JP2578941B2 - Driving method of active matrix liquid crystal display device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a driving method of an active matrix liquid crystal display device using a nonlinear resistance element.

[Related Art] In recent years, applications of liquid crystal display devices (LCDs) centered on twist nematic type (TN type) have been developed and are used in large quantities in the fields of watches and calculators. In addition, in recent years, a matrix type capable of arbitrary display of characters, figures, and the like has begun to be used. In order to expand the application field of the matrix type LCD, it is necessary to increase the display capacity. However, since the rise of the voltage-transmittance change characteristic of the conventional LCD is not so steep, if the number of scans of the multiplex drive is increased in order to increase the display capacity, it affects each of the selected pixel and the non-selected pixel. Since the effective voltage ratio decreases, crosstalk occurs in which the transmittance of the selected pixel increases and the transmittance of the non-selected pixel decreases. As a result, the display contrast is significantly reduced, and the viewing angle at which a certain level of contrast is obtained is also significantly narrowed. Therefore, in the conventional LCD, the number of scanning lines is 6
About 0 is the limit. Such a conventional LCD is referred to as a simple matrix LCD in comparison with an active matrix LCD described below.

In order to greatly increase the display capacity of a matrix type LCD, an active matrix LCD in which switching elements are arranged in series with each pixel of the LCD has been devised. As switching elements of active matrix LCD prototypes that have been announced so far, thin film transistors (TFTs) using amorphous silicon or polysilicon as a semiconductor material are often used. On the other hand, the manufacturing method and structure are relatively simple, so that the manufacturing process can be simplified, and high yield and low cost are expected in thin-film two-terminal devices (TFDs).
(Abbreviated as "abbreviated") has also attracted attention.

Such a thin film two-terminal element type active matrix LC
In D (hereinafter abbreviated as TFD-LCD), an LCD considered to be closest to practical use is an LCD using a metal-insulator-metal element (hereinafter abbreviated as MIM element or MIM) for TFD. TFD
As, in addition to the MIM, amorphous S _i diode ring which is connected in parallel with the polarity opposite pin two diodes, and, a back-to-back diodes likewise connected in series with two pin diode polarity opposite Are known.

The above TFDs are all non-linear resistance elements in terms of circuit, and the current rapidly increases non-linearly with an increase in the voltage applied to both ends of the element. By connecting such a TFD in series with the liquid crystal, the rise of the voltage-transmittance change characteristic becomes steep, and the number of scanning lines can be greatly increased.

A conventional example of an LCD using such a MIM is described in a paper by Di Albaraf and others, "The Optimization of Metal Insulator Metal Nonlinear Devices for Use in Multiplexed." Liquid Crystal Displays ", IEE Transactions on Electron Devices, Vol. 28, No. 6, pp. 736-739 (1981)
DRBaraff, et al., “The Optimization of
Metal-Insulator-Metal Nonlinear Devices for Use
in Multiplexed Liquid Crystal Displays, "IEEE Tran
s.Electron Devices, vol.ED-28, pp.736-739 (198
1)], and Shinji Ryukaku, et al., “Lateral MIM-LCD with 250 × 240 Pixels”, Technical Report of the Institute of Television Engineers of Japan (IPD83-8),
pp. 39-44 (published in December 1983) and disclosed in Japanese Patent Laid-Open Nos. 52-149090 and 55-149090.
It is typically shown in Japanese Patent No. 161273, and its operating principle is also described in detail.

In MIM, the most important material is the material of the insulator layer. In MIMs described in conventional papers and patent publications,
Tantalum (Ta) or silicon oxide or nitride is mainly used as a material of the insulator layer, but is not limited thereto. Almost any metal can be used, but mainly chromium or tantalum is used.

The current-voltage (IV) characteristics of the non-linear resistance element are expressed by various formulas, and the following formulas are known as typical ones.

I = A · V ^α where, I is current, V is voltage, alpha nonlinear coefficient, A is a proportionality constant. In the already described MIM element etc., α
Has a value of 6 or more.

In the TFD-LCD described above, a driving method that is substantially the same as the driving method used in the TN type simple matrix LCD is conventionally used. The conventional driving method is described in detail in Electronics, April 1988, pp. 42-46.

FIG. 2 shows a drive signal when the pixel of interest is the selected pixel and the selected pixel and the non-selected pixel are alternately present on the data signal line. The scanning signal (a) and the data signal (b) take values as shown in Table 1 in each of the negative and positive frames. One pixel is scanned in each of the negative and positive frames, and display contents are written. The address period is a writing period to each pixel, and the non-addressing period is a charge holding period of each pixel. V _D and
The ratio of V _P (V _D / V _P ) is usually constant and is called the bias ratio.

As shown in FIG. 3, the equivalent circuit of one pixel of the TFD-LCD panel has a non-linear resistance element 13 and a liquid crystal element 14 connected in series, and a data signal line 15 and a scanning signal line 16 connected to both ends. There is no problem if this is reversed.

The pixel application signal (c) is (data signal)-(scanning signal), and takes a value shown in Table 2. Correspondingly, the liquid crystal voltage (d) changes, resulting in display contrast. Ideally, the IV characteristics of the nonlinear element should be symmetric with respect to the positive or negative of the voltage, but the actual MIM element has a large asymmetry. That is, the value of A when V> 0 even though α is the same in equation (1)
The value of A at the time of A ⁺ and V <0 A ^- and Kotonariru is. A ^-> A ⁺ when,
The voltage applied to the liquid crystal has a larger absolute value in the negative frame. Since the contrast of the liquid crystal is determined by the effective value of the liquid crystal voltage (d), in such a case, flickering of the screen, that is, flicker, becomes more noticeable. In Table 2, the values at the time of non-addressing are all marked with [] because the content of the data signal is either selected or not selected.
This means that the pixel applied voltage takes a value in [].

[Problems to be solved by the invention]

As described above, the conventional TFD-LCD has a disadvantage that flicker easily occurs.

As a method for solving this, a method of shifting a voltage applied to a pixel in accordance with the asymmetric characteristic of a nonlinear element is described in JP-A-58-14891 and JP-A-61-256325. . However, in this method, since the bias voltage needs to be changed simultaneously with the adjustment of the pixel applied voltage for the contrast adjustment, the driving circuit becomes complicated,
Contrast adjustment is very difficult.

An object of the present invention is to provide a driving method of a TFD-LCD that does not cause flicker and a driving method that can easily adjust contrast.

[Means for solving the problem]

According to the present invention, in a method of driving an active matrix liquid crystal display device using a nonlinear resistance element, biasing is kept constant, the polarity of a pixel applied voltage is inverted for each frame, and the ratio of the absolute value is displayed. A driving method of an active matrix liquid crystal display device characterized in that the pixel applied voltage is set to the optimum ratio value.

[Action]

FIG. 1 shows a driving method according to the present invention. Basically,
2 is the same as the conventional example of FIG. 2, except that the absolute value of the pixel applied voltage (c), which is the difference between the scanning signal (a) and the data signal (b), is changed between the negative frame and the positive frame. It is.
That is, the value of V _P is 'becomes the value of V _D is V _D and V _D' V _P and V _P positive and negative frames becomes. A ^-> Assuming the A ^+,
If V _P > V _P 'and V _D > V _D ', the absolute value of the liquid crystal voltage (d) between the positive and negative frames can be made equal. These values are summarized in Table 3.

Keep the bias ratio constant between positive and negative frames (V _D / V _P =
V _D ′ / V _P ′).

It is the ratio of the absolute value of the pixel applied voltage between the positive and negative frames.
_{V P / V P ', (} V P -2V D) / (V P' by each adjusting -2 V _D '), the ratio of the flicker is eliminated is found. This ratio is called a display optimum ratio value. When the bias ratio is constant, _VP / _VP 'may be set to the display optimum ratio value.

The pixel applied voltage (c) is (data signal)-(scanning signal) and is summarized in Table 4.

[Example] Hereinafter, an example will be described.

Example 1 The structure and manufacturing method of a typical MIM-LCD are as follows.

First, film formation on the lower glass substrate 1 and fabrication of the MIM element will be described. In FIG. 4, the lower glass substrate 1 is
It is covered with a glass protective film 2 such as Ta ₂ O ₅ or SiO ₂ . Since the protective film 2 is not indispensable, the coating can be omitted. Next, a lead electrode 3 and an insulator layer 4 are formed thereon.

Although the silicon nitride of the insulator layer 4 can be formed by various methods, in this embodiment, nitrogen gas,
Plasma CV using mixed gas such as silane gas and hydrogen gas
It was formed to about 1000 ° using the D method.

The upper electrode 5 and C _r, is formed by a resistance heating method, and patterned by a conventional a photolithography. The lower transparent electrode 6 was made of indium oxide-tin oxide (commonly known as ITO), was formed by magnetic sputtering, and was patterned by ordinary photolinography.

Film formation and patterning on the upper glass substrate 7 are almost the same as those of a normal simple multiplex LCD. Although the upper glass substrate 7 is covered with a glass protective film 8 such as SiO ₂ , the protective film 8 is not essential. Upper transparent electrode 9
Similarly, the lower transparent electrode 6 is made of indium oxide-tin oxide, formed by magnetic sputtering, and patterned by a usual photolinography method.

The lower glass substrate 1 and the upper glass substrate 7 were adhered to each other via a spacer such as glass fiber and sealed with a normal epoxy-based adhesive. The cell thickness was 8 μm.

Both glass substrates 1 and 7 were subjected to an alignment treatment by rubbing.
In this case, an alignment treatment film such as polyimide is often applied, but is not indispensable, so that it is omitted in FIG.

Twisted nematic liquid crystal in the above cell
Inject ZLI-1565 (Merck) through the injection hole and add the liquid crystal layer 1
0 was set. TFD-LC by sealing the injection hole with adhesive
D is completed.

FIG. 5 shows an element pattern of one pixel on the lower glass substrate 1. Thus, the lower transparent electrode 6 is separated for each pixel. The front surface of the lead electrode 3 is covered with the insulator layer 4 by anodic oxidation, and small projections are formed from the lead electrode 3 corresponding to each pixel. This protruding electrode
Numeral 11 intersects with the upper electrode 5, and this intersection forms the MIM element.

FIG. 6 shows a part of the structure of the TFD-LCD of this embodiment. In this manner, the pixels on the lower glass substrate 1 shown in FIG. 5 are arranged in a grid, the lead electrodes 3 are connected laterally, and the terminal portions 12 are formed at the ends. The upper transparent electrode 9 on the upper glass substrate 7 in FIG. 4 is formed in a band shape in which each pixel is vertically connected as shown in the figure. The shape of the upper transparent electrode 9 is substantially the same as that of the LCD of the simple multiplex drive.

When the voltage applying method shown in FIG. 3 is applied to the panel having the structure shown in FIG. 6, the upper transparent electrode is a scanning signal line and the lead electrode is a data signal line.

In the driving method of the present invention shown in FIG. 1, V _P = 16 V,
By applying a driving waveform with V _P ′ = 19 V and a bias ratio = 9 to the panel, a high contrast display with a contrast ratio of 20 or more, no crosstalk, and no flicker was obtained. That is, in this case, the display optimum ratio value (=
V _P / V _P ') was 0.842.

Here, in the driving method of the present invention shown in FIG. 1, the scanning signal and the data signal both have a signal centered on O (this voltage is called a center voltage). However, this method is complicated because both positive and negative power supplies are required. Also, usually TN
In the type simple matrix LCD driving method, the number of power supplies required is changed by changing the center voltage of each of the scanning signal and the data signal for each frame without changing the liquid crystal application signal of FIG. 1 as a potential difference. (So-called phase difference driving method). Such a method is also applicable to the driving method of the present invention.

Embodiment 2 Table 5 shows scanning signals and data signals when the phase difference driving method is applied to the driving method according to the present invention. When this is applied to the TFT-LCD panel of Example 1, V _P = 16 V, V _P ′ = 19
Under the conditions of V and bias ratio = 9, a high contrast display with a contrast ratio of 20 or more, no crosstalk, and no flicker was obtained.

Embodiment 3 The method of Embodiment 2 for modulating the time width of the data signal of the selected pixel according to the gradation (that is, the pulse width modulation method) is used.
Halftone is achieved. That is, the video signal was digitized by 4-bit A / D conversion, and the pulse width was changed in accordance with the contrast curve of the liquid crystal to realize 16 gradations.

If the number of A / D conversion bits is further increased, higher gray scale becomes possible.

In each of the above embodiments, the value of V _P / V _P ′ was determined while observing the screen of the liquid crystal display device so as to eliminate flicker.

〔effect〕

By using the driving method of the present invention, a high-contrast display with no flicker can be obtained even in a TFD-LCD panel using a non-linear resistance element having an asymmetric IV characteristic.

Further, in the driving method of the present invention, the ratio of the absolute values of the voltages applied to the pixels having different polarities is adjusted and fixed so as to eliminate flicker, so that flicker does not occur even if the applied voltage is changed. Therefore, by adjusting the applied voltage, the contrast characteristics suitable for each user can be easily obtained. On the other hand, in a conventional method of suppressing the occurrence of flicker by adding an offset voltage to a pixel voltage, such a contrast adjustment is difficult because it is necessary to change the bias voltage at the same time as changing the pixel applied voltage.

Further, in the above-described embodiments, only an example in which a silicon nitride-based MIM element is used as the nonlinear element has been described. However, even when a MIM element of another material, a diode ring, and a back-to-back diode are used as the nonlinear element, almost the same display performance without flicker was obtained.

[Brief description of the drawings]

FIG. 1 is a waveform diagram of one embodiment of a driving method according to the present invention, FIG. 2 is a diagram showing an example of a conventional driving method, FIG. 3 is a diagram showing an equivalent circuit of one pixel, and FIG. FIG. 5 is a cross-sectional view of the MIM-LCD panel used in the example, FIG. 5 is a structural plan view of one pixel of the panel used in the example, and FIG. 6 is a transparent structure plan view of a part of the panel used in the example. 1 ... lower glass substrate, 2, 8 ... glass protective film, 3 ...
Lead electrode, 4 ... insulator layer, 5 ... upper electrode, 6 ...
Lower transparent electrode 7, Upper glass substrate 9, Upper transparent electrode 10, Liquid crystal layer 11, Projecting electrode 12, Terminal part 13, Nonlinear resistance element 14, Liquid crystal element , 15 ... data signal line, 16 ... scanning signal line.

Claims (1)

(57) [Claims] 1. A drive for inverting a polarity of a pixel applied voltage applied to each pixel of an active matrix liquid crystal display device in which a switching element formed of a non-linear resistance element is set in series with each pixel formed of a liquid crystal element for each frame. An active matrix, characterized in that the bias ratio is kept constant and the ratio of the insulation values of the voltages applied to the pixels of different polarities is set such that the absolute value of the voltage applied to the liquid crystal element is equal between each frame. A method for driving a liquid crystal display device.