JPH0758335A - Thin-film transistor and liquid crystal display device - Google Patents

Abstract

PURPOSE:To eliminate the influence due to external electric field by a highly resistant semiconductor film so as to reduce the influence on the bending of an energy band of a channel semiconductor layer by adopting the highly resistant semiconductor having a specific band gap for a protective film which is to be arranged on the upper side of the channel semiconductor layer. CONSTITUTION:A protective film 10 is formed on a thin-film transistor part, a source electrode 8 and a drain electrode 9. Then a highly resistant semiconductor film 10b whose optical band gap 1.9-3.0eV is formed on the film 10. Thus, the influence due to external electric field can be eliminated by the highly resistant semiconductor film, so that it is possible to reduce its influence on the bending of the energy band of a channel semiconductor layer. Since the highly resistant semiconductor film has a high resistance property, no direct leakage through the protective film is generated in stable OFF-state property of the thin-film transistor. On the other hand, current control by using gate electrode electric field is performed against the channel semiconductor layer, thereby hardly reducing the ON-state current.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor and a liquid crystal display device using a thin film transistor as a switching element, and more particularly to the structure of the thin film transistor.

[0002]

2. Description of the Related Art As a liquid crystal display device for displaying characters or symbols, a large number of row electrodes are arranged on two substrates so as to extend in one direction at a predetermined pitch, and to extend in one direction at a predetermined pitch. A matrix type in which a liquid crystal composition is sandwiched between two substrates is generally used, in which a plurality of column electrodes arranged in a row and a minimum section surrounded by the row electrodes and the column electrodes are pixels. In terms of the function of the liquid crystal display device, there are a twisted nematic type with a 90-degree twist and a super twisted nematic type with a birefringence effect of a 180-360 degree twist.

Further, as a large-capacity, high-definition liquid crystal display device for television display, graphic display, etc., a switching element is arranged for each pixel to drive and control, and high-contrast display without crosstalk is possible. An active matrix liquid crystal display device is also used. As a switching element of this active matrix type liquid crystal display device, a three-terminal type thin film transistor is often used because it can perform a transmissive display and can easily have a large area.

Further, many thin film transistors use amorphous silicon for their semiconductor active layers.
Further, the thin film transistor often has an inverted staggered structure in which a gate electrode is arranged in a lower layer and a source electrode and a drain electrode are arranged in an upper layer with an amorphous silicon layer interposed therebetween.

[0005]

In such an inverted staggered thin film transistor, the thin film transistor malfunctions due to a field effect due to external charges through the protective insulating film formed on the channel semiconductor layer between the source electrode and the drain electrode. There is a risk of This problem can be prevented by further providing a fourth electrode on the protective insulating film to control the potential.

However, such a measure is not a very effective means because it causes an increase in the number of manufacturing steps and a decrease in yield due to an interlayer short circuit. Therefore, it is the current situation that a large margin is required for the driving condition so as to prevent such a malfunction.

The present invention has been made in view of the above problems, and a thin film transistor which is not affected by a field effect due to an external charge without increasing a manufacturing process load and reducing a yield, and a thin film transistor using the same. The present invention aims to provide the liquid crystal display device.

[0008]

According to the present invention, there is provided a gate electrode formed on a substrate, a channel semiconductor layer formed on the gate electrode via a gate insulating film, and a central portion on the channel semiconductor layer. A channel protective film formed on
At least a source electrode and a drain electrode formed on both ends of the channel semiconductor layer via a low resistance semiconductor film, and a protective film formed on a surface including the channel protective film and the source electrode and the drain electrode. In the thin film transistor, the band gap 1.9 to 3.0 in at least one of the channel semiconductor layer and the protective film.
A thin film transistor having a high resistance semiconductor film of eV,
Further, it is an active matrix type liquid crystal display device using this thin film transistor.

[0009]

The present invention uses the high resistance semiconductor film having a band gap of 1.9 to 3.0 eV as the protective film disposed on the channel semiconductor layer, so that the influence of the external electric field is absorbed by the high resistance semiconductor film. The influence on the bending of the energy band of the semiconductor layer can be reduced. Since this high-resistance semiconductor film has high resistance, there is no problem of direct leakage through the protective film, so that the off characteristics of the thin film transistor are stable. On the other hand, since the current control by the electric field of the gate electrode is performed on the channel semiconductor layer, the on-current does not decrease.

Further, since the formation of this high-resistance semiconductor film is formed as a part of the manufacturing process of a normal thin film transistor, there is no particular increase in the manufacturing process load and the yield is reduced, and the electric field effect due to external charges is not caused. It is possible to easily manufacture a thin film transistor which is not affected by the above and a liquid crystal display device using the thin film transistor.

[0011]

EXAMPLE An example in which the present invention is applied to an active matrix type liquid crystal display device will be described in detail below. FIG. 1 is a schematic configuration diagram showing a portion including a thin film transistor of an active matrix type liquid crystal display device to which the present invention is applied.

In FIG. 1, for example, 70 manufactured by Corning Co.
On the main surface of the transparent glass insulating substrate 1 made of 59, a molybdenum / tantalum film is formed by a normal sputtering method and processed into a predetermined shape by a photolithography method to form a gate electrode 2. Next, a silicon oxide film 3a having a film thickness of 0.3 μm is formed by a normal temperature thermal CVD method at a substrate temperature of 400 ° C. so as to cover the gate electrode 2, and a silicon nitride film having a film thickness of 0.05 μm is formed by a plasma CVD method at a substrate temperature of 350 ° C. 3b are sequentially formed to form the gate insulating film 3. Further, a channel semiconductor layer 4 made of amorphous silicon having a film thickness of 0.05 μm is formed, and subsequently,
A silicon nitride film is formed as the channel protection film 5 and processed into a predetermined shape.

Next, a low resistance semiconductor film 6 having a film thickness of, for example, 0.05 μm is formed, and the channel semiconductor layer 4 and the low resistance semiconductor film 6 are formed.
Are processed into a predetermined shape to form a channel region at the central portion of the channel semiconductor layer 4 and source and drain regions at both end portions. Further, in the pixel portion adjacent to the channel semiconductor layer 4 on the gate insulating film 3, ITO made of indium tin oxide is formed and processed into a predetermined shape to form the pixel electrode 7. Next, a metal layer made of, for example, chromium or aluminum is formed so as to cover the entire channel semiconductor layer 4 including the channel protective film 5, and processed so that the central portion of the channel protective film 5 is exposed, and the source electrode 8 is formed.
And the drain electrode 9 is formed. Note that one end of the source electrode 8 is connected to the pixel electrode 7.

A protective film is formed on the thin film transistor portion, the source electrode 8 and the drain electrode 9 formed as described above.
Forming 10. The protective film 10 is, for example, a plasma CV.
Silicon nitride insulating film 10a having a film thickness of 0.1 μm and a film thickness of 0.
High resistance semiconductor film 10b made of 1 μm SiCx and film thickness 0.1
It has a laminated structure of a silicon nitride insulating film 10c of μm. Then, an alignment film 16 made of low temperature cure type polyimide is deposited on the entire surface of these, and an array substrate including a thin film transistor as a switching element and a pixel electrode is completed.

On the other hand, a common electrode 14 made of ITO and an alignment film 17 made of polyimide are deposited on another substrate 13,
A counter substrate is prepared. Note that the alignment films 16 and 17 on both substrates are subjected to rubbing alignment treatment by rubbing with cloth or the like in one predetermined direction. The array substrate and the counter substrate thus prepared are arranged facing each other at a predetermined interval so that the rubbing orientations of the alignment film 16 and the alignment film 17 are orthogonal to each other, and some of the injection ports (not shown) are provided. The remaining portion is bonded and fixed at the peripheral portion of the substrate. In addition, in order to keep the distance between both substrates constant, a predetermined spacer is dispersed between both substrates. Then, for example, a nematic liquid crystal composition 18 is injected between both substrates through the injection port, and finally the injection port is also sealed. A polarizing plate 19 is arranged outside the array substrate, and a polarizing plate 20 is arranged outside the counter substrate 13 in accordance with the rubbing alignment of the alignment film set such that the good viewing angle direction is the front direction. Further, an illuminating device is arranged outside one of the substrates (not shown) to effectively operate the display function of the liquid crystal display device.

Next, a method of manufacturing the high resistance semiconductor film 10b included in the protective film 10 of the thin film transistor in the above embodiment will be described. The reaction chamber for forming the high-resistance semiconductor film 10b is equipped with a circular high-frequency electrode having a diameter of 30 cm and a ground electrode facing the circular high-frequency electrode, a gas supply system for SiH ₄ , CH ₄ , and H ₂ and a turbo molecular pump and a rotary pump. Is connected to the exhaust system. The insulating substrate 1, which is a sample, is clamped to a heated ground electrode, and the surface temperature of the substrate is controlled to a desired temperature.

In such a reaction chamber, SiH ₄ 36sccm,
CH ₄ 24sccm and H ₂ 300sccm are introduced, and these gases are exhausted through a turbo molecular pump and a rotary pump. At this time, the pressure in the reaction chamber is controlled to 0.8 Torr by adjusting the opening of the exhaust valve. In this state, a high frequency of 13.56 MHz and 100 W is applied to the high frequency electrode to generate glow discharge, and silicon carbide SiCx is deposited on the insulating substrate 1. In this case, the content of C in the film is X = 0.06.
And the optical bandgap was 2.0 eV.

The content of C can be adjusted by increasing or decreasing the gas flow rate, but in order to reduce the influence on the bending of the energy band of the channel semiconductor layer, which is the object of the present invention, the optical band gap is 1.9 to. It works effectively in the range of 3.0 eV. As in this embodiment, the high resistance semiconductor film 10
When a silicon carbide SiCx film is used as b, the most effective region of the optical band gap is 1.9 to 2.4 eV. Note that the same effect is effectively exerted on other high resistance semiconductor films such as SiNx and SiOx. Further, NH ₃ or N ₂ O can be used as the source gas instead of CH ₄ , but in this case, the gas flow rate is adjusted so that the film formation does not become an insulator and the N and O contents are not changed. It is important to form a Si-rich film with a reduced content.

In the above embodiments, the protective film 10 of the thin film transistor is composed of the insulating film 10a, the high resistance semiconductor film 10b and the insulating film 10c.
Although the example having the three-layer structure described above has been described, the same effect can be obtained even with a single-layer structure having only a high-resistance semiconductor film or a two-layer structure having an insulating film and a high-resistance semiconductor film. Further, the portion to which this high resistance semiconductor film is applied may be not only the protective film 10 but also the channel protective film 5. That is, it goes without saying that various modifications may be made within the range of satisfying the conditions of the present invention.

Furthermore, in the above embodiments, an example in which the thin film transistor of the present invention is applied to a liquid crystal display device has been described, but the present invention is not limited to this and can be applied to a contact sensor using a thin film transistor.

[0021]

As described above, according to the present invention, by using a high resistance semiconductor film having a band gap of 1.9 to 3.0 eV as a protective film covering the upper part of the channel semiconductor layer of the inverted staggered type thin film transistor, it is The influence can be absorbed by the high resistance semiconductor film, and the influence on the band bending of the channel semiconductor layer can be reduced. Therefore, it is possible to easily manufacture a thin film transistor that is not easily affected by the electric field effect due to external charges without increasing the burden of the manufacturing process or affecting the yield.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a portion including a thin film transistor of an active matrix type liquid crystal display device to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Insulating substrate 2 ... Gate electrode 3 ... Gate insulating film 4 ... Semiconductor layer 5 ... Channel protective film 6 ... Low resistance semiconductor region 7 ... Pixel electrode 8 ... Source electrode 9 ... Drain electrode 10 ... Protective film

Claims (2)

[Claims] 1. A gate electrode formed on a substrate, a channel semiconductor layer formed on the gate electrode via a gate insulating film, and a channel protective film formed at a central portion on the channel semiconductor layer. And at least a source electrode and a drain electrode formed on both ends of the channel semiconductor layer with a low resistance semiconductor film interposed therebetween, and a protective film formed on a surface including the channel protective film and the source electrode and the drain electrode. A thin film transistor having a high resistance semiconductor film having a band gap of 1.9 to 3.0 eV in at least one of the channel semiconductor layer and the protective film. 2. A plurality of row electrodes arranged to extend in one direction at a predetermined pitch, a plurality of column electrodes arranged to extend in one direction at a predetermined pitch, and the row electrodes and column electrodes. Each pixel is provided with a thin film transistor as a switching element, and this thin film transistor has a gate electrode, a channel semiconductor layer formed on the gate electrode via a gate insulating film, and a channel. A channel protective film formed in a central portion of the semiconductor layer, a source electrode and a drain electrode formed at both ends of the channel semiconductor layer with a low resistance semiconductor film interposed therebetween, the channel protective film, the source electrode and the drain. In a liquid crystal display device comprising at least a protective film formed on a surface including an electrode, the channel semiconductor layer of the thin film transistor or the A liquid crystal display device comprising a high-resistance semiconductor film having a band gap of 1.9 to 3.0 eV on at least one of protective films.