JPH07120146B2 - Liquid crystal display - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a thin film transistor ( hereinafter
Below, abbreviated as TFT) was used to form a switch array.
The present invention relates to a liquid crystal display device .

FIG. 1A shows a structure of a matrix type liquid crystal display device, 101 to 103 are gate lines, and 104 is a gate line.
˜107 are data lines, and 108˜110 and the like are pixels.
A pixel is a switching TFT 1 formed at an intersection of a gate line 111 and a data line 112, as shown in FIG.
13, a stray capacitance 114 and a liquid crystal cell 115. Reference numerals 116 and 117 are a drive electrode and a common electrode of the liquid crystal cell, respectively.

The data line 112 in FIG. 1B is shown in FIG.
A voltage corresponding to the data signal such as V _D of
A gate line driving voltage such as V _G 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 is supplied during the period when V _G is high.
Twelve signals are written to the liquid crystal cell 115, and the signal written to 115 is held during the period when V _G is low.
In FIG. 1C, A is a negative frame with V _D <0,
B represents a positive frame with V _D > 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. Single crystal silicon MOS transistors have been used. Voltage-current characteristics of single crystal silicon MOS transistor (gate-source voltage V _GS-
The drain / source current I _DS characteristic) has a large ON / OFF ratio as shown by 201 in FIG. 2, the current change is abrupt in the weak inversion region, and the leakage current in the cutoff region is small. Therefore, as shown in FIG. 3, the bias relationship between the gate line drive signal 302 and the data line drive signal 301 is set so that the TFT in the non-conduction state operates in the region 202 in FIG.

However, when a matrix type liquid crystal display device using TFTs as switches is driven by a method similar to that of a matrix type liquid crystal display device using single crystal silicon MOS transistors as switches, the following problems occur. Occurs. FIG. 4 shows an example of the voltage-current characteristics of a TFT made of a silicon thin film.

I _DS is drain / source current, and V _GS is
It is the gate-source voltage. Reference numerals 401 and 402 denote voltage-current characteristics in which the drain-source voltage V _DS is V _DS = V ₁ and V _DS = V ₂ , respectively. However, V ₂ > V ₁ . TFT
The characteristics of the voltage-current characteristics of are that the current change in the weak inversion region 403 is slower than that of the single crystal silicon MOS transistor, the leak current level in the cutoff region is large, and the PN junction leak current in the cutoff region 404. The increase of I _DS due to

When the liquid crystal cell is driven in the bias relationship as shown in FIG. 3, the gate-source voltage V _{GS of} the TFT changes within the range of 405 in FIG. At this time, T when not conducting
The PN junction current flowing through the FT becomes considerably large, and this appears as display unevenness on the screen and greatly impairs the display performance.

[0008]

In a liquid crystal display device of the present invention, liquid crystal is sealed between a pair of substrates, a common electrode is formed on one of the substrates, and a pixel electrode is formed on the other substrate. A thin film transistor connected to the pixel electrode, a data line that supplies a data signal to the pixel electrode through the thin film transistor, and a scanning line that supplies a gate signal to the thin film transistor. In the liquid crystal display device, the data signal applied to the pixel electrode is
The phase is inverted every vertical scanning period, and the electric charge charged in the parasitic capacitance of the thin film transistor during the selection period of the gate signal moves to the pixel electrode side during the non-selection period of the gate signal. It is characterized in that a potential difference for canceling the potential of the pixel electrode which changes due to is formed in advance between the center potential of the data signal and the potential of the common electrode.

In FIG . 5, a constant bias voltage VB matching the voltage-current characteristics of the TFT is provided between the zero level of the gate line driving signal and the zero level of the data line driving signal, so that the TFT in the non-conducting state is It has a minimum leak current within the operating range.

In FIG. 5, reference numeral 501 denotes a gate line, and 502
Is a data line, 503 is a switching TFT, 504 is a liquid crystal display cell, 505 is a data line drive signal source, and 506 is a gate line drive signal source.

6 and 7 show an embodiment of the present invention. Reference numerals 601 and 602 denote 401 and 40 in FIG. 4, respectively.
2 is a curve of the same voltage-current characteristics as No. 2.

It has voltage-current characteristics 601 and 602.
The switch of the matrix type liquid crystal display device is formed by the TFT
In case of non-conduction, the leakage current of TFT at non-conduction becomes minimum.
Such an operation range is the range indicated by 603 in FIG. This
Operating range of the TFT when not conducting, that is, TF when not conducting
Gate-source voltage V of T_GSThe change range of
In order to set a region where the flow becomes a minimum, the present invention
Is the zero level V of the gate line drive signal 702 as shown in FIG.
_G = 0 and zero level V of the data line drive signal 701 _D = 0
Between V_B Bias voltage of

According to the above driving method, the average value of the leak current of the TFT at the time of non-conduction is reduced by 50% to 90% as compared with the driving method as shown in FIG. The circuit configuration for realizing the driving method of the present invention is V _B as shown in the example of FIG.
It is desirable that the value of can be adjusted externally. By allowing V _B to be adjusted from the outside, it is possible to easily deal with variations in TFT characteristics between manufacturing lots. Bias voltage V _B
Another effect of externally setting the value of is that the charging / discharging current to the parasitic capacitance added to the gate and the gate line of the switching TFT is reduced to achieve low power consumption. . Most of the TFT parasitic capacitance and the gate line parasitic capacitance are attached between the gate line and the data line or between the gate line and the liquid crystal drive electrode. Therefore, by setting V _B to a negative value, the current for charging and discharging the parasitic capacitance is reduced and low power consumption is achieved.

8 and 9 show another embodiment of the present invention.

In FIG. 8, reference numeral 801 denotes a gate line, and 802.
Is a data line, 803 is a TFT, 804 is a liquid crystal display cell,
Reference numeral 805 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 C _S is a parasitic capacitance of the TFT. In FIG. 9, 901 is a data line drive signal waveform and 902 is a gate line drive signal waveform.

FIG. 10 shows how the liquid crystal display cell is driven by the gate line drive signal V _G and the data line drive signal V _D , and V _LC shows how the voltage applied to the liquid crystal display cell changes. Due to the influence of the parasitic capacitance C _S of the TFT shown in FIG. 8, the gate line drive signal V _G falls (switches from the selection period of the gate signal to the non-selection period) at the same time as V.
_LC is

[0017]

[Equation 1]

Only changes to the negative side. Therefore, in the negative frame A, the effective value of V _LC increases immediately after the data is written, and in the positive frame B, the effective value of V _LC changes immediately after the data is written. To do. Therefore, the waveform of V _LC contains a constant DC component as shown in FIG. 10, which causes a problem of significantly shortening the life of the liquid crystal.

According to the present invention, a constant bias voltage V _{B2 is} applied between the zero level of the data line drive signal and the common electrode of the liquid crystal cell.
By providing the above, the DC component contained in V _LC is removed to compensate the vertical asymmetry of the V _LC waveform and prevent deterioration of the liquid crystal due to DC drive. As shown in FIG. 8, the bias voltage V _B1 of the gate line drive signal source and the bias potential V _B2 of the data line drive signal source can be set independently from the outside, thereby reducing the leak current of the TFT at the time of non-conduction. It is possible to simultaneously realize the high reliability of the display device by removing the display unevenness and the DC voltage applied to the liquid crystal.

[0020]

As described above, according to the structure of the present invention, it is possible to compensate the asymmetry of the driving voltage generated by the liquid crystal capacitance and the parasitic capacitance of the thin film transistor when the liquid crystal is AC driven. Then, a liquid crystal display device having high reliability and high display quality can be achieved without degrading the liquid crystal by applying a DC voltage to the liquid crystal or causing display unevenness.

[Brief description of drawings]

1A is a diagram showing a configuration of a matrix type liquid crystal display device, FIG. 1B is a diagram for explaining a configuration of a pixel thereof,
(C) is a figure which shows 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 a gate line and a data line.

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

FIG. 5 is a diagram for explaining the first embodiment of the present invention.

FIG. 6 is a diagram for explaining the first embodiment of the present invention.

FIG. 7 is a diagram for explaining the first embodiment of the present invention.

FIG. 8 is a diagram for explaining a second embodiment of the present invention.

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

FIG. 10 is a diagram for explaining the second embodiment of the present invention.

Claims (1)

[Claims] 1. A liquid crystal is sealed between a pair of substrates, a common electrode is formed on one of the substrates, a pixel electrode is formed on the other substrate, and a thin film transistor connected to the pixel electrode. Applied to the pixel electrode in a liquid crystal display device having a data line for supplying a data signal to the pixel electrode through the thin film transistor and a scanning line for supplying a gate signal to the thin film transistor The phase of the data signal is inverted every vertical scanning period, and the electric charge charged in the parasitic capacitance of the thin film transistor during the selection period of the gate signal is changed to the pixel electrode during the non-selection period of the gate signal. A liquid crystal display characterized in that a potential difference for canceling the potential of the pixel electrode which changes by moving to the side is formed in advance between the center potential of the data signal and the potential of the common electrode. Indicating device.