JP2010128004A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain a multi-pixel structure with simpler structure than before. <P>SOLUTION: A pixel 10 includes first and second sub pixels 10a and 10b. A first TFT 16A provided corresponding to the first sub pixel and a second TFT 16B provided corresponding to the second sub pixel are connected to a common gate bus line and a common source bus line. An on-current of the first TFT 16A is higher than that of the second TFT 16B. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a wide viewing angle characteristic.

  The liquid crystal display device is a flat display device having excellent features such as high definition, thinness, light weight and low power consumption. In recent years, the display performance has been improved, the production capacity has been improved, and the price competitiveness with respect to other display devices has been improved. Along with this, the market size is expanding rapidly.

  Further, in recent years, as a liquid crystal display device with improved viewing angle characteristics in a TN mode liquid crystal display device which has been the mainstream in the past, an in-plane switching mode (IPS mode) described in Patent Literature 1 and a patent document 2 A multi-domain vertical aligned mode (MVA mode) liquid crystal display device has been developed.

  In these new mode (wide viewing angle mode) liquid crystal display devices, there is no problem that the display contrast ratio is remarkably reduced or the display gradation is inverted when the display surface is observed from an oblique direction.

  Under the situation where the display quality of liquid crystal display devices is improving, the problem of viewing angle characteristics is that the γ characteristics during frontal observation and γ characteristics during oblique observation are different, that is, the problem of viewing angle dependency of γ characteristics. Has emerged anew. Here, the γ characteristic is the gradation dependency of the display luminance. The fact that the γ characteristic is different between the front direction and the diagonal direction means that the gradation display state differs depending on the observation direction. This is particularly a problem when displaying, or when displaying TV broadcasts and the like.

  The problem of the viewing angle dependency of the γ characteristic is more conspicuous in the display mode in which liquid crystal molecules are aligned in the direction perpendicular to the display surface, as represented by the MVA mode than in the IPS mode. On the other hand, in the IPS mode, it is difficult to manufacture a panel with a high contrast ratio at the time of front observation with high productivity compared to the MVA mode. From these points, it is desired to improve the viewing angle dependency of the γ characteristic particularly in the MVA mode liquid crystal display device.

  Therefore, the present applicant introduces a configuration (referred to as a multi-pixel structure) in which Patent Documents 3 to 5 are provided with a first subpixel and a second subpixel that can apply different voltages to each pixel. Thus, a liquid crystal display device in which the viewing angle dependency of the γ characteristic is improved is disclosed. The entire disclosure of Patent Documents 3 to 5 is incorporated herein by reference.

The liquid crystal display devices disclosed in Patent Documents 3 to 5 include a first TFT provided corresponding to the first subpixel and a second TFT provided corresponding to the second subpixel, and the first TFT And the gate electrodes of the second TFT are connected to a common gate bus line, the source electrodes of the first TFT and the second TFT are connected to a common source bus line, and the drain electrodes of the first TFT and the second TFT are respectively Are connected to the corresponding subpixel electrodes, and after the first and second TFTs are turned off, the voltages of the auxiliary capacitor counter electrodes (so-called CS voltages) of the first and second subpixels change. The amount of change defined by the direction and magnitude of the change is made different between the first subpixel and the second subpixel, so that the change is applied to the first subpixel and the second subpixel. And with different effective voltages (ie brightness) that.
Japanese Examined Patent Publication No. 63-21907 Japanese Patent Laid-Open No. 11-242225 JP 2004-62146 A (US Pat. No. 6,958,791) JP 2005-189804 A JP 2006-39130 A

  In the liquid crystal display devices described in Patent Documents 3 to 5, since it is necessary to provide a circuit for controlling the auxiliary capacitor counter voltage (CS voltage) in order to make the auxiliary capacitor counter voltage different among subpixels, the cost increases. There is a problem.

  Also, as described in Patent Document 5, display streak may occur due to a change in the storage capacitor counter voltage (CS voltage). In Patent Document 5, this problem is solved by switching off the TFT when the potentials of all the auxiliary capacitor counter voltages (CS voltages) are the same, but the auxiliary capacitor counter voltage (CS voltage) is controlled. There is a problem that the circuit to perform becomes complicated.

  The present invention has been made in view of such various points, and its main object is to realize a multi-pixel structure with a simpler structure than before. Another object of the present invention is to improve the display quality of the conventional multi-pixel structure described in Patent Documents 3 and 4.

  The liquid crystal display device of the present invention includes a plurality of pixels, each of the plurality of pixels having a first subpixel and a second subpixel, and each of the first subpixel and the second subpixel is A liquid crystal capacitor formed by a counter electrode, a liquid crystal layer, and a subpixel electrode facing the counter electrode through the liquid crystal layer; an auxiliary capacitor electrode electrically connected to the subpixel electrode; and an insulating layer And a storage capacitor formed by a storage capacitor counter electrode facing the storage capacitor electrode through the insulating layer, wherein the first TFT is provided corresponding to the first sub-pixel. And a second TFT provided corresponding to the second subpixel, the gate electrodes of the first TFT and the second TFT are connected to a common gate bus line, and the source of the first TFT and the second TFT Common electrode The drain electrode of each of the first TFT and the second TFT is connected to the corresponding sub-pixel electrode, and the on-current of the first TFT is higher than the on-current of the second TFT. large.

  In one embodiment, the channel width of the first TFT is larger than the channel width of the second TFT.

  In one embodiment, the channel length of the first TFT is shorter than the channel length of the second TFT.

  In one embodiment, the storage capacitor counter electrode is electrically independent for each of the first subpixel and the second subpixel.

  According to the present invention, a multi-pixel structure can be realized with a simpler structure than in the prior art. Further, by combining with the conventional multi-pixel structure described in Patent Documents 3 and 4, display quality can be improved. In particular, by improving the γ characteristic of a liquid crystal display device having a wide viewing angle characteristic such as the MVA mode or the ASV mode, a display with extremely high display quality can be realized.

  Hereinafter, the configuration and operation of a liquid crystal display device according to an embodiment of the present invention will be described with reference to the drawings.

  First, the structure and operation of the liquid crystal display device 200 having a multi-pixel structure described in Patent Documents 3 and 4 will be described with reference to FIGS.

  As will be described later, the first TFT 16A and the second TFT 16B shown in FIG. 2A or the first TFT 16A ′ and the second TFT 16B ′ shown in FIG. 2B are applied to the first TFT 16a and the second TFT 16b in the liquid crystal surface device 200. By doing so, the display quality can be improved. The description of FIGS. 3 to 7 is also used for description of the liquid crystal display device according to the embodiment of the present invention.

  FIG. 3 schematically shows an electrical configuration of the liquid crystal display device 200. The pixel 10 is divided into sub-pixels 10a and 10b. The sub-pixels 10a and 10b are connected to TFTs 16a and 16b and auxiliary capacitors (CS) 22a and 22b, respectively. The gate electrodes of the TFTs 16 a and 16 b are connected to a common (identical) scanning line (gate bus line) 12, and the source electrodes are connected to a common (identical) signal line (source bus line) 14. Yes. The auxiliary capacitors 22a and 22b are connected to an auxiliary capacitor line (CS bus line) 24a and an auxiliary capacitor line 24b, respectively. The auxiliary capacitors 22a and 22b include an auxiliary capacitor electrode (not shown) electrically connected to the sub-pixel electrodes 18a and 18b, respectively, and an auxiliary capacitor counter electrode (not shown) electrically connected to the auxiliary capacitor wires 24a and 24b. ) And an insulating layer (not shown) provided therebetween. The storage capacitor counter electrodes of the storage capacitors 22a and 22b are independent from each other, and have a structure in which different storage capacitor counter voltages can be supplied from the storage capacitor lines 24a and 24b, respectively.

  Next, the principle by which different effective voltages can be applied to the liquid crystal layers of the two subpixels 10a and 10b of the liquid crystal display device 200 will be described with reference to the drawings.

  FIG. 4 schematically shows an equivalent circuit for one pixel of the liquid crystal display device 200. In the electrical equivalent circuit, the liquid crystal layers of the respective subpixels 10a and 10b are represented as liquid crystal layers 13a and 13b. The liquid crystal capacitance formed by the subpixel electrodes 18a and 18b (see FIG. 1), the liquid crystal layers 13a and 13b, and the counter electrode 17 (common to the subpixels 10a and 10b) is defined as Clca and Clcb. The liquid crystal layers 13a and 13b are formed of a nematic liquid crystal material having a negative dielectric anisotropy, and the liquid crystal display device 200 is a liquid crystal display device that performs display in a normally black mode.

  The capacitance values of the liquid crystal capacitors Clca and Clcb are the same value CLC (V). The value of CLC (V) depends on the effective voltage (V) applied to the liquid crystal layers of the subpixels 10a and 10b. The auxiliary capacitors 22a and 22b that are independently connected to the liquid crystal capacitors of the sub-pixels 10a and 10b are Ccsa and Ccsb, respectively, and their capacitance values are the same value CCS.

  One electrode of the liquid crystal capacitor Clca and the auxiliary capacitor Ccsa of the subpixel 10a is connected to the drain electrode of the TFT 16a provided to drive the subpixel 10a, and the other electrode of the liquid crystal capacitor Clca is connected to the counter electrode. The other electrode of the auxiliary capacitor Ccsa is connected to the auxiliary capacitor line 24a. One electrode of the liquid crystal capacitor Clcb and the auxiliary capacitor Ccsb of the subpixel 10b is connected to the drain electrode of the TFT 16b provided to drive the subpixel 10b, and the other electrode of the liquid crystal capacitor Clcb is connected to the counter electrode. The other electrode of the auxiliary capacitor Ccsb is connected to the auxiliary capacitor line 24b. The gate electrodes of the TFTs 16 a and 16 b are both connected to the scanning line (gate bus line) 12, and the source electrodes are both connected to the signal line (source bus line) 14.

  5A to 5F schematically show the timing of each voltage when driving the liquid crystal display device 200 of the present invention.

  5A shows the voltage waveform Vs of the signal line 14, FIG. 5B shows the voltage waveform Vcsa of the auxiliary capacitance wiring 24a, FIG. 5C shows the voltage waveform Vcsb of the auxiliary capacitance wiring 24b, and FIG. The voltage waveform Vg of the scanning line 12, FIG. 5E shows the voltage waveform Vlca of the subpixel electrode 18a of the subpixel 10a, and FIG. 5F shows the voltage waveform Vlcb of the subpixel electrode 18b of the subpixel 10b. . Moreover, the broken line in the figure indicates the voltage waveform COMMON (Vcom) of the counter electrode 17.

  Hereinafter, the operation of the equivalent circuit of FIG. 4 will be described with reference to FIGS.

  At time T1, the voltage of Vg changes from VgL to VgH, so that the TFT 16a and the TFT 16b are simultaneously turned on (on state), and the voltage Vs of the signal line 14 is applied to the subpixel electrodes 18a and 18b of the subpixels 10a and 10b. Then, the sub-pixels 10a and 10b are charged. Similarly, the auxiliary capacitors Csa and Csb of the respective sub-pixels are charged from the signal line.

Next, when the voltage Vg of the scanning line 12 changes from VgH to VgL at time T2, the TFTs 16a and 16b are simultaneously turned off (off state), and the subpixels 10a and 10b and the auxiliary capacitors Csa and Csb are all turned on. It is electrically insulated from the signal line 14. Immediately after this, due to the pull-in phenomenon due to the influence of the parasitic capacitances of the TFTs 16a and 16b, the voltages Vlca and Vlcb of the respective sub-pixel electrodes decrease by substantially the same voltage Vd,
Vlca = Vs−Vd
Vlcb = Vs−Vd
It becomes. At this time, the voltages Vcsa and Vcsb of the auxiliary capacitance lines 24a and 24b are Vcsa = Vcom−Vad.
Vcsb = Vcom + Vad
It is.

At time T3, the voltage Vcsa of the auxiliary capacitance line 24a connected to the auxiliary capacitance Csa changes from Vcom−Vad to Vcom + Vad, and the voltage Vcsb of the auxiliary capacitance line 24b connected to the auxiliary capacitance Csb changes from Vcom + Vad to Vcom−Vad. It changes by twice Vad. Along with this voltage change of the auxiliary capacitance lines 24a and 24b, the voltages Vlca and Vlcb of the respective subpixel electrodes are Vlca = Vs−Vd + 2 × K × Vad.
Vlcb = Vs−Vd−2 × K × Vad
To change. However, K = CCS / (CLC (V) + CCS).

At time T4, Vcsa changes from Vcom + Vad to Vcom−Vad, Vcsb changes from Vcom−Vad to Vcom + Vad by a factor of two, Vlca and Vlcb also
Vlca = Vs−Vd + 2 × K × Vad
Vlcb = Vs−Vd−2 × K × Vad
From
Vlca = Vs−Vd
Vlcb = Vs−Vd
To change.

At time T5, Vcsa changes from Vcom−Vad to Vcom + Vad, Vcsb changes from Vcom + Vad to Vcom−Vad by a factor of two, Vlca and Vlcb also
Vlca = Vs−Vd
Vlcb = Vs−Vd
From
Vlca = Vs−Vd + 2 × K × Vad
Vlcb = Vs−Vd−2 × K × Vad
To change.

Vcsa, Vcsb, Vlca, and Vlcb alternately repeat the changes in T4 and T5 at intervals of an integral multiple of the horizontal writing time 1H. Whether the repetition interval of T4 and T5 is set to 1 time, 1 time, 2 times, 3 times, or more than 1H depends on the driving method of the liquid crystal display device (polarity inversion method, etc.) It may be set as appropriate in consideration of flickering, display roughness, and the like. This repetition is continued when the pixel 10 is next rewritten, that is, until a time equivalent to T1 is reached. Accordingly, the effective values of the voltages Vlca and Vlcb of the respective subpixel electrodes 18a and 18b are as follows:
Vlca = Vs−Vd + K × Vad
Vlcb = Vs−Vd−K × Vad
It becomes.

Therefore, the effective voltages V1 and V2 applied to the liquid crystal layers 13a and 13b of the subpixels 10a and 10b are
V1 = Vlca-Vcom
V2 = Vlcb-Vcom
That is,
V1 = Vs−Vd + K × Vad−Vcom
V2 = Vs−Vd−K × Vad−Vcom
It becomes.

  Accordingly, the effective voltage difference ΔV12 (= V1−V2) applied to the liquid crystal layers 13a and 13b of the sub-pixels 10a and 10b is ΔV12 = 2 × K × Vad (where K = CCS / (CLC (V ) + CCS)), and different voltages can be applied.

  The relationship between V1 and V2 in FIGS. 3 to 5 is schematically shown in FIG.

  As can be seen from FIG. 6, in the liquid crystal display device 200, the smaller the value of V1, the larger the value of ΔV12. Note that the value of ΔV12 changes depending on V1 or V2 because the capacitance value CLC (V) of the liquid crystal capacitance has voltage dependency.

  FIG. 7 shows the γ characteristic of the liquid crystal display device 200 as “this embodiment”. FIG. 7 also shows γ characteristics when the same voltage is applied to the sub-pixels 10a and 10b for comparison. As will be described later, the liquid crystal display device 100 according to the embodiment of the present invention also has the γ characteristic shown as “this embodiment” in FIG. 7. FIG. 7 shows that the γ characteristic is improved also in the liquid crystal display device 200. Note that FIG. 7 is for more clearly expressing the difference in the γ characteristics, where the value on the horizontal axis = (front viewing angle normalized transmittance ÷ 100) ^ (1 / 2.2), the value on the vertical axis. = (Right 60 degree viewing angle normalized transmittance ÷ 100) ^ (1 / 2.2) and (Upper right 60 degree viewing angle normalized transmittance ÷ 100) ^ (1 / 2.2) It has become. “^” Represents a power, and this index corresponds to the γ value. In a typical liquid crystal display device, the γ value of the front gradation characteristic is set to 2.2.

  Although the liquid crystal display device 200 can improve the viewing angle characteristic of the γ characteristic as described above, it is necessary to provide a circuit for controlling the auxiliary capacitor counter voltage (CS voltage), which increases the cost. There is.

  The liquid crystal display device 100 according to the embodiment of the present invention shown in FIG. 1 can solve this problem.

  In the liquid crystal display device 100, the first TFT 16a and the second TFT 16b in the liquid crystal display device 200 are replaced with the first TFT 16A and the second TFT 16B shown in FIG. 2A, or the first TFT 16A ′ and the second TFT 16B ′ shown in FIG. This corresponds to the auxiliary capacitor wirings 24 a and 24 b in the liquid crystal surface device 200 replaced with the auxiliary capacitor wiring 24.

  That is, in the liquid crystal display device 200, the TFT 16a and the TFT 16b provided for the first subpixel 10a and the second subpixel 10b are equivalent to each other, but in the liquid crystal display device 100, the TFT 16a and the TFT 16b correspond to the first subpixel 10a. The provided first TFT 16A has a larger on-current than the second TFT 16B provided corresponding to the second subpixel 10b. Further, in the liquid crystal display device 200, the auxiliary capacitors 22a and 22b provided corresponding to the first subpixel 10a and the second subpixel 10b are provided electrically independent from each other with respect to the auxiliary capacitor counter voltage. In the liquid crystal display device 100, the auxiliary capacitance lines 24a and 24b are connected to a common auxiliary capacitance counter voltage with respect to the auxiliary capacitances 22a and 22b provided corresponding to the first subpixel 10a and the second subpixel 10b. Instead of the auxiliary capacity wiring 24 to be supplied. Accordingly, in the liquid crystal display device 200, the liquid crystal display device 100 controls the auxiliary capacitor counter voltage (CS voltage) of the auxiliary capacitors 22a and 22b provided corresponding to the first subpixel 10a and the second subpixel 10b. Does not have a circuit for.

  Also in the liquid crystal display device 100, the display signal voltage is supplied from the common source bus line 14 to the source electrodes of the first TFT 16A and the second TFT 16B. And the voltage of 18b are made different. That is, after the first TFT 16A and the second TFT 16B are simultaneously turned on by the scanning signal supplied from the gate bus line 12, the sub-pixel electrode 18a is charged via the first TFT 16A by the display signal voltage supplied from the source bus line 14. The subpixel electrode 18b is charged via the second TFT 16B. At this time, since the on-current of the first TFT 16A is larger than the on-current of the second TFT 16B, more charge is supplied to the subpixel electrode 18a than to the subpixel electrode 18b. Therefore, the sub-pixel 10a displays higher luminance than the sub-pixel 10b.

  Thus, unlike the liquid crystal display device 200, the liquid crystal display device 100 does not need to prepare a plurality of different auxiliary capacitor counter voltages (CS voltages), and only changes the on-current characteristics of the first TFT 16A and the second TFT 16B. Therefore, the improved γ characteristic as shown in “this embodiment” in FIG. 7 can be obtained.

  Next, a specific difference in structure between the first TFT 16A and the second TFT 16B will be described with reference to FIG.

  The first TFT 16 </ b> A has a gate electrode Ga branched from the gate bus line 12, a source electrode Sa configured by a part of the source bus line 14, and a drain electrode Da. The drain electrode Da is connected to the subpixel electrode 18a (see FIG. 1). A semiconductor layer SEa is provided between the gate electrode Ga, the source electrode Sa, and the drain electrode Da.

  Similarly, the second TFT 16B has a gate electrode Gb branched from the gate bus line 12, a source electrode Sb constituted by a part of the source bus line 14, and a drain electrode Db. The drain electrode Db is connected to the subpixel electrode 18b (see FIG. 1). A semiconductor layer SEb is provided between the gate electrode Gb, the source electrode Sb, and the drain electrode Db.

  Here, the channel width CHWa of the first TFT 16A is larger than the channel width CHWb of the second TFT 16B. As a result, the first TFT 16A has a larger on-current than the second TFT 16B.

  Next, a specific difference in structure between the first TFT 16A ′ and the second TFT 16B ′ will be described with reference to FIG.

  The basic configurations of the first TFT 16A ′ and the second TFT 16B ′ are the same as those of the first TFT 16A and the second TFT 16B described with reference to FIG. 2A. It is omitted here.

  The channel length CHLa of the first TFT 16A 'shown in FIG. 2B is shorter than the channel length CHLb of the second TFT 16B'. As a result, the first TFT 16A 'has a larger on-current than the second TFT 16B'.

  Of course, the first TFT 16A and the second TFT 16B used in the liquid crystal display device 100 of the present invention are not limited to the above example, and it is sufficient that the on-current characteristics are different from each other. A pair of TFTs having both of the features shown in FIGS. 2A and 2B may be used.

  Furthermore, the liquid crystal display device according to another embodiment of the present invention includes the first TFT 16A and the second TFT 16B shown in FIG. 2A or the first TFT 16A ′ and the second TFT 16B ′ shown in FIG. The TFTs 16a and 16b (see, for example, FIG. 4) are replaced with a configuration in which different auxiliary capacitor counter voltages are supplied to the auxiliary capacitor line (CS bus line) 24a and the auxiliary capacitor line 24b, similarly to the liquid crystal display device 200. Also good.

  When such a configuration is adopted, the advantage that the circuit for controlling the auxiliary capacitor counter voltage (CS voltage) can be omitted as in the liquid crystal display device 100 is not obtained, but the viewing angle dependency of the γ characteristic is further improved. can do.

  The present invention is used in various liquid crystal display devices. In particular, it is suitably used for a liquid crystal display device for applications requiring wide viewing angle characteristics.

It is a schematic diagram which shows an example of the pixel structure of the liquid crystal display device 100 of embodiment by this invention. (A) And (b) is a typical top view which shows the structure of TFT used for the liquid crystal display device 100. FIG. 3 is a schematic diagram illustrating an example of a pixel structure of a liquid crystal display device 200. FIG. 4 is a diagram showing an electrical equivalent circuit corresponding to the pixel structure of the liquid crystal display device 200. FIG. (A)-(f) is a figure which shows the various voltage waveforms used for the drive of the liquid crystal display device 200. FIG. FIG. 4 is a diagram illustrating a relationship between applied voltages to a liquid crystal layer between sub-pixels in the liquid crystal display device 200. It is a figure which shows (gamma) characteristic of the liquid crystal display device 200, (a) (gamma) characteristic in a right 60 degree viewing angle, (b) shows (gamma) characteristic in a right upper 60 degree viewing angle.

Explanation of symbols

10 pixels 10a, 10b subpixels 12 scanning lines (gate bus lines)
14 Signal line (source bus line)
16A, 16B, 16A ', 16B', 16a, 16b TFT
17 Counter electrode 18a, 18b Subpixel electrode 100, 200 Liquid crystal display device

Claims (4)

A plurality of pixels, each of the plurality of pixels having a first subpixel and a second subpixel;
Each of the first subpixel and the second subpixel is
A liquid crystal capacitor formed by a counter electrode, a liquid crystal layer, and a sub-pixel electrode facing the counter electrode through the liquid crystal layer;
A liquid crystal display device having an auxiliary capacitance formed by an auxiliary capacitance electrode electrically connected to the sub-pixel electrode, an insulating layer, and an auxiliary capacitance counter electrode facing the auxiliary capacitance electrode through the insulating layer Because
A first TFT provided corresponding to the first subpixel; and a second TFT provided corresponding to the second subpixel; and the gate electrode of the first TFT and the second TFT is a common gate bus line. The source electrodes of the first TFT and the second TFT are connected to a common source bus line, and the drain electrodes of the first TFT and the second TFT are respectively connected to the corresponding subpixel electrodes. And
The liquid crystal display device, wherein an on-current of the first TFT is larger than an on-current of the second TFT.   The liquid crystal display device according to claim 1, wherein a channel width of the first TFT is larger than a channel width of the second TFT.   The liquid crystal display device according to claim 1, wherein a channel length of the first TFT is shorter than a channel length of the second TFT.   4. The liquid crystal display device according to claim 1, wherein the storage capacitor counter electrode is electrically independent for each of the first subpixel and the second subpixel. 5.

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