JPH0542833B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the realization of a field-effect thin film transistor with a large on-current and high operating speed.

In recent years, development of thin film transistors that can be formed on insulating substrates such as glass has been active in various places. Active matrix liquid crystal, electrochromic, and electroluminescent 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 are capable of high-speed operation. Enables display of TV 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, on the other hand, the field effect mobility obtained with thin film transistors formed using these amorphous silicon films is
Since it is 0.1 to 1 cm ² (V·sec), it is difficult to realize a transistor that can obtain a current of 10 ⁻⁵ A or more with an operating voltage of about 10 V. For this reason, the operating frequency is several tens of kilometres.
It was considered difficult to implement circuits with frequencies higher than Hz using amorphous silicon transistors. Although amorphous silicon thin film transistors have sufficient operating speed as switch transistors for each pixel in active matrix display devices, they are not suitable for use in peripheral circuits for TV image display, which require an operating frequency of several MHz or more. Insufficient operating speed. In the conventional method, this type of peripheral circuit uses a MOSIC formed on a single-crystal silicon substrate and is connected to the display device through several hundred terminals. (2) The peripheral circuitry cannot be made compact (3) The reliability after mounting is poor.

It had some drawbacks.

Amorphous silicon thin film transistors are also expected to be used as light sensors formed on glass substrates, but in this case as well, the problem of connection with peripheral circuits is similar to that of display devices.

An object of the present invention is to eliminate the above-mentioned conventional drawbacks by realizing a thin film transistor with high operating speed, and to provide a means for simultaneously providing a display device or a sensor and its peripheral circuits on the same insulating substrate. It is to provide.

Hereinafter, the present invention will be explained in detail based on examples and drawings. FIG. 1a is a diagram showing a band structure in a flat band state in a cross section of a channel region of a field effect thin film transistor of the present invention. In Figure 1a, 1 is the gate electrode metal, 2
is the gate insulating film, 3 is the forbidden band width Eg ₂ , and the thickness is 1000 Å
In the above second semiconductor film, Eg ₁ >Eg ₂ . Ec ₁
Ev ₁ is the energy at the conduction band edge and valence band edge of the first semiconductor film 3, respectively, and Ec ₂ and Ev ₂
are the energies of the conduction band edge and the valence band edge of the second semiconductor film 4, respectively. EFG is the Fermi level of the gate electrode 1, and EF is the Fermi level common to the semiconductor films 3 and 4. 1. Examples of gate electrode metal materials include aluminum, chromium, etc. formed by sputtering, vacuum deposition, etc.
Molybdenum etc. are used, and the thickness is usually 500~
It is 3000Å. The gate insulating film 2 is formed by sputtering, vacuum evaporation, plasma CVD, etc., and is made of silicon dioxide, silicon nitride, etc., and has a thickness of usually 500 to 3000 Å.

Since the thin film transistor of the present invention is formed on an insulating substrate such as glass that is not a single crystal,
And as the semiconductor film of 4, plasma CVD method,
An amorphous or microcrystalline semiconductor film is used, which can be formed on a substrate at 500°C or lower using a photo-CVD method or the like. Especially for amorphous silicon, plasma CVD method,
It is known that a good semiconductor film with a localized level density in the forbidden band of 10 ¹⁷ /cm ³ ·ev or less can be obtained when formed by a CVD method, so it is suitable as a semiconductor film for use in the present invention. . It is known that the forbidden band width of an amorphous semiconductor can be made larger or smaller depending on the film formation method. That is, when considering amorphous silicon formed by plasma CVD as a standard, the forbidden band width can be increased by mixing carbon, nitrogen, and enzymes as impurities into the amorphous silicon. Conversely, if germanium, tin, or the like is mixed into amorphous silicon as an impurity, the forbidden band width can be reduced. Furthermore, the plasma CVD method can form a microcrystalline silicon film that is a mixture of amorphous silicon and crystals of about 100 Å, which is known to have a smaller bandgap than an amorphous silicon film. There is. Therefore, various combinations of the semiconductor films 3 and 4 are possible. For example, (1) Microcrystalline silicon as the semiconductor film 3 Amorphous silicon as the semiconductor film 4 (2) Amorphous silicon as the semiconductor film 3 Amorphous silicon containing nitrogen as the semiconductor film 4 (3) Semiconductor film 3, amorphous silicon containing germanium; semiconductor film 4, amorphous silicon (4) semiconductor film 3, amorphous silicon containing germanium; semiconductor film 4, amorphous silicon containing nitrogen, etc. .

Next, the operation of this thin film transistor will be explained. FIG. 1b shows a band diagram when a positive voltage is applied to the gate electrode of the thin film transistor of FIG. 1a to turn it on. Electrons induced in the conduction band of the semiconductor layer 3 are confined in a very thin region with a thickness of 150 Å or less. Therefore, the movement of electrons in the thickness direction of the semiconductor layer 3 is quantized, and the electrons on the conduction band of the semiconductor layer 3 behave as a two-dimensional electron gas. In particular, as the thickness of the semiconductor layer 3 becomes thinner, the quantized energy levels become more separated, and it becomes difficult for electrons to transition to higher energy levels with thermal energy, making it easier to maintain a two-dimensional electron gas state. . The density of states of a two-dimensional electron gas is 0 at the conduction band edge, and takes a finite non-zero value from a certain energy △E above, and has a large free electron density compared to the case where it is not converted into a two-dimensional electron gas. obtain. The semiconductor layer 3 in the present invention basically only needs to have an energy band. However, in actually creating the semiconductor layer 3, the film thickness that can be formed as an energy band gap is, for example, when using Si as semiconductor atoms, a film thickness of about 10 Å, which is the thickness of several layers of Si atoms. becomes.
In addition, the larger the energy difference from the conduction band for electrons flowing through the conduction band in an amorphous material, the less influenced by the potential due to irregular atomic arrangement, and the greater the mobility (>10cm ² /v・sec).
has. Thus, the transistor of FIG. 1b, which has a large electron concentration and mobility, can carry a large current and therefore operates at high speed. The conditions for obtaining a two-dimensional electron gas near room temperature are △E+△ in Figure 1b.
This is the case when Ec is 0.3 eV or more and the thickness of the semiconductor layer 3 is as thin as 150 Å or less. Although holes on the valence band side can also form a two-dimensional gas like electrons, they can be prevented from contributing to conduction by providing an n-type layer at the source and drain of the transistor.

As described above, the thin film transistor according to the present invention can control the channel conductance by applying a gate voltage, can flow a large on-current, and operates at high speed.

FIG. 2 is a diagram showing a cross section of the structure of the first embodiment of the thin film transistor of the present invention. In Figure 2,
5 is an insulating substrate such as glass; 6 is a gate electrode made of aluminum, chromium, etc.; 7 is a gate insulating film made of silicon dioxide, silicon nitride, etc.;
9 and 10 are n ⁺ amorphous silicon films for source and drain contact, respectively; 11,
12 are source and drain electrodes, 13 are
The amorphous silicon film 14, which is a second semiconductor film having a larger band gap than the semiconductor film 8, is a protective insulating film made of silicon dioxide, silicon nitride, or the like. In the structure shown in FIG. 2, after forming the gate electrode 6, the gate insulating film 7, and the first semiconductor film 8, the source and drain electrodes 9, 10, 11, and 12 are formed, and then the second semiconductor film 13 is formed. , protective film 14
is formed.

FIG. 3 is a diagram showing a cross section of the structure of a second embodiment of the thin film transistor of the present invention. In Figure 3,
15 is an insulating substrate such as glass, 16 is a gate electrode, 17 is a gate insulating film, 18 is a first semiconductor film made of amorphous silicon and having a thickness of 150A or less, 19,
20 are n ⁺ for source and drain contact respectively
Amorphous silicon films 21 and 22 are source and drain electrodes, respectively, 23 is a second semiconductor film which is an amorphous silicon film containing nitrogen as an impurity, and 24 is a protective insulating film. In the structure shown in FIG. 3, the gate electrode 16, the gate insulating film 17, the source/drain electrodes 19, 20, 21, and 22 are formed, and then the first semiconductor film 18, the second semiconductor film 23, and the protective film 24 are formed. form.

FIG. 4 shows the results of measuring the relationship between drain voltage and drain current of a single-layer amorphous silicon thin film transistor having a conventional structure by varying the gate voltage. The distance between the source and drain electrodes is
10μm, source, drain electrode length is 100μm
It is. The drain current was 3×10 ⁻⁷ A when the gate electrode voltage was 20 V and the drain voltage was 20 V.

5 and 6 show the electrical characteristics of a thin film transistor having a two-layer structure according to the present invention. The semiconductor film 3 is made of microcrystalline silicon with a thickness of between 100 Å and 150 Å, the semiconductor film 4 is made of amorphous silicon, the distance between the source and drain electrodes is 10 μm, and the length of the source and drain electrodes is is 100μ
It was set as m. Figure 5 shows the dark characteristic of not irradiating light.
Figure 6 shows the result when irradiated with 2000 lux light. In Figure 5, for a gate voltage of 20V and a drain voltage of 20V, the drain current is approximately 2×
10 ^-6 A was obtained.

The thin film transistor of the present invention described above can be formed on an insulating substrate such as glass and can operate at high speed. Therefore, the drive circuit and display section can be formed on the same substrate, making the circuit connection inexpensive and compact. ,
This enables the realization of highly reliable active matrix display devices and devices that have sensors and drive circuits on the same substrate.

[Brief explanation of the drawing]

Figures 1a and 1b are diagrams showing the band structure of the embodiment of the present invention, and Figures 2 and 3 are diagrams showing the cross-sectional structure of the first and second embodiments of the invention, respectively. Figure 4 is a diagram showing the electrical characteristics of a conventional thin film transistor with a single layer of semiconductor film.
6 and 6 are diagrams showing the electrical characteristics of the thin film transistor according to the present invention, and FIG. 5 shows the dark characteristic,
FIG. 6 shows the characteristics under light irradiation. 1... Gate electrode, 2... Gate insulating film, 3,
4... Semiconductor film, 5... Glass substrate, 6... Gate electrode, 7... Gate insulating film, 8... Semiconductor film,
9, 10...n ⁺ amorphous silicon film, 11... source electrode, 12... drain electrode, 13... semiconductor film, 14... insulating film, 15... glass substrate, 1
6... Gate electrode, 17... Gate insulating film, 18
... Semiconductor film, 19, 20 ... n ⁺ amorphous silicon film, 21 ... Source electrode, 22 ... Drain electrode, 23 ... Semiconductor film, 24 ... Insulating film.

Claims (1)

[Claims] 1. A field effect thin film transistor comprising a gate electrode, a gate insulating film, a semiconductor film in contact with the gate insulating film, etc. formed on an insulating substrate, wherein the semiconductor film is at least a first semiconductor film. and a second semiconductor film, the first semiconductor film is in contact with the gate insulating film and has a thickness of 10 Å to 150 Å, and the band gap of the first semiconductor film is the same as that of the first semiconductor film. A field-effect thin film transistor characterized by having a bandgap smaller than that of the semiconductor film described in No. 2. 2. The field effect thin film transistor according to claim 1, wherein the first semiconductor film is a microcrystalline silicon film, and the second semiconductor film is an amorphous silicon film. . 3. The first or second semiconductor film is an amorphous silicon film containing one or more of carbon, nitrogen, oxygen, germanium, and tin as an impurity. The field effect thin film transistor according to item 1.