JPH02165118A - Driving method for liquid crystal display device - Google Patents

Abstract

PURPOSE:To evade an undesirable influence caused by irradiating with light by inverting the polarity in a prescribed period against the potential of a counter electrode, and also, applying a video signal in which the maximum value of each polarity is different by its polarity to a picture element electrode through a thin film transistor. CONSTITUTION:The polarity is inverted in a prescribed period agains the potential Vc of a counter electrode, and also, a video signal in which the maximum value of each polarity is different by its polarity is applied to a picture element electrode through a thin film transistor (TFT). Accordingly, by reducing an absolute value of a negative polarity voltage of a video signal for driving an AC of a TFT of an active matrix, namely, a drain voltage Vd in the case of negative, an undesirable influence to the TFT against a light beam can be made extremely small. Also, by checking up the influence of an initial characteristic against a light beam of the TFT, a voltage value of the drain voltage Vd which takes the irradiation with light into consideration can be determined. In such a way, it is entirely unnecessary to change a material of the TFT, the structure and the process, and the light counter-measure can be realized without necessitating a conventional structural light countermeasure means.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Industrial Application Field The present invention is directed to a plurality of display pixels arranged in a matrix on an insulating substrate, and each display pixel is equipped with a thin film transistor (hereinafter referred to as TPT).
The present invention relates to a method for driving an active matrix liquid crystal display device in which a pixel electrode substrate formed by bonding a pixel electrode substrate with a counter electrode substrate and a counter electrode substrate provided with a counter electrode are opposed to each other, and a liquid crystal material is filled between the two substrates.

(B) Prior Art In recent years, active matrix liquid crystal display devices using TFT arrays have been developed and put into practical use as displays for televisions.

In order to prevent deterioration in display quality due to deterioration of liquid crystal characteristics due to voltage application, such liquid crystal display devices employ AC drive in which the polarity of the applied video signal is reversed for each field. No. 61-18755].

Historically, in such AC driving, in consideration of the fact that the level of the AC video signal decreases due to the capacitance component between the gate and source of the TPT, as shown in Figure 4, the potential of the counter electrode is lower than Vc. It has been proposed to apply a video signal Vd with a constant high voltage level to the pixel electrode [JP-A-61
-116392]. In such a driving method, as is clear from FIG. 4, the maximum value of the video signal Vd having a drain reference voltage higher than the counter electrode level has the same amplitude whether the polarity is positive or negative.

However, such conventional driving methods are based on the idea that the TPT always performs stable switching operations, and the TPT used in liquid crystal display devices for televisions is based on the idea that the TPT always performs stable switching operation. Second, the following inconveniences occur because the device is exposed to light irradiation from a backlight, external light, etc.

That is, since the active layer of TPT is a non-crystalline semiconductor film such as amorphous, photocarriers are generated by light irradiation, the OFF current increases, and the source-drain current changes from an ohmic current to a non-ohmic current. There was an inconvenience.

Conventionally, as measures against light, attempts have been made to reduce the thickness of the semiconductor of the TPT and to select a semiconductor material with a large optical band gap to construct the TPT in order to reduce the photosensitivity of the semiconductor.

Furthermore, as a structural measure against light, it is common to form a light-shielding film over the channel of the TPT.

(c) Problems to be Solved by the Invention The thinning and widening of the semiconductor film and widening of the bandgap as described above as a countermeasure against light in the TPT of an active matrix liquid crystal display device essentially sacrifices the characteristics of the TFT itself. However, it is not a fundamental solution. Also, TPT
The structure in which a light shielding film is provided is 1. This complicates the process and causes a decrease in the yield of liquid crystal display devices.

The present invention has been made in view of the above, and provides a driving method that can realize light countermeasures without requiring conventional structural light countermeasure means by providing drive conditions that satisfy predetermined conditions. It is.

(d) Means for Solving the Problems The method for driving a liquid crystal display device of the present invention is a method for driving an active matrix type liquid crystal display device equipped with a TPT, in which polarity is reversed at a predetermined period with respect to the potential of a counter electrode. At the same time, a video signal in which the maximum value of each polarity differs depending on the polarity is applied to the image Xi pole via the TPT.

(e) Effect According to the present invention, by using a video signal whose maximum polarity differs depending on the polarity, the mechanism of flow between the source and drain caused by light incident on the TPT semiconductor can be prevented from ohmic current. It is possible to prevent the current from changing to a non-ohmic current, and avoid the adverse effects of light irradiation.

(f) Example The present applicant measured the photosensitivity characteristics of TPT made of an amorphous semiconductor in order to investigate the adverse effects of light irradiation on the TPT.

Figure 2 is a graph showing the relationship between the drain current 1d and the drain voltage Vd in the OFF state of a TFT made of amorphous silicon (hereinafter referred to as a-5i) under light irradiation, using the gate voltage Vg as a parameter. 2, (a) shows a condition in which the drain voltage Vd is positive, and (b) in the same figure shows a condition in which the drain voltage Vd is negative.

The TPT used for the measurements in the same figure has a channel length of 1011 m.
, the channel width is 70 μm, and the measurement circuit is the third one.
As shown in the figure, it was assumed that a drain current 1d, a drain voltage Vd, and a gate voltage Vg could be measured while the TPT was irradiated with light B.

In addition, (G) in Fig. 3 is the gate, (D) is the drain, and (S
) is the source, and (A) is the ammeter.

As is clear from the data in both figures in Figure 2, when the drain voltage Vd is positive, Vd and Id have a linear relationship even under light irradiation, but when the drain voltage Vd is negative, the drain voltage The drain current 1d rises steeply in response to a change in Vd.

These phenomena are caused by carrier transport in amorphous semiconductors. In other words, since the a-5i used as an example is a high-resistance semiconductor made of an amorphous material, it can be made from conventional single-crystal silicon (hereinafter referred to as a-5i) by injection of carriers through the contacting metal electrode and light irradiation. This shows different carrier transport with low-resistivity materials represented by . a- shown earlier
5i T FT is generally used as an n-channel, so in order to form an ohmic relationship with the metal electrodes at the source and drain, a high concentration of P is applied between the semiconductor and the metal electrode.
An n-type semiconductor doped with an impurity such as (phosphorus) is formed. Therefore, the distance between the drain and source is n-
It has a 1-n junction structure, and when a drain voltage Vd is applied, the ohmic current changes to a non-ohmic current depending on the electric field strength between the drain and source. Conditions that cause such a change vary depending on the thickness of the i-layer and the channel length of the TPT, and further vary depending on the voltage applied to the gate.

However, as seen in the 21st ffi (b), even when the drain voltage Vd is negative, the transition to a non-ohmic current can be prevented by reducing its absolute value.

The present invention has been made based on the above-mentioned measurement results, and FIG. 1 shows a drive signal diagram employing the method for driving a liquid crystal display device of the present invention.

The TPT of the liquid crystal display device using the driving method shown in the figure is the same as the above-mentioned TPT [channel length: 10 μm, channel width near 0 μm], and according to the conventional driving method as shown in FIG. is in the range of -10 to -15V, and the drain voltage is mainly used in the range of IOV in absolute value.

If the driving method of the present invention is adopted under such conditions, the second
According to the characteristics shown in the figure, by setting the maximum value of the negative voltage of the drain voltage Vd to about -3V with respect to the maximum value of the positive voltage of the drain voltage Vd of 10V as shown in Fig. 1, the IV characteristic becomes approximately linear. Considering this, a negative voltage region of the drain voltage Vd can be used.

Moreover, the maximum positive voltage of the drain voltage Vd is 1jE10V.
Drain current 1d (OFF current) and drain voltage V
Drain current 1d (OF
The order of F current) is almost equal to A(]Q −” −I
Q-') The TPT can be driven with the same characteristics regardless of whether the polarity of the train voltage Vd, that is, the AC (frame inversion or field inversion) video signal is positive or negative.

In the above description, the case where the drain reference level and the counter electrode level are equal is exemplified based on FIG. 4, but in the present invention, it is not always necessary to make the above two levels equal.

In addition, the present invention has been developed so far that data 1-OFF! in positive and negative states of the drain voltage Vd! The explanation was given assuming that the pressures are the same, but as you can see from Figure 2, data 1 is only for negative cases.
By simultaneously increasing the absolute value of the -OFF voltage, it is possible to better bring out the effects of the present invention.

The driving method described above is effective for liquid crystal display devices such as LCD televisions where light irradiation is performed regularly, but it can be used as a light transmission type with strong light irradiation and as a reflection type with weak light irradiation. For selectable liquid crystal display devices, it is also possible to automatically change the voltage in the negative region of the drain voltage Vd depending on the usage situation. For example, when the backlight of an LCD TV is turned on, it is possible to provide a voltage conversion circuit or a voltage switching circuit that simultaneously compresses the absolute value of the negative region value of the video signal supplied as the drain voltage Vd. can.

(G) Effects of the Invention According to the method for driving a liquid crystal display device of the present invention, the negative polarity voltage of the video signal for AC driving of the active matrix TPT, that is, the absolute value of the drain voltage in the negative case, is reduced. By simply using this method, the adverse effect of light on the TPT can be extremely minimized. Furthermore, in the present invention, by examining the influence of the initial characteristics of the TPT on light, it is possible to determine the voltage value of the drain voltage in consideration of light irradiation, so there is no risk of making the initial design of the TPT equally complicated.
There is no LL. Furthermore, according to the present invention, there is no need to change the material, structure, or process of TPT at all.

Therefore, if the present invention is implemented in a prototype projection using a liquid crystal display device in which intense light is thought to be irradiated onto the TPT, the effect will be significant.

[Brief explanation of the drawing]

FIG. 1 is a drive signal waveform diagram illustrating the driving method of the liquid crystal display device of the present invention, and FIGS. 2(a) and (b) are TPT
FIG. 3 is a measurement circuit diagram, and FIG. 4 is a drive signal waveform diagram showing a conventional drive method. G...gate, D...drain, S...source,
A...it current meter, B...light, Vd...drain voltage, Id...drain iWt, Vg...gate voltage. Figure 1 Figure 3 Figure 4

Claims (1)

[Claims] (1) A pixel electrode substrate formed by arranging a plurality of display pixels in a matrix on an insulating substrate and coupling a thin film transistor to each display pixel is opposed to a counter electrode substrate equipped with a counter electrode, and a liquid crystal display is placed between the two substrates. In a method for driving an active matrix type liquid crystal display device filled with a substance, a video signal whose polarity is reversed at a predetermined period with respect to the potential of the counter electrode and whose maximum value of each polarity is made different depending on the polarity is transmitted to the thin film transistor. A driving method characterized in that the voltage is applied to the pixel electrode via the pixel electrode.