JPH0545958B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a display device in which a switch array is formed using thin film transistors (hereinafter abbreviated as TFT).

[Prior art]

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 the data line 112, a switch TFT 113, a capacitor 114, and a liquid crystal cell 11 are formed at the intersection of 1 and the data line 112.
It consists of 5. 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. 1 c is applied to the data line 112 in FIG. 1 b, and a gate line drive voltage such as V _g in FIG. 1 c is applied to the gate line 111 in FIG. is applied. TFT113
When is an N-type TFT, the signal on the data line 112 is written to the liquid crystal cell 115 and the capacitor 114 during the period when V _g is high, and the signal written to 115 is held during the period when V _g is low. . In Fig. 1c, A denotes a negative frame with V _d <0, and B
represents a frame with 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
MOS transistor voltage-current characteristics (gate/current characteristics)
Source voltage V _GS - drain-source current I _DS characteristics)
The ON/OFF ratio is large as shown at 201 in Figure 2. The current change in the weak reversal region is abrupt, and the leakage current in the weak reversal region is small.

[Problems and objectives of conventional technology]

However, when trying to drive a matrix-type liquid crystal display device that uses thin film transistors (hereinafter referred to as TFTs) made of non-single-crystal silicon as switches instead of the above-mentioned single-crystal silicon MOS transistors, the following problems occur. . In this regard, let us compare the case of silicon single crystal and the case of thin film transistor. When a silicon single crystal substrate 1 is used as shown in FIG. 3a, when a scanning signal voltage is applied to the gate 2 (on state), an inversion layer is formed in the channel region. Charges move through this inversion layer. Next, as shown in Figure b, when the gate 2 becomes the same potential as the silicon single crystal substrate without applying the scanning signal voltage (off state), the inversion layer disappears and the charge as a carrier is transferred to the silicon single crystal substrate. Dispersion disappears. Therefore, in the off state, the region where a capacitance is formed within the transistor is the portion l where the gate 2 and the diffusion region 3 overlap.

On the other hand, in a display device using a thin film transistor shown in FIG. 3c, a gate electrode 6 is formed on a glass substrate 4 via an insulating film and an island-shaped semiconductor region 5 of the thin film transistor. Here, by applying a scanning signal to the gate electrode 6, an inversion layer is formed in the island-shaped semiconductor region 5 and carriers are generated, as in the case of a silicon single crystal substrate.
However, in the case of a thin film transistor, when the scanning signal voltage is no longer applied to the gate 2 (off state) as shown in d of the figure, the inversion layer disappears, but the charges cannot escape. This is because the island-shaped semiconductor region 5 is formed on the glass substrate 4, which is an insulator, and does not have a specific potential, so that charges cannot move rapidly.
At most, this charge is attracted to the diffusion region. Therefore, in the off state, in the regions a and b, which are approximately half the width of the gate electrode,
A capacitive component is formed.

Therefore, if we compare the capacitance between the gate and the drain between the silicon substrate case b and the thin film transistor case d, the capacitance in the case of the thin film transistor is about ten times larger than that in the case of the silicon substrate. That is, in the case of thin film transistors,
This means that the influence of capacitance between the gate and drain cannot be ignored.

The existence of such capacity has conventionally caused the following unavoidable problems.

When the TV video signal is input to the liquid crystal display device,
A flicker phenomenon occurs in which the LCD screen flickers. The reason is as follows.

As shown in FIG. 4a, it is assumed that the aforementioned capacitance C _TFT exists between the gate and the drain. If the voltage drop V _G due to the pulse signal voltage change on the gate line G
When there is a voltage drop, the effect of this voltage drop appears at the liquid crystal electrode point P due to the existence of this capacitance C _TFT . The amount of change △V _GC is △V _GC =V _{GC TFT} /C _LC +C _TFT . Here, _CLC is the capacitance of the liquid crystal.

On the other hand, the video signal input via the source line
The relationship between V _IN and the voltage V _P applied to the liquid crystal electrode point P is a one-to-one relationship, as shown by the straight line d in FIG. 4b, unless the existence of the capacitance C _TFT is taken into account. However, when considering the voltage drop due to the presence of the capacitance _CTFT , V _P =V _IN -ΔV _GC , and this curve becomes like C.

The video signal is generally a positive field V _IN ⁺
and a negative field V _IN ^- are formed alternately at a frequency of 60Hz. Also, between adjacent fields, approximately |V _IN ⁺ |=|V _IN ^- |. However,
As can be seen from FIG. 4b, in the case of curve C, the voltage applied to the liquid crystal is |V _p ⁺ |>|V _p ^- |. Therefore, as mentioned above, when an AC drive signal consisting of a positive field V _p ⁺ and a negative field V _p ^- is applied to the liquid crystal with a cycle of 60 Hz, a strong voltage level V _p ⁺ appears every 30 Hz, which causes the flickering phenomenon. will be recognized as.

The present invention overcomes the above problems by providing an arbitrary bias potential between the center potential of the external AC inversion signal and the common electrode potential level.
The purpose of this invention is to reduce flicker that occurs in a display device using the above-mentioned TFT.

〔Example〕

Embodiments of the present invention will be described below with reference to FIGS. 5 to 7.
This will be explained based on the diagram.

In FIG. 5, 401 is a gate line, 402 is a data line, 403 is a TFT, 404 is a liquid crystal display cell, 405 is a data line drive signal source, 406 is a gate line drive signal source, 407 is a first bias power supply, 408 is a pixel electrode, and 409 is a common electrode, which is the parasitic capacitance of _CTFT . In this embodiment, a DC bias V _B is applied to an external AC inverted video signal Vd obtained by AC inverting an external video signal, and the TFT 40
3. In FIG. 6, an external video signal 501 has a center potential of zero level Vd.
The AC current is inverted around =0. A DC bias V _B is applied here. Therefore, the non-selection level V _G of the gate line drive signal 502 and the center potential
A potential difference V _B occurs between Vd=0 and Vd=0. Furthermore, this level V _G is also the potential of the common electrode 409 . 7a, b, and c show the relationship between the input external video signal and the voltage actually applied to the liquid crystal.

That is, in FIG. 1A, the external video signal is AC inverted to become an external AC inverted signal A. By applying a DC bias V _B to this external AC inverted signal A, an AC inverted signal B with a DC component superimposed is obtained. Therefore, by sampling this AC inverted signal B using the sampling clock signal shown in FIG. 5B and applying it to the pixel electrode 408 of the liquid crystal display cell 404 via the TFT 403 shown in FIG. A vertically symmetrical sampled video signal is applied to the liquid crystal display cell 404 .

In this way, in this embodiment, by providing a constant bias voltage V _B between the common level, which is the zero level of the data line drive signal based on the external AC inversion signal, and the common electrode of the liquid crystal cell, the above-mentioned V _LC can be reduced. This removes the DC component contained in the _VLC waveform to compensate for the vertical asymmetry of the VLC waveform, thereby preventing the liquid crystal from deteriorating due to DC drive. Although a DC bias is applied to the external AC inversion signal in this embodiment, it is sufficient that the common potential of the external AC inversion signal and the potential of the common electrode have a different potential difference. The same effect can be obtained even if a DC bias is applied to the common electrode 409 without applying a bias.

Furthermore, in this embodiment, since the voltage applied to the liquid crystal shown in FIG. 4 is vertically symmetrical, the curve C' shown in FIG. It will never be recognized.

〔effect〕

As described above, the present invention includes a pair of substrates in which an electro-optical conversion substance is sealed, a common electrode on one of the substrates, pixel electrodes arranged in a matrix on the other substrate, and a plurality of pixel electrodes arranged in a matrix on the other substrate. In a display device having thin film transistors connected to each other, a data line supplying an external AC inverted video signal to the source electrode of the thin film transistor, and a gate line supplying a gate signal to the gate electrode of the thin film transistor, the common electrode Since there is a constant potential difference between the potential level of the external AC inverted video signal and the common potential of the external AC inverted video signal, it is possible to suppress the flickering phenomenon caused by the AC inverted signal, and the electro-optic Since no DC component is superimposed on the conversion substance, the life of the display device can be significantly extended.

[Brief explanation of the drawing]

1a, b, and c show conceptual diagrams of a display device according to the present invention. FIG. 2 shows a V-characteristic diagram of a conventional single crystal silicon substrate. 3A to 3D are conceptual diagrams showing the parasitic capacitance of a transistor. FIGS. 4a and 4b show an equivalent circuit diagram of the display device according to the present invention and a conventional input-output characteristic diagram. FIG. 5 shows an equivalent circuit diagram showing an embodiment of the present invention. 6th
7a to 7c show waveform diagrams showing one embodiment of the drive waveform of the present invention. 401...Gate line, 402...Data line, 4
03...TFT, 408...pixel electrode, 409...
...Common electrode.

Claims (1)

[Claims] 1. A liquid crystal is sealed in a pair of substrates, a common electrode is on one of the substrates, a pixel electrode is on the other substrate, a thin film transistor connected to the pixel electrode, and a thin film transistor connected to the pixel electrode. In a liquid crystal display device having a data line for supplying a video signal and a scanning line for supplying a gate signal to the thin film transistor, the video signal supplied to the data line changes phase every vertical scanning period. is inverted, and there is a potential difference between the center potential of the video signal and the potential level of the common electrode, which is proportional to the product of the parasitic capacitance of the thin film transistor and the amplitude of the gate signal. A liquid crystal display device.