JPH0126077B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a matrix type liquid crystal display device in which a switch array is formed using thin film transistors (hereinafter abbreviated as TFT).

FIG. 1a shows the configuration of a matrix type liquid crystal display device, in which 101 to 103 are gate lines, 104
107 to 107 are data lines, 108 to 110, etc. are pixels. The pixel is connected to the gate line 11 as shown in FIG. 1b.
1 and a data line 112, a floating capacitance 114, and a liquid crystal cell 115. 116 and 117 are a drive electrode and a common electrode of the liquid crystal cell, respectively.
A voltage corresponding to a data signal such as V _D in FIG. 1c is applied to the data line 112 in FIG.
A gate line driving voltage such as V _G in FIG. 1c is applied to the gate line 111 in FIG. 1b. If the TFT 113 is an N-type TFT, the signal on the data line 112 is written to the liquid crystal cell 115 while V _G is high, and the signal written to 115 is held while V _G is low. In Figure 1c, A is V _D
B represents a negative frame where V D >0, and B represents a positive frame where V _D >0.

Conventionally, MOS transistors formed in a single-crystal silicon substrate have been used as switches in matrix-type liquid crystal display devices. single crystal silicon
Voltage-current characteristics of MOS transistors (gate and
Source voltage V _GS - Drain-source current I _DS characteristics)
As shown at 201 in FIG. 2, the ON/OFF ratio is large, the current change in the weak inversion region is abrupt, and the leakage current in the inversion 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 was determined so that the TFT operates in the region 202 in FIG. 2 when non-conducting.

However, when attempting to drive a matrix type liquid crystal display device using TFT switches in the same manner as a matrix type liquid crystal display device using single crystal silicon MOS transistors as switches, the following problems occur. By silicon thin film
Figure 4 shows an example of the voltage-current characteristics of a TFT.

I _DS is the drain-source current and V _GS is the gate-source voltage. Reference numerals 401 and 402 indicate voltage-current characteristics where the drain-source voltages V _DS are V _DS =V ₁ and V _DS =V ₂ , respectively. However, V ₂ > V _1p TFT
The characteristics of the voltage-current characteristics are that the current change in the weak inversion region 403 is slower than that of a single-crystal silicon MOS transistor, that the leakage current level in the shearing region is large, and that
4, an increase in _IDS due to PN junction leakage current is observed. When the liquid crystal cell is driven with the bias relationship as shown in FIG. 3, the gate-source voltage _VGS of the TFT changes within the range 405 in FIG. 4. At this time, the PN flowing through the TFT when non-conducting
The junction current becomes considerably large, which appears as display unevenness on the screen and greatly impairs display performance.

The present invention provides an appropriate bias voltage between the zero level of the gate line drive signal V _G and the zero level of the data line drive signal V _D in accordance with the voltage-current characteristics of the TFT.
By providing V _B , the present invention provides a method for driving a matrix type liquid crystal display device that solves the above-mentioned drawbacks and has good display performance.

The driving method of the present invention is achieved by providing a constant bias voltage V _B in accordance with the voltage-current characteristics of the TFT between the zero level of the gate line drive signal and the zero level of the data line drive signal, as shown in FIG. , the TFT has minimal leakage current within its operating range when non-conducting.

In FIG. 5, 501 is a gate line, 502 is a data line, 503 is a switching TFT, and 50
4 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. 601 and 602 are respectively shown in Fig. 4 40
This is the same voltage-current characteristic curve as No. 1,402. Has voltage-current characteristics of 601 and 602
When a switch of a matrix type liquid crystal display device is formed using a TFT, the operating range in which the leakage current of the TFT when non-conducting is minimal is the range shown in FIG. 6 603. In this way, in order to set the operating range of the TFT when non-conducting, that is, the variation range of the gate-source voltage V _GS of the TFT during non-conducting, to a region where the leakage current is minimal, in the present invention, 7th
As shown in the figure, the zero level of the gate line drive signal 702
V _G = 0 and the zero level V _D of the data line drive signal 701
A bias voltage of V _B is provided between 0 and 0.

According to the above driving method, compared to the driving method shown in FIG. 3, the average value of the leakage current of the TFT during non-conduction is reduced by 50% to 90%. It is desirable that the circuit configuration for realizing the driving method of the present invention allows the value of V _B to be adjusted externally, as shown in the example of FIG. By making V _B adjustable externally, variations in TFT characteristics between manufacturing lots can be easily dealt with. Another effect of arbitrarily setting the value of the bias voltage _VB externally is that it reduces power consumption by reducing the charging and discharging current to the gate of the switching TFT and the parasitic capacitance added to the gate line. Is Rukoto. Most of the TFT parasitic capacitance and 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, achieving lower power consumption.

An electro-optical conversion substance is sealed in a pair of substrates, and one of the substrates has pixel electrodes arranged in a matrix, thin film transistors connected to the pixel electrodes, and data on the source electrodes of the thin film transistors. In a display device having a data line for supplying a signal and a gate line for supplying a gate signal to the gate electrode of the thin film transistor,
The thin film transistor is formed of a non-single-crystal silicon thin film, and the voltage level of the gate signal when not selected is such that the gate-source voltage range (V _GS ) of the thin film transistor corresponds to the minimum value of the source-drain current. Since the bias voltage value is set to be within the gate-source voltage range including the source voltage, it is possible to limit the leakage current peculiar to the thin film transistor when it is off to a minimum. Therefore, since crosstalk due to leakage can be prevented, the image signal accumulated in the pixel electrode can be maintained reliably, and a clear image corresponding to this signal can be obtained.

[Brief explanation of drawings]

FIG. 1a is a diagram showing the configuration of a matrix type liquid crystal display device. FIG. 1b is a diagram for explaining the configuration of the pixel. FIG. 1c is a diagram showing drive signals. FIG. 2 is a diagram showing the voltage-current characteristics of single-crystal silicon MOS transistors used in conventional matrix-type liquid crystal display devices. FIG. 3 is a diagram showing a conventional example of a method for driving gate lines and data lines. FIG. 4 is a diagram for explaining driving of a switching TFT using a conventional driving method. Figures 5, 6, and 7 are
FIG. 1 is a diagram for explaining a first embodiment of the present invention.

Claims (1)

[Claims] 1. An electro-optical conversion substance is sealed in a pair of substrates, and on one of the substrates are pixel electrodes arranged in a matrix, thin film transistors connected to the pixel electrodes, and source electrodes of the thin film transistors. In a display device having a data line for supplying a data signal and a gate line for supplying a gate signal to the gate electrode of the thin film transistor,
The thin film transistor is formed of a non-single-crystal silicon thin film, and the voltage level of the gate signal when not selected is such that the gate-source voltage range (V _GS ) of the thin film transistor corresponds to the minimum value of the source-drain current. A display device characterized in that a bias voltage value is set to be within a gate-source voltage range that includes a source voltage.