JPH01291216A - Active matrix type liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the deterioration of the drain current-gate voltage characteristics of TFTs from an initial state and to stabilize the contrast of a liquid crystal cell by turning on and off at least one of the two TFTs connected in series with a voltage of about 50% duty ratio. CONSTITUTION:The TFTs 1, 2 are connected in series. One end of the liquid crystal cell 3 is connected to a counter electrode 7 and the other end to the TFT 2. The TFT 1 is connected to a data electrode 4 and the gates of the TFTs 1, 2 are respectively connected to independent scanning electrodes 5, 6. The voltage set at about 50% duty ratio is impressed to at least either of the electrodes 5, 6. Shifting of the drain current-gate voltage characteristics of the TFTs to a plus direction and shifting thereof to a minus direction are thereby canceled and the drain current-gate voltage characteristics of the TFTs do not change from the initial characteristics. The degradation in the contrast of the liquid crystal cell 3 is obviated.

Description

[Detailed Description of the Invention] [Summary] Regarding an active matrix liquid crystal display device in which data applied to data electrodes is written into a liquid crystal cell via a switching element, the active matrix type liquid crystal display device prevents deterioration of the switching element and writes the data applied to a data electrode into a liquid crystal cell via a switching element.
In order to stabilize the display quality of the IJ type liquid crystal display device, the switching element of the active matrix type liquid crystal display device is composed of two thin film transistors connected in series, and the gates of these two thin film transistors are It is connected to independent scanning electrodes of the tree, and the duty ratio of the voltage of at least one of the scanning electrodes is set to about 50% to write data into the liquid crystal cell.

[Industrial application field]

The present invention relates to improvements in active matrix liquid crystal display devices.

In recent years, active matrix drive type flat displays have been actively researched and developed. Since this active matrix drive type flat display can have a smaller depth than a cathode ray tube, it has also been commercialized as a display device for pocket TVs, boot-top computers, and the like.

However, in the active matrix drive system, thin film transistors used as switching elements, especially amorphous silicon thin film transistors (A-3i
The display quality of a liquid crystal display device may deteriorate due to the deterioration of the characteristics of TFTs, and improvement of this problem is desired.

[Conventional technology]

FIG. 5 shows the configuration of one element of a conventional active matrix liquid crystal display device. Generally, a conventional active matrix display device is constructed by sandwiching a display layer in which liquid crystal cells are arranged between two glass substrates. On one glass substrate, switching elements such as thin film transistors, scanning electrodes, and liquid crystal cells are placed on one glass substrate. One electrode of the cell (hereinafter referred to as the counter electrode) is formed, and a data electrode is formed on the other glass substrate. -A common electrode of the liquid crystal cell is formed.
Here, only the connection of one liquid crystal cell in the display layer to the thin film transistor and the connection to each electrode will be explained.

As shown in FIG. 5, an active matrix type liquid crystal display device has three electrodes, a data electrode 4, a scanning electrode 5, and a counter electrode 7, and one liquid crystal cell ( LC) 3 and a thin film transistor (hereinafter referred to as TPT) 8 for controlling the liquid crystal cell 3 are provided. The control terminal (gate) of the TFT 8 is connected to the scanning electrode 5, one of the two controlled terminals (drain) is connected to the data electrode 4, and the other (source) is connected to one terminal of the liquid crystal cell 3. . Further, the other terminal of the liquid crystal cell 3 is connected to a counter electrode 7.

The operation of the active matrix liquid crystal display device configured as described above will be explained using FIG. 6.

Since the liquid crystal cell 3 apparently deteriorates unless an alternating current bias is applied, the counter electrode 7 is held at an intermediate potential as shown by the waveform A, and the voltage of the data electrode 4 is lower than that of the counter electrode as shown by the waveform opening. It is an alternating voltage centered on the potential of 7. The period of the alternating voltage of the data electrode 4 is twice the length of one vertical scanning time (16.7 m5) of a rusk scan in a television image.

As shown by the waveform C, the voltage of the scanning electrode 5 is normally maintained at less than 07, and the voltage becomes high only when the TFT 8 is turned on. Generally, the time for turning on the TFT 8 is 60 μs. Then, each time the TFT 8 is turned on, the data on the data electrode 4 at that time is written into the liquid crystal cell 3 as shown by waveform 2.

[Problem to be solved by the invention]

However, in the above-mentioned active matrix type liquid crystal display device, the TFT 8 is considerably longer in the off state than in the on state, so the drain current-gate voltage characteristic of the TFT 8 changes from the initial state as shown in FIG. It gradually shifts to the negative side. However, even if a negative potential (-αV) is applied to the scanning electrode 5 to the gate in order to turn off the TFT 8, a minute drain current β will flow to the liquid crystal cell 3, and the liquid crystal cell 3 will be turned off first. The written data potential cannot be held. That is, the conventional active matrix type liquid crystal display device has a problem in that the contrast of the liquid crystal cell 3 gradually decreases.

The phenomenon described above is that when ten voltages are applied to the gate of the a-3i TFT, the TPT turns on and drain current flows, but the accumulated electrons are trapped at the gate insulating film/active layer interface. When the threshold value is shifted to the positive side and, conversely, one voltage is applied to the gate, the TPT turns off, the drain current becomes less than 10-'2A, and the trapped electrons are released. This is due to the characteristic that the threshold value is shifted to the negative side.

An object of the present invention is to eliminate shifts in the negative direction of the threshold value due to changes over time in TPT used in active matrix liquid crystal display devices, and to constantly stabilize the contrast of liquid crystal cells in active matrix liquid crystal display devices. The purpose is to do so.

[Means to solve the problem]

The structure of an active matrix type liquid crystal display device of the present invention that achieves the above object is shown in FIG.

In the figure, 1.2 is TPT, which are connected in series. 3 is a liquid crystal cell, one end of which is connected to the counter electrode 7, and the other end connected to the TPT 2 described above. Further, the TFTI is connected to the data electrode 4,
The gates of TFTI,2 are each connected to two independent scan electrodes 5.6. And the scanning electrode 5,
6, a voltage with a duty ratio of approximately 50% is applied to at least one of them.

[For production]

According to the active matrix liquid crystal display device of the present invention, there are two TPTs connected in series between the liquid crystal cell and the data electrode, and at least one of the TPTs is
Since a gate voltage with a duty ratio of approximately 50% is applied from the scanning electrode, its drain current-gate voltage characteristics do not deteriorate, and when a negative potential is applied to the gate from the scanning electrode to turn off the TPT. drain current does not increase. Therefore, the data potential previously written to the liquid crystal cell is held, and the contrast of the liquid crystal cell does not deteriorate.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 2 shows the configuration of an embodiment of the active matrix liquid crystal display device of the present invention. In this embodiment as well, the connection of one liquid crystal cell in the display layer to the TPT and the connection to each electrode are shown. I will only explain about.

In FIG. 2, 1 and 2 are rFTs (
fi film transistor), and the source S of its source 2 is
2 is connected to the other end of the liquid crystal cell 3 whose one end is connected to the counter electrode 7 . In addition, the gate G1 of TFTl and TP
Gates 1-G2 of T2 are independent scanning electrodes 5.
and scan electrode 6, and two TFTIs, 2
are controlled to turn on and off independently.

An example of driving the active matrix liquid crystal display device configured as described above will be explained with reference to FIG.

Since the liquid crystal cell 3 apparently deteriorates unless an alternating current bias is applied, the counter electrode 7 is held at an intermediate potential as shown by the waveform (dl) in this embodiment as well, and the voltage of the data electrode 4 is maintained at an intermediate potential as shown by the waveform (dl). As shown in C), it is twice as long as one vertical scanning time (16.7 m5) of the Rusk scan in this facing TV image.

An alternating voltage with a waveform (al) centered at 0 is applied to the scanning electrode 5, and its duty ratio is 50.
It is slightly larger than the percentage.

The scan electrode 6 is applied with an alternating voltage shown in a waveform (b) having exactly the same period as the voltage waveform (a) of the scan electrode 5,
The periods of these two waveforms (al, (bl) are shifted by one vertical scanning time (16,7 m5) of the rask scan.

Therefore, the voltage waveform of the scanning electrode 5 (al high level “1”)
If we pay attention to the period of ('', the waveform (al) overlaps with the high level "11" voltage of the scanning electrode 6 for a predetermined time T from its rise and for the same time γ just before it falls, and during this period γ only TFTl and T in FIG.
Both PT2 are turned on.

That is, the AND waveform of the waveforms of scanning electrode 5 and scanning electrode 6 becomes as shown by waveform (e) in FIG. In this embodiment, at the above-mentioned time γ, that is, the high level "I" in the waveform tel.
+” time is set to 60 μs, and this waveform (e)
is the waveform of the scanning electrode 5 in a conventional active matrix liquid crystal display device (the waveform shown by C in FIG. 6). Therefore, in this embodiment, both TFTl and TPT2 are turned on every vertical scanning time (16,7m5), and during that period, data on the data electrode 4 is written into the liquid crystal cell 3 in the waveform (as shown by fl). The driving waveform of the liquid crystal cell 3 is the same as that of the conventional device shown in FIG.

By the way, in conventional active matrix liquid crystal display devices, the TPT is much longer in the off state than in the on state, so the drain current-gate voltage characteristics of the TPT are gradually shifted to the negative side from the initial state, and this state De TPT
Even if a negative potential is applied to the scanning electrode 5 to the gate of the liquid crystal cell 3, a small drain current flows to the liquid crystal cell 3, causing a problem in that the contrast of the liquid crystal cell 3 gradually decreases. However,
As shown in the waveforms (al and (b)), TFT1 and TFT2 in the device of this embodiment have a duty ratio of approximately 5.
Since it is turned on/off with 0% drive voltage, TF
The positive and negative shifts in the drain current-gate voltage characteristics of T are canceled,
The drain current-gate voltage characteristics of the TPT do not change from the initial characteristics shown in FIG.

Therefore, when TFT1 or TFT2 is turned off, no minute drain current flows through the liquid crystal cell 3, so that the contrast of the liquid crystal cell 3 does not deteriorate.

FIG. 4 shows another example of driving the active matrix liquid crystal display device of FIG. 2. In FIG. In this example, the alternating voltage applied to the scanning electrode 5 has a period as shown in the waveform (the alternating voltage applied to the scanning electrode 6 is the same as the embodiment shown in FIG. 3, as shown in the waveform). The pulse is set to a high level "H" for 60 μs every 16.7 n+s, and this pulse is output in synchronization with the alternating voltage of the scanning electrode 5. Therefore, in this example as well, the A of the waveform (a) of the scanning electrode 5 and the waveform (b) of the scanning electrode 6 is
Since the ND waveform is the same as the waveform +e) in FIG. 3, the driving waveform of the liquid crystal cell 3 is the same as (fl) in FIG.

In the driving example of this embodiment, the TFT connected to the scanning electrode 6
As for I, since the off state is longer than the on state, its drain current-gate voltage characteristic gradually shifts to the negative side from the initial state, and TFTl applies a negative potential to the scanning electrode 5. However, since TFT2 is driven with a voltage with a duty ratio of 50%, its drain current-gate voltage characteristics do not deteriorate from the initial state, and a minute drain current can flow. Since the drain current is blocked by the TFT 2, deterioration of the contrast of the liquid crystal cell 3 is prevented.

Note that the voltages of scanning electrode 5 and scanning electrode 6 in FIG. 4 are exactly the same even if they are interchanged.

〔Effect of the invention〕

As explained above, according to the active matrix liquid crystal display device of the present invention, two TPTs connected in series are used.
Since at least one of them is turned on/off with a voltage with a duty ratio of approximately 50%, the drain current-gate voltage characteristics of the TPT do not deteriorate from the initial state, and the liquid crystal cell 3
This has the effect of constantly stabilizing the contrast of the images.

[Brief explanation of the drawing]

FIG. 1 is a principle block diagram of an active matrix type liquid crystal display device of the present invention, FIG. 2 is a circuit diagram showing the configuration of an embodiment of an active matrix type liquid crystal display device of the present invention, and FIG. 3 is a block diagram of the principle of an active matrix type liquid crystal display device of the present invention. FIG. 4 is a waveform diagram showing another driving example of the circuit in FIG. 2. FIG. 5 is a circuit diagram showing the configuration of a conventional active matrix liquid crystal display device. FIG. 6 is a waveform diagram showing an example of driving the circuit. is a driving waveform diagram of the circuit of FIG. 5, and FIG. 7 is a diagram illustrating the shift from the initial state of the drain current-gate voltage characteristics of the thin film transistor. 1.2.8... Thin film transistor (TPT), 3...
-Liquid crystal cell, 4...data electrode, 5.6...scanning electrode, 7...counter electrode.

Claims (1)

[Claims] Active matrix type liquid crystal in which a liquid crystal cell is formed by interposing a liquid crystal between a data electrode and its counter electrode, and data given to the data electrode is written into the liquid crystal cell via a switching element. In the display device, the switching element is constituted by two thin film transistors connected in series, the gates of these two thin film transistors are connected to two independent scan electrodes, and the duty of the voltage of at least one scan electrode is adjusted. The ratio is about 5
An active matrix type liquid crystal display device characterized by a 0% display.