JPH0262077A - Thin film transistor - Google Patents

Abstract

PURPOSE:To improve a characteristic and uniformity by making a operating semiconductor layer of a thin film transistor double-layered, while an electric characteristic is mainly held by a first layer and a second layer plays an auxiliary role. CONSTITUTION:A gate electrode 2 is formed on a transparent insulating substrate 1 while forming a gate electrode 3. An operating semiconductor layer 4 is made a double-layer construction of a non-doped a-Si layer 4a connecting to the gate insulating film 3 and of a second semiconductor layer 4b having a band gap larger than a-Si, accordingly lower photosensitivity to white light than a-Si such as an a-Si1-xCx layer (carbon-doped amorphous silicon) layer and an a-Si1-xNx (nitrogen amorphous silicon) layer. The film thickness of the layer 4a is made thin less than 300Angstrom for making a photocurrent, while the film thickness of the layer 4b is made sufficiently thicker than that of the layer 4a in order to form both together of the required film thickness as the operating semiconductor layer 4. Accordingly, photoabsorption can be made extremely little.

Description

[Detailed Description of the Invention] [Summary] Regarding the structure of a thin film transistor for driving a liquid crystal, the present invention aims to provide an improved structure of a thin film transistor having good on/off characteristics without increasing the number of manufacturing steps. In the structure of a thin film transistor in which a gate electrode is provided on one main surface side of an active semiconductor layer via a gate insulating film, and a source and drain electrode are provided on the other main surface side, the active semiconductor layer is
The structure has a two-layer structure including a non-doped amorphous silicon layer disposed on the gate insulating film side and a second semiconductor layer having a larger band gap than the non-doped amorphous silicon layer disposed on the opposite side.

[Industrial Application Field] The present invention relates to the structure of a thin film transistor for driving a liquid crystal.

Liquid crystal display devices are popular among pocket TVs because they have characteristics such as low power consumption, light weight, and easy color display.

It is gaining a wide market as a flat display device for OA terminal equipment and the like. In particular, active matrix display devices driven by thin film transistors are capable of displaying clear gradations.
There are high expectations for the future.

In active matrix display devices, it is necessary to form the drive TPT element with uniform characteristics over a large area and without defects, and a major breakthrough is needed in terms of manufacturing technology. .

[Conventional technology]

Conventionally, thin film transistors for driving liquid crystals have a stacked structure of a gate electrode made of a metal film, a gate insulating film such as a 5iN (silicon nitride) film, and an active semiconductor layer made of a-3i (amorphous silicon) on a transparent insulating substrate. The non-doped a-3i layer, which is the operating semiconductor layer formed in the above, absorbs white light well, so when light is incident, the off-state current increases and the on/off characteristics as a driving TPT switch deteriorate. do.

As a means to solve this problem, the thickness of the a-3i layer was increased to 300 mm.
It is common practice to make the film as thin as possible and to form a light-shielding film.

[Problem to be solved by the invention]

However, when the thickness of the a-3i layer is reduced as described above,
The uniformity and reproducibility of the film thickness of the A-3I layer over a large area and other properties are reduced, and the number of steps required to form a light-shielding film is increased, leading to a decrease in manufacturing yield and making it difficult to reduce costs. It became.

SUMMARY OF THE INVENTION In order to eliminate these drawbacks, it is an object of the present invention to provide an improved structure of a thin film transistor having good on/off characteristics without increasing the manufacturing process.

[Means to solve the problem]

As shown in FIG.
Si layer-xcx layer (carbon doped amorphous
2 with a second semiconductor layer 4b, such as a silicon layer or an a-3i NX (nitrogen-doped amorphous silicon) layer, which has a larger band gap than a-3i and therefore has lower photosensitivity to white light than a-3i. It has a layered structure.

The thickness of the non-doped Me3i layer 4a is made as thin as 300 Å or less in order to reduce the photocurrent, as in the conventional case, and the thickness of the second semiconductor layer 4b is made sufficiently thicker than the non-doped A-3iN4a. The active semiconductor layer 4 is formed to a required thickness.

With this configuration, the non-doped semiconductor layer 4a
For example, it is possible to have a thickness of 100 Å or less,
Therefore, light absorption can be extremely reduced.

[For production]

Gate insulating film 3 that determines the main characteristics of thin film transistors
The interface characteristics between the gate insulating film 3, which is a SiN film or SfO, and the non-doped Mi layer 4 are as follows.
Since it is configured so that a is in contact, it is the same as before,
Non-uniformity in properties such as film quality, breakage of the active semiconductor layer 4, etc. caused by thinning the a-3i layer 4a can be compensated for by laminating the second semiconductor layer 4b.

The laminated structure of the a-3i layer and the a-3i First, by making the a-3i layer 4a thinner, the amount of absorption of illumination light (natural light) for liquid crystal display is reduced, so it is possible to prevent an increase in off-state current due to incident light, and therefore, light shielding is achieved. A membrane becomes unnecessary, and the manufacturing process is simplified in this respect as well.

〔Example〕

Embodiments of the present invention will be described below with reference to FIGS. 2 and 3.

FIG. 2 shows the photoconductivity of the a5in-xCx layer 4b for white light. When the value of X increases, that is, the amount of C (carbon) doped increases, the photoconductivity increases to
ie, several orders of magnitude lower than the case where x=0. Therefore,
A Si layer formed on non-doped Si54a
Even if the thickness of the xCx layer 4b is at most 1000 F degrees,
Sensitivity to light can be sufficiently suppressed.

An example using this is shown in FIG. 3.

In this embodiment, the active semiconductor layer 4 includes a non-doped 3i layer 4a with a thickness of about 100 mm and a layer 4a with a thickness of about 500 mm.
This is an example of a two-layer structure including the S I I-X Cx layer 4b.

In addition, using a glass substrate 1 as a transparent insulating substrate,
The gate electrode 2 is made of a Ti film, and the gate insulating film is made of 5iN.
(silicon nitride) film, contact layer is n'a-3i layer,
The electrode metal film is a Ti film6. Channel protective film is 5iOz film 7
It was formed using

There is no particular difference between these and ordinary thin film transistors.

Note that the contact layer 5 is made of nya-Si of this embodiment.
Instead of layer 5, it is also possible to use an n' a-31t-XCX layer.

a-Si layer 4a with photosensitivity to white light for liquid crystal display
Since the H'J thickness is only 100, the off-state current does not increase even when exposed to white light, allowing stable operation. Further, the a-3it-xCxCx layer which is the second semiconductor layer 4b is
By changing the amounts of Si and C, it is also possible to control photosensitivity and conductivity, making it possible to control the characteristics of thin film transistors over a wide range.

Note that although the thin film transistor in the above-described embodiment has been described as an inverted staggered type, the present invention can also be applied to a staggered type.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the operating semiconductor layer of the thin film transistor is made of two layers, and the electrical characteristics are
The second layer serves as an auxiliary layer, which improves the uniformity of characteristics, and since there is no photosensitivity, there is no need for a light-shielding film, which simplifies the manufacturing process.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the structure of the present invention, FIG. 2 is a diagram showing a change in photoconductivity of an a S j IX Cx layer, and FIG. 3 is a diagram illustrating an embodiment of the present invention. In the figure, 1: Transparent insulation 1 (Glass substrate) 2; Gate electrode 3; Gate insulating film (SiN film) 4: Operating semiconductor layer 4a: Non-doped a-3i layer 4b: Second semiconductor layer (a S i 1-x Cx layer) 5 : Contact layer (n″a-Si layer) 6 : Electrode metal film (Ti film) 7 : Channel protective film (S i O, film). 7 'fff*I4 ( 7 t
z # 1 Shil # Seven heads 1 day e4 - Pointed mirror EJE (Ko [
1Figure 3

Claims (1)

[Claims] A gate electrode (2) is arranged on one main surface side of the active semiconductor layer (4) via a gate insulating film (3), and source and drain electrodes (S, D) are arranged on the other main surface side. In the structure of the thin film transistor provided, the active semiconductor layer (4) includes a non-doped amorphous silicon layer (4a) disposed on the gate insulating film side (3), and a non-doped amorphous silicon layer (4a) disposed on the opposite side. 4a) the second with larger bandgap
A thin film transistor characterized by having a two-layer structure including a semiconductor layer (4b).