JPH0283526A - Method for driving active matrix liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain display with high contrast without generating flicker at all even in a TFD-LCD pannel using a nonlinear resistance element having asymmetric I-V characteristics by constituting a voltage impressed to a picture element whose polarity is inverted every frames and whose ratio of the absolute values is set at an optimum ratio of the display of the device. CONSTITUTION:Each of the picture elements mounted on a lower part glass substrate 1 are arranged in a lattice state, and lead electrodes 3 are connected with each other in a transverse direction. An upper transparent electrode 9 mounted on an upper glass substrate 7 is formed belt-likely in a state of connecting to each of the picture elements in a longitudinal direction. The electrode 9 is constituted of a scanning signal (a) line, and the electrode 3 is constituted of a data signal (b) line. Thin film two terminal element type active matrix LCD(TFD-LCD) constituted as mentioned above is driven by changing the absolute value of the voltage (c) impressed to the picture element which is difference between the signals (a) and (b) in a negative and a positive frames, respectively. The ratio (optimum ratio of the display) which does not generate the flicker is detected by controlling the ratio of the absolute value of the voltage impressed to the picture element between the positive and negative frames, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for driving an active matrix liquid crystal display device using a nonlinear resistance element.

[Conventional technology]

In recent years, applications of liquid crystal display devices (LCDs), mainly twisted nematic type (TN type), have been developed and are being used in large quantities in the fields of wristwatches and calculators. In addition, in recent years,
Matrix types that can display arbitrary characters such as characters and figures are also beginning to be used. In order to expand the field of application of this matrix type LCD, it is necessary to increase the display capacity. but,
The rise of the voltage-transmittance change characteristic of conventional LCDs is not very steep, so when the number of scans in multiplex drive is increased to increase display capacity, the effective voltage applied to each selected pixel and non-selected pixel increases. Since the ratio decreases, crosstalk occurs in which the transmittance of selected pixels increases and the transmittance of non-selected pixels decreases. As a result, display contrast is significantly reduced, and the viewing angle at which a certain degree of contrast can be obtained is also significantly narrowed. Therefore, in a conventional LCD, the maximum number of lines that can be scanned is about 60 lines. Such a conventional LCD is called a simple matrix LCD in comparison with an active matrix LCD described below.

In order to significantly increase the display capacity of matrix WLCDs, active matrix LCDs have been devised in which switching elements are arranged in series in each pixel of the LCD. The switching elements of active matrix LCD prototypes that have been announced so far, thin film transistor elements (T) using amorphous silicon or polysilicon as semiconductor materials
PT) is often used. On the other hand, since the manufacturing method and structure are relatively simple, the manufacturing process can be simplified, and thin film two-terminal elements (
Active matrix LC using TFD (hereinafter abbreviated as TFD)
D is also attracting attention.

Such a thin film two-terminal element type active matrix LC
D (hereinafter abbreviated as TFD-LCD), the LCD considered to be closest to practical use is an LCD using a metal-insulator-metal element (hereinafter abbreviated as MIM element or MIM) in a TFD.

In addition to MIM, amorphous S1 p
A diode ring in which two IN diodes are connected in parallel with opposite polarities, and a pack-to-back diode in which two PIN diodes are connected in series with opposite polarities are known.

All of the TFDs described above are nonlinear resistance elements in circuit terms, and as the voltage applied to both ends of the element increases, the current increases rapidly in a nonlinear manner. By connecting such a TFD in series with a liquid crystal, the rise of the voltage-transmittance change characteristic becomes steeper, making it possible to significantly increase the number of scans.

A conventional example of an LCD using such an MIM is described in a paper by D. R. Paraf et al. Flex) “-IJ Kid Crystal Displays”, IE Transactions on Electron Devices, Volume 28, No. 6, Page 73
6-739 (published in 1981) [D, R, Baraff
, et al.

” The Optimization of Met
al-Insulator-Metal Non1in
ear Devices for Use inMul
Tiplexed Liquid Crystal D
isplays.

I EEE Trans, Electron De
vices, vol, ED-28, pp, 736-
739 (1981)] and Nobuharu Morozumi, et al. r250
x240 pixel 2-channel MIM-LCD J Television Society Technical Report (I PD83-8).

1) I) 39-44. (published in December 1983), and in the patent publication publication JP-A-52-14909.
0 and Japanese Patent Application Laid-Open No. 55-161273, the principle of operation thereof is also described in detail.

In MIM, the most important material is that of the insulator layer. In MIMs described in conventional papers, patent publications, etc., tantalum (Ta) or silicon oxide or nitride is mainly used as the material for the insulating layer, but the material is not limited thereto. Furthermore, although almost any metal can be used, chromium or tantalum is mainly used.

The current-voltage (I-V) characteristics of a nonlinear resistance element can be expressed mathematically in various ways, but the following equation is known as a typical one.

I=A・■" Here, Δ is the current, ■ is the voltage, α is the nonlinear coefficient, and A is the proportionality constant. In the MIM elements etc. already mentioned, α
has a direct value of 6 or more.

In the TFD-LCD described above, a driving method that is substantially the same as that used for conventional TN-type simple matrix (IJ) LCDs has been used. Regarding the conventional drive method, see Electronics April 1988, 9fp.
1) 42 46.

FIG. 2 shows drive signals when the pixel of interest is a selected pixel, and selected pixels and non-selected pixels are mutually present on the data signal line. Traffic signal (a), data signal (
b) takes values as shown in Table 1 for each negative and positive frame. One screen is scanned in each negative and positive frame, and the display contents are written. The address time is a write period for each pixel, and the non-address time is a charge retention period for each pixel. The ratio between VD and VP (VD/VP) is usually constant and is called the bias ratio.

As shown in FIG. 3, the equivalent circuit of one pixel of a TFD-LCD panel has a nonlinear 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 even if this is the other way around.

The second surface element applied signal (c) in Table 1 is (data signal) - (scanning signal)
and takes the values shown in Table 2. Correspondingly, the liquid crystal voltage (d) changes, resulting in display contrast.

Ideally, the IV characteristics of a nonlinear element should be symmetrical with respect to the positive and negative voltages, but actual MIM elements have a large degree of asymmetry. That is, in equation (1), even though α is the same, the value A+ of A when ■>0 is different from the value A- of A when v>0.

A-) At A+, the absolute value of the voltage applied to the liquid crystal becomes larger in the negative frame. The contrast of the LCD is
Since it is determined by the effective value of the liquid crystal voltage (d), in such a case, flickering on the screen becomes more noticeable.

In Table 2, all non-address values are marked with [ ] because the pixel applied voltage changes to the value in [ ] depending on whether the data signal is selected or not selected. It means to take.

[Problem to be solved by the invention]

As described above, conventional TFD-LCDs have the disadvantage of being susceptible to flickering.

An object of the present invention is to provide a method for driving a TFD-LCD that does not cause 7-flash.

[Means to solve the problem]

According to the present invention, in a method for driving an active polymer (IJ) liquid crystal display device using a non-linear resistance element, the polarity of the pixel applied voltage is reversed for each frame, and the ratio of the absolute values is set to the optimal display ratio value. A method for driving an active matrix IJ liquid crystal display device is obtained, which is characterized in that a set pixel applied voltage is applied.

[Effect]

A driving method according to the present invention is shown in FIG. Basically, it is the same as the conventional type shown in Fig. 2, but the absolute value of the pixel applied voltage (c), which is the difference between the scanning signal (a) and the data signal (b), is calculated between the negative frame and the positive frame. The feature is that it has changed. That is, the value of vP is Vp and ■P' for positive frames and negative frames.
Therefore, the value of VD becomes VD and Vn'. A->A”
t - Assumption t le(!: , Vp>Vp'.

If vD>vD' is set, the absolute values of the liquid crystal voltage (d) between positive and negative frames can be made equal.

These values are summarized in Table 3.

Normally, the bias ratio is kept constant for positive and negative frames (Vo
/Vp=Vn'/Vp') However, this is not essential.

V, which is the ratio of the absolute value of the pixel applied voltage between positive and negative frames
p/Vp', (Vp-2Vo)/(Vp'-2v,,'
), a ratio that eliminates flicker can be found. This ratio is called the display optimum ratio value.

If the bias ratio is constant, %VP/VP' may be set to the display optimum ratio value.

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

Table 3 Table 4 〔Example〕 The following will explain based on examples.

[Example 1] The structure and manufacturing method of a typical MIM-L CD are as follows.

First, film formation on the lower glass substrate 1 and fabrication of the MIM element will be described. In FIG. 4, a lower glass substrate 1 is coated with a glass protective film 2 such as Tag Os, 5ift or the like. Since this Hoto membrane 2 is not essential, it may be possible to omit the coating. Next, on top of this, lead electrode 3
, to form the insulator layer 4.

The silicon nitride of the insulator layer 4 can be formed by various methods, but in this example, plasma CV using a mixed gas such as nitrogen gas, silane gas, and hydrogen gas is used.
It was formed to about 100 OA using method D.

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

The film formation and patterning on the upper glass substrate 7 are almost the same as those of the ordinary simple multiplex LCD #1. The upper glass substrate 7 is coated with a glass protective film 8 such as 5iO-, but this protective film 8 is not essential. The upper transparent electrode 9 was also made of indium oxide-tin oxide, like the lower transparent electrode 6, and was formed by magnetic sputtering and patterned by the usual photolithography method.

The lower glass substrate 1 and the upper glass substrate 7 were pasted together with a spacer such as glass fiber interposed therebetween, and sealed with a common epoxy adhesive.

The cell thickness was 8 μm.

Both glass substrates 1 and 7 were subjected to alignment treatment by rubbing.

In this case, an alignment treatment film such as polyimide is often applied, but it is not essential, so it is omitted in Figure 81.

In the above cell, there is Z, which is a twisted nematic type liquid crystal.
LI-1565 (manufactured by Merck) was injected through the injection hole to form the liquid crystal layer 10. A TFD-LCD was completed by sealing the injection hole with adhesive.

FIG. 5 shows the element pattern of one pixel on the lower glass substrate l. In this way, the lower transparent electrode 6 is separated for each pixel. The front surface of the lead electrode 3 is covered with an insulating layer 4 by anodic oxidation, and a small projection is formed from the lead electrode 3 corresponding to each pixel. This protruding electrode 1
1 intersects with the upper electrode 5, and this intersection is the MIM
It has become an element.

FIG. 6 shows a part of the structure of the TFD-LCD of this embodiment. In this way, each pixel on the lower glass substrate 1 shown in FIG. 5 is arranged in a lattice pattern, and the lead electrodes 3 are connected laterally to form a terminal portion 12tl at the end. The upper transparent electrode 9 on the upper glass substrate 7 in FIG. 4 is formed into a band shape in which each pixel is vertically connected as shown. The shape of this upper transparent electrode 9 is L
The shape of the electrode is almost the same as that of a CD.

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

In the driving method of the present invention shown in FIG.
By applying a drive waveform with v, VP' = 19V, and bias ratio = 9 to the above panel, the contrast ratio is 2.
0 or more, a high contrast display with no crosstalk and no flicker was obtained. That is, in this case, the display optimum ratio value (=Vp/Vp') was 0.842r.

Here, in the driving method of the present invention shown in FIG. 1, both the scanning signal and the data signal fluctuate around O (this voltage is called the center voltage). However, this method is complicated because it requires positive and negative power supplies. Also, usually TN
In the W simple matrix LCD driving method, the number of required power supplies can be reduced by changing the center voltage of each scanning signal and data signal for each frame without changing the liquid crystal applied signal shown in Fig. 1 as a potential difference. (so-called phase difference drive method). Such a method is also applicable to the driving method of the present invention.

[Example 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. Example 1
When applied to a TPT-LCD panel of
Under the conditions of 6V, Vp' = 19V, and via x ratio = 9, a high contrast display with a contrast ratio of 20 or more, no crosstalk, and no flicker was obtained.

Table 5 [Example 3] Halftones were realized by the method of modulating the time width of the data signal of the selected pixel according to the gradation (ie, pulse width modulation method) of Example 21-. In other words, the video signal is converted into a 4-bit A
/D conversion was used to digitize the image, and by changing the pulse width to match the contrast curve of the liquid crystal, 16 gradations were achieved.

However, if the number of bits of A/D conversion is increased, the VI will be higher.
T key is now possible.

In addition, in any of the above embodiments, V p / V p
The value was determined by adjusting y4 to eliminate flicker while looking at the screen of the liquid crystal display device.

〔effect〕

If the driving method of the present invention is used, a high-contrast display with no flicker can be obtained even in a TFD-LCD panel using a nonlinear resistance element with an asymmetric IV characteristic.

Further, in the above embodiments, only examples in which a silicon nitride MIM element is used as a nonlinear element have been described.

However, even when MIM elements made of other materials, diode rings, and back-to-back φ diodes were used as nonlinear elements, almost the same flicker-free display performance was obtained.

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
...Protruding electrode, 12...Terminal part,
13...Nonlinear resistance element, ]4...Liquid crystal element, 15...Data signal line, 16...
...Scanning signal line.

Claims (1)

[Claims] In a driving method in which the polarity of the pixel applied voltage applied to each pixel of an active matrix liquid crystal display device is inverted every frame, in which a switching element consisting of a nonlinear resistance element is connected in series to each pixel consisting of a liquid crystal element, 1. A method for driving an active matrix liquid crystal display device, characterized in that a ratio of absolute values of voltages applied to different pixels is set such that the absolute values of voltages applied to a liquid crystal element are equal between frames.