JPH0556027B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thin film transistor in which leakage current does not increase even when external light is incident.

In recent years, development of thin film transistors that can be formed on insulating substrates such as glass has been active in various places. Display devices such as active matrix liquid crystal, electroluminescent, and electrochromic display devices, in which switch elements made of thin film transistors are arranged in an array on an insulating substrate, have no crosstalk between pixels and can operate at high speed, making them ideal for TVs. Enables display of images, etc. Promising semiconductor films for use in thin film transistors include hydrogenated amorphous silicon films and fluorinated amorphous silicon films, which can be formed at low temperatures, over a large area, and at low cost, on substrates such as glass using plasma CVD methods. It is said that However, these amorphous silicon films have high photoconductivity, as is known from their application to solar cells, so when applying thin film transistors as switch elements for display devices or optical sensors,
In order to prevent leakage current, it is necessary to shield the product from light when using it.

FIG. 1 shows a cross-sectional structural diagram of a conventional light-shielded thin film transistor. In FIG. 1, 1 is a glass substrate, and 2 is a gate electrode made of a metal material such as aluminum, chromium, or molybdenum. Reference numeral 3 denotes a gate insulating film made of silicon dioxide, silicon nitride, or the like. 4 is a hydrogenated amorphous silicon film, 5 is a phosphorus-doped n ⁺ hydrogenated silicon film, 6 and 7
are source and drain electrodes, respectively, and are made of metal materials such as aluminum and chromium. 8 and 9 are insulating films made of silicon dioxide, silicon nitride, or the like. Reference numeral 10 denotes a light shielding film made of a metal material such as aluminum or chromium.

In the thin film transistor shown in FIG. 1, a channel is formed at the interface between the amorphous silicon film 4 and the gate insulating film 3 between the source electrode 6 and the drain electrode 7 by the voltage applied to the gate electrode 2, resulting in an insulated gate field effect. It operates like a type transistor.

In this transistor, incident light from above is blocked by the light shielding film 10, and incident light from below is blocked by the gate electrode 2, so no photocurrent flows between the source and drain electrodes 6 and 7, and the transistor is bright. No leakage current occurs even in places.

However, the above-mentioned thin film transistor cannot avoid the following drawbacks.

(1) The thin film forming process and photo process for forming the light shielding film 10 and the insulating film 9 are performed twice more than in the case where no light shielding is required.

(2) Due to the capacitive coupling between the light shielding film 10 and the amorphous silicon film 4, the operating speed of the transistor becomes slow.

(3) Light shielding film 10 and source/drain electrodes 6, 7
This leads to a decrease in manufacturing yield due to short cuts during the process.

An object of the present invention is to provide a thin film transistor that eliminates the above-mentioned conventional drawbacks, can be formed by an easy process, and does not require separate formation of a light shielding film.

Hereinafter, the present invention will be explained with reference to the drawings based on examples.

FIG. 2 is a diagram showing a cross-sectional structure of a thin film transistor provided with a semiconductor film having light-shielding properties according to the present invention.

11 is a glass substrate, and 12 is a gate electrode, which is made of a metal material such as aluminum, chromium, or molybdenum. A gate insulating film 13 is made of silicon dioxide, silicon oxide, or the like.

14 is a semiconductor film made of hydrogenated amorphous silicon, and near the interface with the gate insulating film, the dark conductivity is 10 ^-8 (Ω cm) ^-1 or less and the photo conductivity is
10 ^-7 (Ω・cm) ^-1 or more, and near other surfaces of the semiconductor film 14 that are not in contact with the gate insulating film, the dark conductivity and photoconductivity are 10 ^-7 (Ω・cm) ^-1 or less. It changes monotonically and continuously within the amorphous semiconductor film so that . However, the photoconductivity is the conductivity when irradiated with light with a spectrum AMI of 1 mW/cm ² or more.
Amorphous silicon films with high photoconductivity, manufactured by glow discharge method etc., have a low local level density and exhibit good characteristics even when used in field effect transistors, with low on-current and off-current. The ratio becomes 5 digits or more. 15 is an n ⁺ hydrogenated amorphous silicon film doped with phosphorus, and 16 and 17 are source and drain electrodes, respectively, which are made of a metal material such as aluminum or chromium. Reference numeral 19 denotes an insulating film made of silicon dioxide, silicon dioxide, or the like. The operation as a transistor is similar to that of the conventional thin film transistor shown in FIG.

Before explaining the production of the amorphous semiconductor film 14, FIG. 3 shows the dependence of photoconductivity and dark conductivity on film-forming speed when amorphous silicon is produced by glow discharge of silane and disilane. .

In Figure 3, the horizontal axis is the film forming rate, the vertical axis is the conductivity,
σd indicates dark conductivity, and Δσp indicates photoconductivity. The graph takes the flow rate of silane or disilane as a parameter.

(Matsuda, A.etal: Jpn.J.Appl.Phys, Vol.22,
No. 2, 1983), (Scott, BAetal.: Appl, Phys.
Lett.Vol.37:727, 1980), (Ogawa, etal: Jpn.
J. Appl. Phys., Vol. 20: L639, 1981) As is clear from Figure 3, for example, in disilane, a gas flow rate of 15 sccm is applied on the gate insulating film.
By adjusting the gas flow rate to a film forming rate of 5 Å/sec to start film forming, and then increasing the gas flow rate sequentially to accelerate the film forming rate to a final stage of 20 Å/sec or more, the above-mentioned non-conformity can be achieved. A crystalline semiconductor film 14 can be obtained.

Furthermore, the type of gas used is not limited to disilane, but can also be used in combination with silane.

For example, by initially supplying silane and then switching to disilane, or by sequentially adding disilane while supplying silane,
The characteristics of the amorphous semiconductor film 14 can be obtained.

As shown in the above example, the film forming speed, gas flow rate,
Alternatively, desired continuous characteristics can be obtained by appropriately selecting and adjusting the type of gas.

By the way, amorphous silicon has high photoconductivity.
For example, in the vicinity of 10 ^-7 (Ω·cm) ^-1 , the region where it absorbs light exhibits an absorption coefficient of 10 ⁴ to 10 ⁵ cm ^-1 .

By the way, the photoconductivity is 10 ^-7 (Ω・cm) ^-1 and the absorption coefficient is
At 10 ⁵ cm ^-1 , the sheet resistance of a film thickness of 0.5 μm is approximately 10 ¹¹
(Ω・cm), and the light transmittance is 10 ^-5 .

In this way, both dark conductivity and photoconductivity are 10 ^-7
(Ω・cm) Amorphous semiconductor film 1 with characteristics of ^-1 or less
The upper surface of 4 has a sufficient function as a light shielding film of the thin film transistor.

Therefore, the amorphous semiconductor film 14 of the present invention is
This surface layer absorbs most of the incident light and exhibits light-shielding properties, so it acts as a light-shielding film, and in addition,
This light absorbing portion has insulating properties.

In the embodiment shown in FIG. 2, the insulating film 19 is formed, but as described above, the upper surface of the amorphous semiconductor film 14 of the present invention has insulating properties. Therefore, it is fully functional even without this.

As described above, the thin film transistor according to the present invention does not require the formation of a light shielding film on the upper surface of the semiconductor film, and
In addition, since an insulating film is not necessarily required,
The formation process is simplified and the manufacturing process is reduced, resulting in a high product yield.

In addition, a transistor with a normal light-shielding film is
Since the light-shielding film is made of metal, there is a risk that the stray capacitance of the transistor will increase, but such a drawback does not occur in the present invention, which does not have a light-shielding film.

Furthermore, the present invention exhibits remarkable effects in an environment of light irradiation, such as eliminating the occurrence of transistor defects due to shorting, which is common with metal light-shielding films.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a thin film transistor having a conventional light-shielding film, FIG. 2 is a cross-sectional view of a thin film transistor of the present invention, and FIG. 3 is a diagram showing the relationship between photoconductivity and dark conductivity with film manufacturing conditions. It is a diagram. 1, 11... Glass substrate, 2, 12... Gate electrode, 3, 13... Gate insulating film, 4, 14...
Amorphous silicon film, 5, 15...n ⁺ amorphous silicon film, 6, 16... source electrode, 7, 17...
Drain electrode, 8, 9, 19...insulating film.

Claims (1)

[Scope of Claims] 1. An insulated gate field effect thin film transistor provided on an insulating substrate and having a gate electrode, a gate insulating film, an amorphous semiconductor film containing silicon, and a source/drain electrode, comprising: The quality semiconductor film has a dark conductivity of 10 ^-8 (Ω cm) ^-1 or less and a photoconductivity of 10 ^-7 (Ω cm) ^-1 or more near the interface with the gate insulating film, and other An insulated gate electric field characterized in that the insulated gate electric field changes continuously within the amorphous semiconductor film so that dark conductivity and photoconductivity are 10 ^-7 (Ω cm) ^-1 or less near the surface. Effective thin film transistor.