effect transistor composed of a gate electrode, a drain electrode, a source electrode, and an amorphous semiconductor layer, each electrode being a scanning line, It is connected to the video line and the pixel electrode. The scanning line group is selected in a line-sequential manner, and all the TFTs on one scanning line are turned on for a predetermined scanning time. During this ON period, a data signal is input to each pixel electrode via each video line. The A voltage is set to the common electrode in synchronization with the scanning signal, and a liquid crystal capacitor serving as a pixel is formed between the opposing pixel electrodes to hold the voltage. This holding voltage drives the liquid crystal in the gap and maintains the driving state of the liquid crystal for a predetermined one scanning period until it is rewritten by reversing the polarity in the next field.

A general configuration and equivalent circuit of the field sequential type liquid crystal display device will be briefly described with reference to FIGS. FIG. 2 is a plan view of one pixel of the field sequential type liquid crystal display device, and FIG. 3 is a diagram showing an equivalent circuit of several pixels. Each pixel electrode LP is provided in a section surrounded by scanning lines Xn, Xn + 1... And video lines Ym, Ym + 1... On the liquid crystal display device. Is represented by the liquid crystal capacitance _CLC . Usually the liquid crystal capacitance C _LC auxiliary capacitor C _S is connected in parallel. One end of the liquid crystal capacitance C _LC is connected to the drain electrode D of the TFT for driving and the other end is connected to the counter electrode a predetermined reference voltage Vcom is applied.

The TFT is an insulated gate field effect type thin film transistor, and its source electrode S is connected to the video lines Ym, Ym + 1... And supplied with the image signal Vsig, and the drain electrode D is a liquid crystal capacitor _CLC . One end is connected to the pixel electrode LP. Further, the gate electrode G of the TFT is connected to the scanning lines Xn, Xn + 1... So that a gate pulse GP having a predetermined gate voltage Vgate is applied. Then, Kamata parasitic capacitance C _DS between the drain and source electrodes, the coupling capacitance C _GD is formed between the liquid crystal capacitor C _LC and the gate electrode G. The coupling capacitance _CGD is a combination of the stray capacitance component between the pixel electrode and the scanning line Xn and the parasitic capacitance component between the drain region and the gate region inside the TFT. since dominant, _hereinafter referred to as _{C GD} and the parasitic capacitance with _{C DS.}

  An image signal Vsig is written to a pixel of a liquid crystal display device during a selection period, and an image signal written during a subsequent non-selection period is held to form one field. The transmittance of the liquid crystal pixel in one field is Is determined by the effective voltage applied to the liquid crystal. Therefore, the TFT must be able to secure an ON current necessary for completing writing within the selection period, and obtain an effective voltage sufficient to keep the liquid crystal pixels lit during one field period. In order to achieve this, the leakage current during the non-selection period or the holding period needs to be as small as possible.

In the liquid crystal display device, when the TFT is turned on, the image signal Vsig applied to the source electrode S of the TFT is applied to the pixel electrode LP. The voltage across the pixel electrode LP is the liquid crystal capacitance C _LC , Since the image signal Vsig is approached while charging and discharging the auxiliary capacitance C _S and the parasitic capacitances C _GD and C _DS , at least as the parasitic capacitances C _GD and C _DS are smaller and the ON current of the TFT is larger, The response speed of the liquid crystal display device is increased.

Therefore, the liquid crystal molecules do not move immediately even when a driving voltage is applied, but it is more advantageous as the ON current of the TFT is larger in order to drive the liquid crystal display device at high speed. In order to increase the ON current of the TFT, referring to FIG. 4 showing the configuration of the TFT of the conventional active matrix liquid crystal display device, the TFT is increased in size so that the source electrode S and the drain electrode D face each other. This can be achieved by increasing the length, that is, the channel width W, and shortening the distance between both electrodes, that is, the channel length L. However, the channel length L has a certain limit in circuit design, and increasing the channel width W increases the parasitic capacitance components C _GD and C _{DS accordingly} (see FIG. 2). This is a contradictory factor in terms of high-speed driving of the apparatus.

  Accordingly, as a result of various studies on the configuration of the TFT of the field sequential type liquid crystal display device, the present inventors have increased the size of the TFT by devising the shape of the source electrode S and the drain electrode D of the TFT. The present inventors have found that the ON current of the TFT can be increased without doing so, and have completed the present invention.

  That is, an object of the present invention is to provide a liquid crystal display device that can increase the ON current without increasing the TFT size of each pixel and has a high response speed in a field sequential type liquid crystal display device. .

  The above object of the present invention can be achieved by the following configurations. That is, the field sequential type liquid crystal display device according to claim 1 of the present application is a field sequential type liquid crystal display device having a thin film transistor as a driving element for each pixel. The shape of the opposing surface is characterized in that the source electrode has a semicircular arc shape and the drain electrode has a semicircular shape.

  Further, in the invention according to claim 2 of the present application, in the field sequential type liquid crystal display device according to claim 1, the distance between the opposed surfaces of the source electrode and the drain electrode of the thin film transistor is constant. It is characterized by that.

  The invention according to claim 3 of the present application is the field sequential type liquid crystal display device according to claim 1 or 2, wherein the thin film transistor includes the source electrode having the semicircular arc shape with respect to one gate electrode, and the source electrode having the semicircular arc shape. A plurality of semicircular drain electrodes are formed, and each semicircular source electrode and each semicircular drain electrode are connected in parallel to each other.

  According to the above configuration of the present invention, the following excellent effects can be obtained. That is, according to the field sequential type liquid crystal display device according to claim 1 of the present application, the length of the opposed surfaces of the source electrode and the drain electrode can be increased without increasing the parasitic capacitance without particularly increasing the shape of the TFT. In other words, since the channel width W can be widened, a TFT with a large ON current can be obtained, so that a field sequential type liquid crystal display device with high-speed response can be obtained.

  In addition, according to the field sequential type liquid crystal display device according to claim 2 of the present application, it is advantageous that the parasitic capacitance is small, and a field sequential type liquid crystal display device with little variation in ON current for each pixel is obtained. It is done.

  Further, according to the field sequential type liquid crystal display device according to claim 3 of the present application, since a plurality of sets of semicircular arc-shaped source electrodes and semicircular drain electrodes are provided in parallel, the parasitic capacitance is increased so much. Accordingly, the ON current of the TFT can be further increased, so that a field sequential type liquid crystal display device with a faster response speed can be obtained.

  Embodiments of the present invention will be described below with reference to FIGS. However, the embodiment shown below exemplifies a field sequential type liquid crystal display device for embodying the technical idea of the present invention, and the present invention is specified as the field sequential type liquid crystal display device of this embodiment. It is not intended to be applied, but is equally applicable to what is included in the technical scope described in the claims.

  FIG. 5 shows an enlarged plan view of a TFT of the field sequential type liquid crystal display device corresponding to the first embodiment of the present invention. This TFT 10 is provided with an insulating film on the surface of a glass substrate (both not shown), and a gate electrode G and a semiconductor layer 12 such as amorphous silicon are sequentially laminated on the insulating film, and the surface of this semiconductor layer is formed. A source electrode S and a drain electrode D are arranged to face each other. The distal end portion 14 of the drain electrode D has a semicircular shape, and the distal end portion 16 of the source electrode S facing the distal end portion of the drain electrode D has a semicircular arc shape, and the distance between the two electrodes, That is, the channel length L is constant.

  In this case, in order to reduce the parasitic capacitance between the drain electrode D and the gate electrode G, the length of the portion 18 other than the semicircular tip portion 14 of the drain electrode D located on the gate electrode G is made as short as possible. Is good. The shape of the source electrode S other than the portion facing the drain electrode D is arbitrary, but in order to increase the ON current of the TFT 10, it is preferable to increase the thickness to some extent to reduce the resistance value.

With such a configuration, it is possible to increase the channel width W, since the parasitic capacitance C _GD between parasitic capacitance C _DS and the gate electrode and the drain electrode between the drain electrode D and the source electrode S may both small, Even if the size of the TFT 10 is the same as that of the conventional example, the ON current can be increased, so that the response speed of the liquid crystal display device can be improved, and the liquid crystal display device applicable as a field sequential type liquid crystal display device Is obtained.

  An enlarged plan view of the TFT 20 of the field sequential type liquid crystal display device corresponding to the second embodiment of the present invention is shown in FIG. In FIG. 6, the same reference numerals are given to the same components as those of the TFT 10 of Example 1 shown in FIG. 5, and detailed description thereof is omitted.

  The TFT 20 of Example 2 is different from the TFT 10 of Example 1 in that the gate electrode G is greatly extended along the signal line Ym, and a plurality of combinations of the source electrode S and the drain electrode D are provided. In the example, four sets are provided, and the source electrodes S are electrically connected to each other, and the drain electrodes D are also electrically connected to each other. Therefore, in the field sequential type liquid crystal display device according to the second embodiment, the TFT 20 has a plurality of semicircular source electrodes S and semicircular drain electrodes D formed on one gate electrode G, and each semi-circular drain electrode D is formed. The arc-shaped source electrode S and the semicircular drain electrodes D are connected in parallel to each other.

In this case, in order to suppress variations in TFT characteristics for each pixel, the length of the portion where each source electrode S faces the drain electrode D, that is, the channel widths W1 to W4 are all made equal, and the source electrode It is desirable that the distance between the arc-shaped portion 16 and the semicircular tip portion 14 of the drain electrode D, that is, the channel length L are all equal. In addition, the portion other than the semicircular tip portion 14 of the drain electrode D is preferably as short as possible in order to reduce the parasitic capacitance C _GD between the gate electrode and the drain electrode.

With this configuration, the ON current of the TFT 20 can be significantly increased as compared with that of the first embodiment, and in addition, the length of the portion where the source electrode S faces the drain electrode D, that is, the channel Since the width W = W1 + W2 + W3 + W4 can also be increased, even if the channel width W is the same as that of the conventional TFT having the configuration shown in FIG. Since the parasitic capacitance _CDS and the parasitic capacitance _CGD between the gate electrode and the drain electrode can be reduced, the response speed of the liquid crystal display device can be improved, and the liquid crystal display device applicable as a field sequential type liquid crystal display device Is obtained.

It is a figure explaining the display principle of the liquid crystal display device of a field sequential system. FIG. 6 is a plan view of one pixel of a field sequential type liquid crystal display device. It is a figure which shows the equivalent circuit for several pixels of the liquid crystal display device of a field sequential system. It is a figure explaining the structure of TFT of the active matrix type liquid crystal display device of a prior art example. FIG. 2 is an enlarged plan view showing a configuration of a TFT of a field sequential type liquid crystal display device corresponding to Embodiment 1 of the present invention. It is an enlarged plan view which shows the structure of TFT of the field sequential type liquid crystal display device equivalent to Example 2 of this invention.

Explanation of symbols

Xn, Xn + 1... Scanning line Ym, Ym + 1... Video line C _LC liquid crystal capacitance C _S auxiliary capacitance D drain electrode G gate electrode S source electrode _CGD , _CDS parasitic capacitance 10, 20 TFT
12 Semiconductor layer 14 Semi-circular drain electrode D tip portion 16 Source electrode S tip portion

Claims (3)

  In the field sequential type liquid crystal display device having a thin film transistor as a driving element for each pixel, the shape of the opposing surface of the thin film transistor in the source electrode and the drain electrode is a semicircular arc shape of the source electrode and a semicircular shape of the drain electrode. A field-sequential liquid crystal display device characterized by that.   2. The field sequential liquid crystal display device according to claim 1, wherein a distance between opposing surfaces of the source electrode and the drain electrode of the thin film transistor is constant.   In the thin film transistor, a plurality of sets of the semicircular arc source electrode and the semicircular drain electrode are formed for one gate electrode, and each semicircular arc source electrode and each semicircular drain electrode are mutually connected. 3. The field sequential type liquid crystal display device according to claim 1, wherein the field sequential type liquid crystal display devices are connected in parallel.

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