JPH05333316A - Liquid crystal display device and method therefor - Google Patents

Abstract

PURPOSE:To realize a uniform picture having no high contrast and no crosstalk by optimizing the structure, picture element circuit and driving method of a TFT and impressing a driving voltage sufficient for liquid crystal. CONSTITUTION:A TFT 3 is arranged at the intersection of a signal line 1 and a scanning line 2 which are arranged in a grid-shape and the TFT 3 is used as a switch and writes image signals in a liquid crystal capacitor and holding capacitor 6. At the TFT 3, resistor parts 4 are provided between a drain electrode D and the signal line 1 and between a source electrode S and a picture element electrode, and the pressure resistance of the TFT 3 is improved by decreasing this resistance value so as to decrease a leak current. Namely, two inequalities are established among a resistance value RT between the source and drain of the TFT 3 in the ON state, resistance value RL of the resistor parts 4, resistance value RO of a liquid crystal layer, capacity value C0, holding capacity Cs, one horizontal period TH and one vertical period T.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device and a driving method thereof.

[0002]

2. Description of the Related Art The conventional TN type liquid crystal used in many active matrix type liquid crystal display devices has the drawbacks that the screen is dark and the viewing angle is narrow because two polarizing plates are required. Therefore, attempts have been made to use polymer-dispersed liquid crystals that do not use polarizing plates or guest-host liquid crystals that have little viewing angle dependence. An example of an active matrix type liquid crystal display device using polymer dispersed liquid crystal is “SI
D (S.I.D.) 90 digest, p. 22
7-230, Kunigita and others ”. FIG. 2 is an equivalent circuit of the pixel portion, in which a TFT 23, a liquid crystal 25, and a storage capacitor 26 are arranged at the intersection of the signal line 21 and the scanning line 22.

[0003]

However, the above-mentioned prior art has the following problems. Generally, polymer-dispersed liquid crystals and guest-host liquid crystals have a higher threshold voltage and a smaller change in transmittance with respect to applied voltage than TN liquid crystals. A voltage that is several times that of the mold must be applied. However, since the withstand voltage of the TFT is not so large, the applied voltage has an upper limit in order to ensure reliability. Further, when the driving voltage is high, the leak current of the liquid crystal or the TFT increases, which causes problems such as deterioration of contrast and crosstalk.

The liquid crystal display device of the present invention solves such a problem, and an object thereof is to provide an active matrix liquid crystal display device using polymer dispersed liquid crystal or guest-host liquid crystal. Is to realize a high contrast and a uniform screen without crosstalk.

[0005]

A liquid crystal display device of the present invention is provided with a resistance portion made of a semiconductor thin film between a channel portion and a drain electrode of a thin film transistor and between a channel portion and a source electrode of the thin film transistor, and Resistance value RT, resistance value RL of resistance part, resistance value R0 of liquid crystal layer between pixel electrode and counter electrode, capacitance value C0, storage capacitance Cs,
It is characterized in that the following two expressions are established during one horizontal scanning period TH and one vertical scanning period T.

(RT + 2 · RL) × (Cs + C0) <T
H (C0 + Cs) × R0> T Further, two potentials are repeatedly applied to the counter electrode with an integral multiple of one horizontal scanning period or one vertical scanning period as a half cycle,
The two potential difference Vc is the threshold voltage Vth of the liquid crystal layer.
And the image signal voltage range Vid applied to the signal line, the following expression is established.

2Vth + Vid> Vc> 2Vth

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS This embodiment will be described below with reference to the drawings.

FIG. 4 shows the applied voltage dependence of the transmittance of the liquid crystal display device. Since the TN type liquid crystal can obtain a high contrast ratio at a low voltage, most TF
Used in T liquid crystal display devices. However, since this liquid crystal requires two polarizing plates, there is a problem that the transmittance is low and the viewing angle is narrow. On the other hand, since the polymer dispersed liquid crystal does not use a polarizing plate, it is possible to obtain the brightness twice or more that of the TN liquid crystal. However, since the scattering of light is used, a high contrast ratio cannot be obtained unless a parallel light source is used as a light valve of a light valve of a liquid crystal projector when used in a transmission type. Although the guest-host type liquid crystal using the dichroic dye has the same transmittance as the TN type, it can obtain a high contrast ratio in a very wide viewing angle and is excellent as a direct-view display device. In particular, it is best suited to convey information to multiple people at the same time.

Generally, polymer-dispersed liquid crystals and guest-host liquid crystals require a driving voltage that is more than twice that of the TN liquid crystal, but it is extremely difficult to lower this voltage. For example, in a polymer dispersed liquid crystal, increasing the dielectric anisotropy lowers the threshold voltage, but also lowers the specific resistance of the liquid crystal, making the holding operation impossible. In the case of guest-host type liquid crystal, if the amount of the dye is increased, the threshold voltage decreases, but the specific resistance of the liquid crystal also decreases, and the holding operation becomes impossible. Therefore, in the present application, the structure of the TFT, the pixel circuit and the driving method thereof are optimized so that a sufficient driving voltage can be applied to the liquid crystal.

FIG. 1 is an example of an equivalent circuit diagram of the liquid crystal display device of the present invention. TFTs 3 are arranged at the intersections of the signal lines 1 and the scanning lines 2 arranged in a lattice, the drain electrodes (D) are the signal lines, the gate electrodes (G) are the scanning lines, and the source electrodes (S) are the pixel electrodes. Respectively connected to. There is a liquid crystal 5 between the pixel electrode and the counter electrode, and the capacitance of this liquid crystal is C
0 and resistance is R0. Here, the TFT is used as a switch and has a function of writing an image signal in the liquid crystal capacitance and the storage capacitance 6. There are two types of storage capacitors, an additional capacitance system that is added to the preceding scanning line as shown in the figure, and a storage capacitance system that is created between independent storage lines. TF of this embodiment
At T, there is a resistance portion 4 between the drain electrode and the signal line and between the source electrode and the pixel electrode. By optimizing this resistance value, the breakdown voltage of the TFT can be improved and the leak current can be reduced. Specifically, when the TFT is in the ON state, the resistance value between the source and the drain is RT, and the resistance unit 4
If the resistance value of RL is RL, then (RT + 2 · RL) × (Cs + C0) <TH (1). Here, TH is one horizontal scanning period, and for example, in order to write a voltage of 95% or more, the left side of expression (1) must be at least 1/3 or less of TH. On the other hand, if RL is too small, the TFT becomes O
In the FF state, the electric field strength of the drain part cannot be reduced, and the effect of suppressing the leak current is lost. Therefore, the size of RL is usually one tenths of that of RT.
It is practical to have a range of from several times.

In general, polymer-dispersed liquid crystals and guest-host liquid crystals have a smaller specific resistance than TN liquid crystals and cannot hold a written voltage unless a sufficient storage capacitance is added. However, the increase of the storage capacity leads to a decrease of the aperture ratio and the brightness of the screen is reduced, and therefore the size should be set to the minimum necessary. (C0 + Cs) × R0> T (2) may be satisfied as a condition for reducing the contrast ratio and not impairing the brightness. Here, T is one vertical scanning period, and this expression means that the voltage written in the liquid crystal is held at 60% or more, and the effective value is that 70% or more of the voltage is applied to the liquid crystal. Become.

FIG. 3 shows T in order to further improve the aperture ratio.
It is an example of an equivalent circuit of a pixel portion when a resistance portion is provided only on one side of an FT. In general, the resistance portion 34 is provided on the drain side of the TFT 33 to reduce the leak current. Therefore, strictly speaking, in a liquid crystal display device that is premised on AC driving, it must be attached to both sides as shown in FIG. 1, but even if it is attached only to the signal line side in this way, there is a considerable effect. This is because the average potential of the signal line 31 is actually higher than the average potential of the pixel electrode due to the voltage generated by the parasitic capacitance of the TFT and the capacitive coupling between the liquid crystal capacitance and the storage capacitance.

Next, an example of the structure of an actual TFT will be described.
FIG. 5 shows a typical TFT having an LDD structure. The semiconductor thin film deposited on the insulating substrate is divided into three regions according to the impurity implantation concentration. That is, the intrinsic semiconductor region 51, the low concentration impurity implantation region 52 and the high concentration impurity implantation region 53.
Is. Of these, the intrinsic semiconductor region 51 is a channel region of the TFT, and by ion-implanting through the gate insulating film 54 using the gate electrode 55 as a mask, the low-concentration impurity implantation region 52 and the intrinsic semiconductor region 51 are self-aligned.
Is divided into There are cases where an intrinsic semiconductor layer is used as described above and cases where a low-concentration impurity is implanted in the channel portion. Further, if the low-concentration impurity region 52 in this figure is in the same intrinsic semiconductor state as that of the channel portion, it becomes an offset gate TFT. The high-concentration impurity-implanted region 53 needs to have a sufficiently low resistance in order to make ohmic contact with the semiconductor thin film, the transparent conductive film 58, and the metal thin film 57. If the distance between the high-concentration impurity region 53 and the channel portion is unnecessarily large, not only the RL of the formula (1) is increased but also the aperture ratio is lowered. Therefore, the self-alignment should be controlled to the minimum required distance. Is desirable. As a specific method, there is a method of using the side wall portion of the insulating film deposited on the gate electrode as a mask, a method of over-etching the gate electrode after implanting impurities, and the like. The transparent conductive film 58 constitutes a pixel electrode, the metal thin film 57 constitutes a signal line, and these are insulated from the gate electrode 55 by the interlayer insulating film 56.

FIG. 6 is a diagram showing an example of the structure of another TFT. In this example, an impurity semiconductor thin film 63 is deposited in advance in order to reduce the resistance of the contact portion of the TFT, and then a thin semiconductor thin film is deposited to form an intrinsic semiconductor region 63 and a low concentration impurity implantation region 62 to be a channel portion. To form. According to this method, the thickness of the channel portion can be reduced and the electrical characteristics of the TFT can be improved.

Finally, a method of driving this liquid crystal display device will be described. FIG. 7 is an example of a timing chart of drive waveforms for applying a sufficient voltage to the liquid crystal with a small image input signal amplitude. In general, since the liquid crystal needs to be driven by an alternating current, in this example, the potential 73 of the counter electrode is switched between two levels VC1 and VC2 for each field. As a result, the potential 72 of the signal line can be set to almost the same voltage range when writing with positive polarity and writing with negative polarity. The potential 71 of the scanning line is given a selection pulse for one horizontal scanning period once in each field,
The potential VH required to turn on the TFT is maintained. In the non-selection period, two potentials VL1 and VL are synchronized with the counter electrode.
Repeat 2 alternately. This is a driving method in the case where a storage capacitor is added to the previous stage gate, and in the case of a storage capacitance method in which it is added to an independent capacitance line, the same potential as that of the counter electrode may be applied to that capacitance line. The potential 74 of the pixel electrode becomes equal to the potential of the signal line during the selection period, and approaches the potential of the counter electrode with the time constant defined by the left side of the equation (2) during the non-selection period. The voltage actually applied to the liquid crystal is the potential difference between this pixel electrode potential 74 and the counter electrode potential 73, and the counter electrode potential 73 is set so that this voltage is symmetrical in both positive and negative polarities. This is to prevent flicker. Comparing the average potential of the signal line potential 72 and the average value of the counter electrode potential 73, in the case of an N-channel TFT, the latter must be set lower than the former by a certain voltage. This is T
This is because when the FT is turned off, the pixel electrode potential shifts due to capacitive coupling between the parasitic capacitance of the TFT and the liquid crystal capacitance and the storage capacitance, and it is necessary to compensate for this shift voltage.

In general, driving the signal line with low impedance by amplifying the image signal amplitude including halftone increases the load on the external circuit and increases the power consumption. Therefore, in order to suppress the image signal amplitude to a necessary minimum and apply a sufficient voltage to the liquid crystal, the driving voltage is set so as to satisfy the following expression in this embodiment.

2Vth + Vid>VC1-VC2> 2Vth (3) where Vid is the amplitude of the image signal applied to the signal line, and Vth is the threshold voltage of the liquid crystal layer. Under this condition, the levels of the image signals of the positive and negative signal lines overlap each other, the dynamic range of the signal line drive circuit can be minimized, and the drive circuit can be made compact and have low power consumption. Cost reduction is possible.

Although the polarity of the image signal is inverted every one vertical scanning period in FIG. 7, this period may be one horizontal scanning period or an integral multiple thereof. In this case, about half of the pixels always drive the liquid crystal with opposite polarities, which has the effect of suppressing flicker and crosstalk.

In the driving method of the present invention, a TFT made of a non-single-crystal Si thin film or a compound semiconductor thin film, a MOSFET formed on a single-crystal Si substrate, or the like is applied to all scanning lines to turn them on and off. It can be applied to an active matrix type liquid crystal display device using an element.

[0021]

As described above, the liquid crystal display device of the present invention realizes a very bright liquid crystal projector by using polymer dispersion type liquid crystal, and a direct view type of wide viewing angle by using guest-host type liquid crystal. A display can be realized. Moreover, it can be realized at the same cost as the TN type and consumes less power than the TN type.
Since it is similar to a mold, it becomes a practical liquid crystal display device. As for display quality, a high contrast ratio can be obtained because a sufficient drive voltage can be applied, and since leakage current is extremely small, crosstalk does not occur and an image of the highest quality can be obtained.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of a pixel portion of a liquid crystal display device.

FIG. 2 is an equivalent circuit diagram of a pixel portion of a conventional liquid crystal display device.

FIG. 3 is an equivalent circuit diagram of a pixel portion of a liquid crystal display device.

FIG. 4 is a diagram showing applied voltage dependency of transmittance of a liquid crystal display device.

FIG. 5 is a cross-sectional view of a TFT.

FIG. 6 is a cross-sectional view of a TFT.

FIG. 7 is a timing chart showing a driving method of a liquid crystal display device.

[Explanation of symbols]

 1, 21, 31 Signal line 2, 22, 32 Scan line 3, 23, 33 TFT 4, 34 Resistor part 5, 25, 35 Liquid crystal 6, 26, 36 Storage capacitor 51, 61 Intrinsic semiconductor region 52, 62 Low concentration impurity Injection region 53 High-concentration impurity injection region 63 Impurity semiconductor thin film 54, 64 Gate insulating film 55, 65 Gate electrode 56, 66 Interlayer insulating film 57, 67 Metal thin film 58, 68 Transparent conductive film 71 Scan line potential 72 Signal line potential 73 potential of counter electrode 74 potential of pixel electrode

Claims (3)

[Claims] 1. A plurality of scanning lines and signal lines on a first substrate, a thin film transistor having a drain electrode connected to the signal line and a gate electrode connected to the scanning line, and a source electrode of the thin film transistor. A pixel electrode and a storage capacitor are provided, a counter electrode is provided on a second substrate, and a polymer-dispersed or guest-host liquid crystal is sandwiched in a space where the first substrate and the second substrate face each other. A liquid crystal display device comprising: a thin film transistor, wherein a resistance portion made of a semiconductor thin film is provided between the channel portion and the drain electrode and between the channel portion and the source electrode of the thin film transistor, and the resistance value RT of the ON state channel portion of the thin film transistor, Resistance value RL, resistance value R0 of the liquid crystal layer between the pixel electrode and the counter electrode, capacitance value C0, the storage capacitance Cs, 1 horizontal scanning period TH, and 1 vertical scan. A liquid crystal display device characterized in that the following two expressions are established during a check period T. (RT + 2 · RL) × (Cs + C0) <TH (C0 + Cs) × R0> T 2. The liquid crystal display device according to claim 1, wherein the resistance portion formed of the semiconductor thin film exists only between the channel portion and the drain electrode of the thin film transistor. 3. A plurality of scanning lines and signal lines, a thin film transistor arranged at an intersection of the signal line and the scanning line, and a pixel electrode connected to the thin film transistor are provided on a first substrate, and a second electrode is provided. A method for driving a liquid crystal display device, comprising a counter electrode on a substrate, and sandwiching a polymer-dispersed type or guest-host type liquid crystal in a space in which the first substrate and the second substrate face each other. Two potentials are repeatedly applied to the counter electrode with an integral multiple of one horizontal scanning period or one vertical scanning period as a half cycle, and the two potential differences Vc,
A method of driving a liquid crystal display device, wherein the following formula is established between a threshold voltage Vth of the liquid crystal layer and an image signal voltage range Vid applied to the signal line. 2Vth + Vid>Vc> 2Vth