JP2626651B2 - Liquid crystal device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal device in which a switch array is formed using thin film transistors (hereinafter, abbreviated as TFTs).

[0002]

2. Description of the Related Art FIG. 1A shows a configuration of a conventional matrix type liquid crystal display device, wherein 101 to 103 are gate lines, 104 to 107 are data lines, and 108 to 110 are pixels. The pixel has a gate line 11 as shown in FIG.
It comprises a switching TFT 113, a floating capacitor 114, and a liquid crystal cell 115 formed at the intersection of 1 and the data line 112. Reference numerals 116 and 117 denote a drive electrode and a common electrode of the liquid crystal cell, respectively.

The data line 112 shown in FIG. 1B is connected to the data line 112 shown in FIG.
A voltage corresponding to a data signal such as VD is applied,
A gate line driving voltage such as VG in FIG. 1C is applied to the gate line 111 in FIG. When the TFT 113 is an N-type TFT, the data line 1 during the period when VG is high is high.
Twelve signals are written to the liquid crystal cell 115, and the signal written to 115 is held while VG is low.

In FIG. 1C, A represents a negative frame with VD <0, and B represents a positive frame with VD> 0.

Conventionally, a MOS transistor formed in a single crystal silicon substrate has been used as a switch of a matrix type liquid crystal display device. The voltage-current characteristic (gate-source voltage VGS-drain-source current IDS characteristic) of the single-crystal silicon MOS transistor has a large ON / OFF ratio as shown by 201 in FIG. 2 and the current change in the weak inversion region is sharp. The leakage current in the cut-off region is small. Therefore, as shown in FIG. 3, the gate line drive signal 302 and the data line drive signal 301 are changed so that the TFT in the non-conductive state operates in the region 202 in FIG. A bias relationship was defined.

[0006]

However, when trying to drive a matrix type liquid crystal display device using TFTs as switches in the same manner as a matrix type liquid crystal display device using single crystal silicon MOS transistors as switches, The problem described above arises. TFT with silicon thin film
FIG. 4 shows an example of the voltage-current characteristics of FIG.

IDS is a drain-source current, and VGS is a gate-source voltage. Reference numerals 401 and 402 denote voltage-current characteristics when the drain-source voltage VDS is VDS = V1 and VDS = V2, respectively. However, V2> V1. TFT
Are characterized by the fact that the current change is slower in the weak inversion region 403 than in the single-crystal silicon MOS transistor, the leakage current level in the cutoff region is large, and the PN junction leakage current in the cutoff region 404. There is an increase in IDS.

When the liquid crystal cell is driven in a bias relationship as shown in FIG. 3, the gate-source voltage VGS of the TFT changes in the range of FIGS. At this time, T
The PN junction current flowing through the FT becomes considerably large, which appears as display unevenness on the screen and greatly degrades the display performance.

[0009]

A liquid crystal device according to the present invention comprises a plurality of gate lines and a plurality of data lines crossing each other, a plurality of thin film transistors respectively connected to the gate lines and the data lines; A plurality of pixel electrodes connected to the thin film transistor; and a liquid crystal interposed between the pixel electrode and the common electrode. A scanning signal is supplied to the gate electrode of the thin film transistor from the gate line. ,
A data signal is supplied to the pixel electrode from the data line through the thin film transistor, and a polarity of a voltage applied to the pixel electrode by the data signal supplied through the thin film transistor with respect to a potential of the common electrode is provided. Is 1
In a liquid crystal device which is inverted every vertical scanning period, the change range of the gate-source voltage during the non-selection period of the scanning signal is set to a region where the leak current is minimized, and The potential difference that compensates for the potential of the pixel electrode, which changes as the charge charged in the parasitic capacitance of the thin film transistor moves to the pixel electrode side during the non-selection period of the scanning signal during the selection period, is the center of the data signal. It is characterized by being set between a potential and the common electrode potential.

[0010]

[Function] By applying a constant bias voltage between the center voltage of the data line drive signal and the common electrode, the DC component of the voltage VLC applied to the liquid crystal is removed, and the asymmetry of the VLC waveform is compensated. However, deterioration of the liquid crystal due to DC driving can be prevented. Also, the voltage level when the scanning signal is not selected
Is the voltage value as much as possible within the specified gate-source voltage range.
Because it is set, the thin-film transistor-specific off-state
Leakage current can be limited to a minimum.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a gate line drive signal VG.
Between the zero level of the data line and the zero level of the data line drive signal VD.
Appropriate bias voltage V according to the voltage-current characteristics of FT
By providing B, a method of driving a matrix-type liquid crystal display device which solves the above-mentioned drawbacks and has good display performance is provided, and will be described in detail below.

A reference example of the driving method according to the present invention will be described with reference to FIG. As shown in FIG. 5, between the zero level of the gate line driving signal and the zero level of the data line driving signal, the voltage of the TFT-
By providing a constant bias voltage VB in accordance with the current characteristics, the non-conducting TFT has a minimal leakage current within its operating range.

In FIG. 5, reference numeral 501 denotes a gate line;
Is a data line, 503 is a switching TFT, 504 is a liquid crystal cell, 505 is a data line drive signal source, and 506 is a gate line drive signal (scanning signal) source.

FIGS. 6 and 7 show a reference example of the present invention. 601 and 602 correspond to FIGS.
2 is a curve of the same voltage-current characteristic as FIG. 601, 60
When a switch of a matrix type liquid crystal display device is formed by TFTs having a voltage-current characteristic of 2
The operating range in which the leakage current of FIG.
The range shown in FIG. In this manner, in order to set the operation range of the TFT in the non-conducting state, that is, the change range of the gate-source voltage VGS of the TFT in the non-conducting state, in a region where the leak current is minimized, As shown in FIG. 7, a bias voltage of VB is provided between the zero level VG = 0 of the gate line drive signal 702 and the zero level VD = 0 of the data line drive signal 701.

According to the above-described driving method, the average value of the leak current of the TFT when the TFT is not conductive is reduced by 50% to 90% as compared with the driving method as shown in FIG. It is desirable that the circuit configuration for realizing the driving method of the present invention can externally adjust the value of VB as shown in the example of FIG. By enabling VB to be adjusted externally, it is possible to easily cope with variations in TFT characteristics between manufacturing lots. Another effect of setting the bias voltage VB arbitrarily from the outside is that low power consumption is achieved by reducing the charge / discharge current to the parasitic capacitance added to the gate and gate line of the switching TFT. It is. Most of the TFT strange capacitance and the parasitic capacitance of the gate line are provided between the gate lines or between the gate line and the liquid crystal driving current. Therefore, by setting VB to a negative value, the current for charging and discharging the parasitic capacitance is reduced, and low power consumption is achieved.

FIGS. 8 and 9 show an embodiment of the present invention.

In FIG. 8, reference numeral 801 denotes a gate line;
Is a data line, 803 is a TFT, 804 is a liquid crystal cell, 80
5 is a data line drive signal source, 806 is a gate line drive signal source, 807 is a first bias power supply, 808 is a second bias power supply, and CS is a parasitic capacitance of the TFT. 9, reference numeral 901 denotes a data line driving signal waveform, and 902 denotes a gate line driving signal waveform.

FIG. 10 shows that the liquid crystal cell has a gate line driving signal V
G and the data line drive signal VD indicate a state of being driven, and VDC indicates a state of a change in the voltage applied to the liquid crystal cell. Due to the influence of the parasitic capacitance CS of the TFT shown in FIG.
At the same time as the fall of the gate line drive signal VG, VLC becomes

[0019]

(Equation 1)

Only changes to the negative side. Therefore, in the negative frame A, the effective value of VLC changes so as to increase immediately after the data is written, and in the positive frame B, the effective value of VLC changes so as to decrease immediately after the data is written. Therefore, the waveform of VLC includes a constant DC component as shown in FIG. 10, which causes a problem that the life of the liquid crystal is significantly shortened.

In the embodiment of the present invention, a constant bias voltage VB2 is also provided between the zero level of the data line drive signal and the common electrode of the liquid crystal cell, thereby removing the DC component contained in the VLC. In this way, the vertical asymmetry of the VLC waveform is compensated to prevent the deterioration of the liquid crystal due to the DC drive. As shown in FIG. 8, the bias voltage VB1 of the gate line drive signal source and the bias potential VB2 of the data line drive signal source can be independently set from the outside, thereby reducing the leak current of the TFT when the TFT is off. High reliability of the display device can be simultaneously realized by removing the unevenness and removing the DC voltage applied to the liquid crystal.

[0022]

As described above, the liquid crystal display device of the present invention has the following features.
The potential difference between the center potential of the data signal and the potential of the common electrode is different.
VLC becomes substantially positive / negative symmetric during the vertical scanning period
Because it is characterized by being pierced in the direction,
DC component contained in the VLC can be removed.
To prevent the liquid crystal from deteriorating due to movement,
The effect that the display performance can be greatly improved
There is. The voltage level when the scanning signal is not selected is
The gate-source voltage range of the thin film transistor is
Since the voltage value is set,
The off-state leakage current can be limited to a minimum. Obedience
Can prevent crosstalk due to leaks
You.

[Brief description of the drawings]

FIG. 1A illustrates a configuration of a matrix liquid crystal display device. FIG. 2B is a diagram illustrating the configuration of the pixel.
(C) is a diagram showing a drive signal.

FIG. 2 is a diagram showing voltage-current characteristics of a single crystal silicon MOS transistor used in a conventional matrix type liquid crystal display device.

FIG. 3 is a diagram showing a conventional example of a method for driving gate lines and data lines.

FIG. 4 is a diagram illustrating driving of a switching TFT by a conventional driving method.

FIG. 5 is a diagram for explaining a reference example of the present invention.

FIG. 6 is a diagram illustrating a reference example of the present invention.

FIG. 7 is a diagram illustrating a reference example of the present invention.

FIG. 8 is a diagram illustrating an embodiment of the present invention.

FIG. 9 is a diagram for explaining an embodiment of the present invention.

FIG. 10 is a diagram illustrating an embodiment of the present invention.

[Explanation of symbols]

101, 102, 103, 111, 404, 501, 8
01 gate line 104, 105, 106, 107, 112, 502, 8
02 Data lines 108, 109, 110 Pixels 113, 503, 803 TFT 114 Stray capacitance 115, 504, 804 Liquid crystal cell 116 Drive electrode of liquid crystal cell 117 Common electrode 401 Drain-source voltage is weak when VDS is VDS = V1 Inversion region 402 Drain-source voltage VDS is VDS = V2 Voltage-current characteristics Weak inversion region 403 Blocking region 505, 805 Data line drive signal source 506, 806 Gate line drive signal source 807 First bias power supply 808 Second Bias power supply CS Parasitic capacitance of TFT 901 Data line drive signal waveform 902 Gate line drive signal waveform

Claims (1)

(57) [Claims] A plurality of gate lines and a plurality of data lines crossing each other; a plurality of thin film transistors connected to the gate lines and the data lines; and a plurality of pixel electrodes connected to the plurality of thin film transistors. Having a liquid crystal interposed between the pixel electrode and the common electrode, a scanning signal is supplied to the gate electrode of the thin film transistor from the gate line, and the pixel electrode is provided through the thin film transistor through the thin film transistor. A data signal is supplied from the data line, and the polarity of the voltage applied to the pixel electrode by the data signal supplied through the thin film transistor with respect to the potential of the common electrode is inverted every vertical scanning period. In the liquid crystal device, the change range of the gate-source voltage during the non-selection period of the scanning signal is set to a region where the leak current is minimized. And compensating for the potential of the pixel electrode, which changes when the charge charged in the parasitic capacitance of the thin film transistor moves to the pixel electrode side during the non-selection period of the scanning signal during the scanning signal selection period. A liquid crystal device, wherein a potential difference is set between the central potential of the data signal and the common electrode potential.