JPH11186553A - Active matrix type liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce fluctuations of threshold voltage by forming a gate insulating layer with at least three layers which have differing band gaps and using a thin-film transistor, of which threshold voltage is shifted in the positive direction as a switching element. SOLUTION: A gate insulating layer is formed of a gate insulating layer 3 and a gate insulating layer 2, a gate insulating layer 4 is formed of a silicon film and a gate insulating layer 1 is formed of a silicon oxide film with the band gap sequentially reduced from the insulating layer 1 to 4. When a threshold control voltage is applied, a gate electrode 2 becomes positive for the drain electrode 9. Therefore electrons in the semiconductor layer pass through the gate insulating layer 1 having a large band gap and is then trapped by the insulating layer 2 having small band gap, and the threshold voltage of thin film transistor is shifted to the positive side, showing the enhancement type characteristic. When threshold voltage applied to each thin-film transistor is adjusted individually, threshold voltage of each transistor can be shifted to the same value and the fluctuations of threshold voltage can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an active matrix type liquid crystal display device having a wide viewing angle and low power consumption.

[0002]

2. Description of the Related Art An active matrix type liquid crystal display device using a switching element represented by a thin film transistor (TFT) has begun to be widely used as a display terminal of OA equipment and the like. The display method of this liquid crystal display device is roughly classified into the following two types. One is a transparent electrode 2
In this method, the liquid crystal is sandwiched between a plurality of substrates, operated by a voltage applied to the transparent electrode, and light transmitted through the transparent electrode and incident on the liquid crystal is modulated for display. The other is to sandwich the liquid crystal between two substrates and operate the liquid crystal by an electric field substantially parallel to the substrate surface between two electrodes (pixel electrode and counter electrode) formed on one substrate, This method modulates light incident on the liquid crystal from the gap between the two electrodes and displays the light. It has features such as a wide viewing angle and a low load capacitance, and is a promising technology for an active matrix liquid crystal display device. Hereinafter, the latter method is called a horizontal electric field method.

[0003] The lateral electric field method has the above characteristics,
Since the opaque electrode is formed in the shape of a comb, the aperture area through which light can be transmitted is small, and the display screen is dark. Therefore, there is a problem in that a bright backlight having high power consumption needs to be used.

On the other hand, as described in Japanese Patent Application No. 6-199247, the role of the counter electrode wiring for supplying a voltage to the counter electrode from the outside is also used for the scanning electrode wiring, so A method has been proposed in which the electrode wiring is omitted and the opening area of the lateral electric field method is increased. Hereinafter, the above technique will be referred to as a counter-electrode wiring-less horizontal electric field technique.

[0005]

In the lateral electric field method without the counter electrode wiring, a thin film transistor as a switching element has a threshold voltage higher than a maximum liquid crystal operating voltage required for optical modulation of the liquid crystal. It must exhibit enhancement-type switching characteristics. As a method of realizing a thin film transistor exhibiting an enhancement type switching characteristic, as described in Japanese Patent Application No. 6-199247, a method of thinning an amorphous silicon semiconductor layer which is a semiconductor layer, or a gate scanning electrode of the thin film transistor A method of controlling the voltage of the back electrode provided at a position opposed to the above.

However, any of the methods has a problem that the threshold voltage varies greatly, and the display quality of the active matrix type liquid crystal display device of the lateral electric field type without the counter electrode wiring is low.

SUMMARY OF THE INVENTION An object of the present invention is to provide an active matrix type liquid crystal display device of the horizontal electric field type without a common electrode wiring which has high image quality using a thin film transistor having a small variation in threshold voltage and a simple structure. .

[0008]

According to the present invention, as a thin film transistor used for a switching element of an active matrix type liquid crystal display device, a gate electrode is formed on a transparent glass substrate, and three layers having different band gaps are formed on the gate electrode. A thin film transistor is used in which a gate insulating film including the above is formed, and a semiconductor layer to which a source electrode and a drain electrode are connected through a contact layer is formed over the gate insulating layer. The gate insulating layer 1, the gate insulating layer 2, and the gate insulating layer 3 were formed from the side closer to the semiconductor layer, and their band gaps were Eg1, Eg2, and Eg.
Assuming that 3, Eg1>Eg3> Eg2.

The material of these layers includes silicon oxide and SiN _x . The band gap of silicon oxide is 7 eV, and the band gap of SiN _x increases with the increase of x, and is 5.3 eV when x = 1.3 and 4.5 eV when x = 1. Therefore, the structure of the present invention can be manufactured by using silicon oxide for the gate insulating layer 1, SiN _{x having} a small x for the gate insulating layer 2, and SiN _x having a large _{x for} the gate insulating layer 3.

Regarding the composition x of SiN _x , the plasma C
When forming a film by VD (Chemical Vapor Deposition) method,
It can be controlled by changing the ratio of the source gas. That is, when SiH ₄ and NH ₃ gas are used as source gases, x can be increased by increasing NH ₃ / SiH ₄ .

As a configuration similar to the present invention, MNOS (Me
tal Nitride Oxide Semiconductor). Regarding the MNOS structure, for example,
It is described in JP-A-4-334063. In this structure, silicon oxide and silicon nitride are stacked as a gate insulating film from the side closer to the semiconductor layer. However, in the conventional invention, the silicon oxide film thickness corresponding to the gate insulating layer 1 of the present invention is 20 ° or less. In the present invention, the threshold value of the transistor is stabilized by setting the thickness of the insulating layer 1 to 30 ° or more.

Further, according to the present invention, the threshold voltage of the thin film transistor is controlled as follows. With the drain electrode or the drain electrode and the source electrode grounded, a positive threshold control voltage sufficiently higher than the liquid crystal operating voltage (about ± 10 V) is applied to the gate electrode. Further, when the thin film transistors of the present invention are arranged in a matrix at the intersection of the scanning (gate) electrode wiring and the signal (drain) electrode wiring in the active matrix liquid crystal display device, the threshold voltage of each thin film transistor is equal. In order to achieve this, the above-described threshold control voltage is individually applied between the gate / drain electrodes of each thin film transistor using, for example, a line sequential driving method. At this time, in order to reduce variation in threshold voltage, control is performed while monitoring the luminance distribution of the liquid crystal display device, thereby ensuring uniformity of display quality.

When the threshold control voltage is applied as described above, the gate electrode has a positive voltage with respect to the drain electrode, so that the electrons in the semiconductor layer penetrate through the gate insulating layer 1 having a large band gap and have a large band gap. Trapped in the small insulating layer 2. By the action of the electrons trapped in the gate insulating layer 2, the threshold voltage of the thin film transistor shifts to the plus side, so that the thin film transistor exhibits enhancement-type characteristics.

As the threshold control voltage increases, the amount of trapped electrons in the gate insulating layer 2 increases, and the threshold voltage increases. Therefore, if the threshold control voltage applied to each thin film transistor is individually adjusted, the threshold voltage of each thin film transistor can be shifted to the same value.
Variation in threshold voltage can be significantly reduced. On this occasion,
When the gate insulating layer 2 having a small band gap is thickened, light absorption increases and the transmittance of the film decreases.

For this reason, by setting this layer to 1000 ° or less and forming the gate insulating layer 3 having a large band gap to be thick, a laminated gate insulating layer having high transmittance and high electric breakdown voltage can be obtained.

Further, in the thin film transistor of the present invention,
Since the thickness of the gate insulating layer 1 is 30 ° or more, when used for a switching element of the in-plane switching method without the counter electrode wiring, the threshold value does not fluctuate during the display operation and stable enhancement-type characteristics are provided. it can. Hereinafter, this operation will be described in detail.

In general, when a voltage having a polarity opposite to that described above is applied between the gate / drain electrodes of a thin film transistor having an MNOS structure so that the gate electrode becomes negative, electrons in the silicon nitride film penetrate through the silicon oxide film to form a semiconductor. The threshold voltage shifts to the minus side because the ions are released into the layer or holes in the semiconductor layer penetrate the silicon oxide film and are trapped in the silicon nitride film. When the thin film transistor of the present invention is used as a switching element of a lateral electric field method without a counter electrode wiring, a gate / gate is used during a display operation.
A bipolar voltage of about the liquid crystal operating voltage (about ± 10 V) is applied between the drain electrodes, and it is required that the threshold voltage of the thin film transistor does not fluctuate due to the applied voltage.

FIG. 7 shows the dependency of the absolute value of the minimum gate / drain applied voltage (hereinafter referred to as threshold shift start voltage) at which the threshold voltage shift of the thin film transistor of the present invention occurs, on the silicon oxide film thickness. . Silicon oxide film thickness is 20
In the case of Å or less, the threshold shift start voltage is about 5 V, while in the case of a silicon oxide film thickness of 30 Å or more,
The threshold shift start voltage is equal to or higher than the maximum value of the liquid crystal operation voltage.

This is because when the silicon oxide film thickness is less than 20 °, electrons or holes pass through the thin silicon oxide film by a tunnel effect when a voltage of about 5 V is applied, whereas when the silicon oxide film thickness is more than 30 °. This is because a higher electric field is required for electrons or holes to pass through the silicon oxide film. Therefore, when a conventional thin-film memory transistor having a silicon oxide film thickness of 20 ° or less is used as a switching element of a lateral electric field method without a counter electrode wiring, the liquid crystal operation voltage applied between the gate / drain electrodes during the display operation makes it easy. Therefore, stable enhancement-type characteristics cannot be provided.

On the other hand, in the thin film transistor of the present invention, the threshold voltage once shifted to the positive side is maintained constant without fluctuating during the display operation. A liquid crystal display device can provide stable enhancement-type characteristics. In particular, by reducing the band gap of the gate insulating layer 2 that retains electric charges, the potential barrier can be increased and the loss of electric charges can be reduced; thus, a thin film transistor with a stable threshold can be manufactured.

Further, in the active matrix display device of the present invention, the threshold voltage can be made uniform by controlling the threshold value while monitoring the display luminance. At the time of normally black display, a high-quality active matrix display device having uniform threshold characteristics can be provided by further applying a threshold control voltage to a portion having low luminance.

[0022]

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a cross-sectional structure of a thin film transistor used in the present invention. 1 is a glass substrate, 2
Is a gate electrode made of Al or Cr, 3 is a gate insulating layer 3 made of a silicon nitride film, 4 is a gate insulating layer 2 made of a silicon nitride film, 5 is a gate insulating layer 1 made of a silicon oxide film, and 6 is amorphous. A semiconductor layer 7 made of silicon, 7 is a contact layer made of n + type amorphous silicon doped with phosphorus, 8 and 9 are source and drain electrodes made of Cr, and 10 is a passivation film made of a silicon nitride film.

The thin film transistor having the above structure was manufactured as follows. First, an Al film or a Cr film having a thickness of about 300 nm is formed on a Corning 7059 glass substrate 1 by a sputtering method. The gate electrode 12 is formed by patterning Al by photoetching. Si on top
Plasma CVD using a mixed gas of H ₄ , NH ₃ , N _2, etc.
A gate insulating layer 33 made of a 2500-nm-thick SiN _x film is formed by the method. At this time, a SiN _1.3 film having a band gap of 5.3 eV with NH ₃ / SiH ₄ being 3 was obtained.

Further, a gate insulating layer 24 of 200 ° SiN _x was formed by a plasma CVD method. At this time, NH ₃ / SiH ₄ was set to 2 and the band gap was 4.5 e.
A SiN ₁ film of V was obtained. A gate insulating layer 15 made of a silicon oxide film having a thickness of 100 ° is formed thereon by a plasma CVD method using a mixed gas such as TEOS (tetraethyl orthosilicate) and O ₂ . On top of that, Si
2000 thickness by plasma CVD using H ₄ gas
An amorphous silicon film having a thickness of 膜 and an n + -type amorphous silicon film having a thickness of 300 に よ り are formed by a plasma CVD method using a mixed gas of SiH ₄ and PH ₃ . It is desirable that the thin film forming process using the plasma CVD method be continuously performed without breaking the vacuum.

The semiconductor layer 6 is formed by subjecting the amorphous silicon film to island processing simultaneously with the n + type amorphous silicon film by photoetching. The source electrode 8 and the drain electrode 9 are formed by patterning Cr deposited thereon by photo-etching using a sputtering method. Further, the contact layer 7 is formed between the semiconductor layer 6 and the source electrode 8 and the drain electrode 9 by etching away the n + type amorphous silicon film between the source / drain electrodes. Further, a 5000 nm thick silicon nitride film deposited by a plasma CVD method thereon is patterned by photoetching to form a protective insulating film 10, whereby a thin film transistor is completed.

The threshold voltage of the thin film transistor manufactured as described above was controlled as follows. That is,
With the drain electrode or the drain electrode and the source electrode grounded, a positive voltage of +80 V was applied to the gate electrode for 2 seconds.

FIGS. 2A and 2B show the gate voltage dependence (Id-Vg characteristics) of the drain current before and after the threshold voltage control in the thin film transistor of the present invention. The above voltage application increases the threshold voltage from 2V to 10V,
It began to show enhancement-type characteristics.

Next, the change with time of the threshold voltage of the thin film transistor whose threshold voltage was shifted as described above was examined. In order to simulate the situation where the thin film transistor of the present invention is used as a switching element of an in-plane switching type active matrix liquid crystal display device, a rectangular AC voltage of ± 15 V and ± 10 V is applied to a gate electrode and a drain electrode, respectively.
The threshold voltage was measured at regular intervals.

FIG. 3 shows the change over time of the threshold voltage measured as described above. For comparison, the gate insulating layer 1
Also, the results of a thin film transistor having a conventional structure without the gate insulating layer 2 and a thin film memory transistor having a silicon oxide film thickness of 20 ° of the gate insulating layer 1 are shown. While the threshold voltage of the thin film transistor of the present invention does not change for 10 ⁴ hours or more, the threshold voltage of the conventional thin film transistor monotonically decreases in a short time. In a thin-film memory transistor, the threshold voltage greatly varies depending on the applied voltage.

In this embodiment, the semiconductor layer 6 made of amorphous silicon is formed continuously on the gate insulating layer 15 made of a silicon oxide film, but the surface of the gate insulating layer 15 made of a silicon oxide film is formed. Is exposed to N ₂ plasma for a certain period of time, the semiconductor layer 6 is formed. As a result, in the dependency of the drain current on the gate voltage in FIG. In addition, the switching characteristics of the thin film transistor have been improved.

In the present invention, the thin film transistor has an inverted staggered structure, but may have a normal staggered structure or a coplanar structure. Also, the semiconductor layer may be other than amorphous silicon, for example, polysilicon or microcrystalline silicon.

(Embodiment 2) FIG. 4 shows a sectional structure of a thin film transistor of this embodiment. A gate electrode 2 made of Al or Cr was formed on a glass substrate 1 in the same manner as in Example 1. A gate insulating layer 4 made of silicon oxide is formed thereon.
11 is 5 by plasma CVD using TEOS and O _2.
It was formed to a thickness of 00 °. Then, a gate insulating layer 3 made of SiN _1.3 was formed to a thickness of 1500 ° in the same manner as in Example 1. Further, the gate insulating layer 2, the gate insulating layer 1,
A semiconductor layer, a contact layer, a source electrode and a drain electrode, and a passivation film were sequentially formed and processed in the same manner as in Example 1.

The threshold voltage of the thin film transistor manufactured as described above was controlled in the same manner as in the first embodiment.
That is, while the drain electrode or the drain electrode and the source electrode are grounded, a positive voltage of +80 V is applied to the gate electrode for 2 hours.
For 2 seconds.

FIGS. 5A and 5B show the gate voltage dependence (Id-Vg characteristics) of the drain current before and after the threshold voltage control in the thin film transistor of the present invention. The above voltage application increases the threshold voltage from 2V to 11V,
It began to show enhancement-type characteristics.

In the present invention, the thin film transistor has an inverted staggered structure, but may have a normal staggered structure or a coplanar structure. Also, the semiconductor layer may be other than amorphous silicon, for example, polysilicon or microcrystalline silicon.

(Embodiment 3) FIGS. 6 (a) and 6 (b) show planes of a pixel portion in a TFT substrate of a lateral electric field type liquid crystal display device without an opposing electrode, using a thin film transistor of the present invention as a switching element. 1 shows a configuration and a cross-sectional configuration. The thin film transistor 12 of the present invention has a scanning electrode (gate electrode) 1
3, a signal electrode (drain electrode) 14, a pixel electrode (source electrode) 15, a gate insulating film 1 composed of a laminated film of three or more layers
6 and a semiconductor layer 17 made of amorphous silicon. The scanning electrode 13 was formed in the lowermost layer, and the signal electrode 14 and the pixel electrode 15 were formed by patterning the same metal layer via the gate insulating film 16 and the semiconductor layer 17. The storage capacitor 18 is composed of the pixel electrode 15 and the scanning electrode 1 of the previous row.
9, a gate insulating film 16 is formed from a stacked film of a silicon oxide film and a silicon nitride film.

The orientation direction of the liquid crystal layer is determined by the scanning electrode 19 in the preceding row.
Is controlled by an electric field applied between the projection 20 extending in the direction of the signal electrode and the pixel electrode 15 extending in parallel between the projections 20. The light passes between the protruding portion 20 and the pixel electrode 15, enters the liquid crystal layer, and is modulated.
In the counter-electrode-less horizontal electric field type liquid crystal display device, there is no counter electrode wiring because the scanning electrode 19 in the preceding row also serves as the counter electrode wiring. Therefore, the aperture ratio is about 10 times smaller than that of a conventional horizontal electrolytic liquid crystal display device having a counter electrode.
% Can be improved.

An alignment film was formed on the above-mentioned substrate, an opposing substrate was bonded via a spacer, and liquid crystal was sealed to produce a liquid crystal display device. The brightness was improved by about 10%.

[0039]

According to the in-plane switching mode liquid crystal display device using the thin film transistor of the present invention as a switching element, since the threshold voltage of the thin film transistor is uniform without variation and high in stability, the display quality is improved. .

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a thin film transistor used in the present invention.

FIG. 2 shows Id-Vg of a thin film transistor used in the present invention.
Characteristic diagram.

FIG. 3 is a characteristic diagram showing a change over time of a threshold voltage of a thin film transistor used in the present invention.

FIG. 4 is a sectional structural view of a thin film transistor used in the present invention.

FIG. 5 shows the Id-Vg of the thin film transistor used in the present invention.
Characteristic diagram.

FIGS. 6A and 6B are a plan view and a cross-sectional configuration of a pixel portion of a liquid crystal display device of the present invention.

FIG. 7 is a characteristic diagram showing a dependency of a threshold shift start voltage on a silicon oxide film thickness.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Glass substrate, 2 ... Gate electrode, 3 ... Gate insulating layer 3, 4 ... Gate insulating layer 2, 5 ... Gate insulating layer 1, 6 ... Semiconductor layer, 7 ... Contact layer, 8 ... Source electrode, 9 ... Drain electrode 10, passivation film, 11 gate insulating layer 4, 12 thin film transistor of the present invention, 13 scan electrode (gate electrode), 14 signal electrode (drain electrode), 1
5: pixel electrode (source electrode), 16: gate insulating film composed of three or more layers, 17: semiconductor layer, 18: storage capacitor,
19: scanning electrode of the previous row, 20: projection part.

Claims (7)

[Claims] A liquid crystal is sandwiched between two substrates, and a plurality of pixel portions are constituted by scanning electrodes (gate electrodes) and signal electrodes (drain electrodes) arranged in a matrix on the one substrate. A thin film transistor including a gate insulating layer and a semiconductor layer is formed in the pixel portion, and a pixel electrode (source electrode) is connected to the thin film transistor,
An active matrix for operating a liquid crystal by an electric field substantially parallel to a substrate surface between the pixel electrode and at least one of the scanning electrodes, and modulating and displaying light incident on the liquid crystal from a gap between the two electrodes. In the active matrix type liquid crystal display device, the gate insulating layer is composed of at least three layers having different band gaps, and a thin film transistor having a threshold voltage shifted in a positive direction is used as a switching element. 2. The active matrix liquid crystal display device according to claim 1, wherein the gate insulating layer is a gate insulating layer, a gate insulating layer, and a gate insulating layer in order from the side in contact with the semiconductor layer. An active matrix liquid crystal display device comprising a thin film transistor, wherein the band gap of 1 is larger than the other two layers, and the band gap of the gate insulating layer 2 is smaller than the other two layers. 3. The active matrix type liquid crystal display device according to claim 2, wherein the gate insulating layer 1 has a thickness of 30 ° or more. 4. The active matrix type liquid crystal display device according to claim 2, wherein said gate insulating layer 1 is made of silicon oxide. 5. The active matrix type liquid crystal display device according to claim 2, wherein the gate insulating layer 2 and the gate insulating layer 3 are provided.
An active matrix type liquid crystal display device comprising a thin film transistor, wherein X of the gate insulating layer 2 is smaller than X of the gate insulating layer 3 using SiN _x . 6. The active matrix type liquid crystal display device according to claim 1, further comprising a thin film transistor, wherein the semiconductor layer is made of amorphous Si. 7. An active matrix type liquid crystal display according to claim 1, wherein the threshold voltage of the thin film transistor used as a switching element is larger than the maximum value of the liquid crystal operating voltage. apparatus.