JPS60160170A - Thin film transistor - Google Patents

Abstract

PURPOSE:To enable formation of a thin film transistor on an insulating substrate by comprising a semiconductor layer by lamination of a thin semiconductor layer doped with impurity and a thick semiconductor layer an impurity concentration of which is lower than the thin semiconductor layer and of inverse conductive type. CONSTITUTION:On an insulating substrate 5, a gate electrode 6, a gate insulating film 7, the first semiconductor layer 8, the second semiconductor layer 13, N<+> amorphous Si layers 9 and 10 for source and drain contacts and source and drain electrodes 11 and 12 are formed. The layer 8 consists of N type amorphous Si or the like of 150Angstrom thick or under and the layer 13 consists of non- doped amorphous Si of 1,000Angstrom thick or above. The thin film transistor having a channel part of such structure can be formed on an insulating substrate and it is capable of flowing of a large on-state current and high-speed operation. Accordingly, it is possible to realize the circuit connection in which a display part and a drive circuit are formed on the same surface at low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to realizing a thin film transistor with a large on-current and a 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. on an insulating substrate,
Display devices such as active matrix liquid crystal, electrochromic, and electroluminescent display devices in which switch elements made of thin film transistors are arranged in a play pattern are capable of high-speed operation without any black talk between pixels.77
Enables display of images, etc. Semiconductor films used in thin film transistors are made by plasma OVD method, etc.
Hydrogenated amorphous silicon films and fluorinated amorphous silicon films, which can be formed at low temperatures, over large areas, and at low cost, on substrates such as glass are promising.

However, the field effect mobility obtained with a thin film transistor formed of 17M amorphous material is ~0.11a.
l/V, s o-c, xOV, Ktv operation 'R pressure 10
It is difficult to realize a transistor that can obtain a current greater than ``Im''. For this reason, it has been difficult to realize a circuit with an operating frequency of several tens of KH2 or higher 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,
This operating speed is insufficient for application to peripheral circuits for TV image display, which require an operating frequency of Hz or higher. In the conventional method, this type of peripheral circuit is formed on expanded monocrystalline silicon, and is connected to the display device through several hundred terminals, thereby driving a tape matrix display device. . Therefore, conventional tape matrix type display devices have disadvantages such as (1) the cost of circuit connection cannot be made low, C) peripheral circuits cannot be isolated, and (3) reliability after mounting is poor. I had.

Amorphous silicon thin film transistors are also expected to be used as sensors for light and the like 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 peripheral circuits on the same insulating substrate. It is to provide.

The present invention will be described below with reference to the drawings based on examples.

FIG. 1(B) 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 1 (α), 1 is the gate electrode metal, 2 is the gate insulating film, and 3 is the forbidden band width "Is".
A first semiconductor layer doped to n-type with a thickness of 150A or less, 4 is the same forbidden band EI as 3, and a second semiconductor with a thickness of tooZ or more and a small amount of impurity doping (3). It's layered. E('l5IIlv1 is n first semiconductor layer 3
Energy at the conduction band edge and valence band edge. B6. , I!
iv@ is the energy of the conduction band edge and the valence band edge of the second semiconductor layer, respectively. If! to is the lumi level of the gate electrode 1, 11/, By is common to the semiconductor layer 3.4, and *7 is the r-luminium level. An example of the gate electrode metal material (1) is aluminum, chromium, molybdenum, etc., which are formed by a spitting method, a vacuum evaporation method, etc., and the thickness thereof is usually 500 to 3000 μm. The gate insulating film in step 2 was made using the %Tsubatsuta method, vacuum evaporation method, and plasma O.
Silicon dioxide, silicon nitride, or the like, which is formed by a VD method or the like, is used, and the thickness is usually 500 to 3000 μm. Since the thin film transistor of the present invention is formed on an insulating substrate that is not a single crystal such as glass, it is possible to form a 3 and 40 semiconductor film on a substrate at 500° C. or lower using a plasma AV'N method, a photoG'VD method, etc. An amorphous or microcrystalline semiconductor film that can be formed is used. In particular, it is known that when formed by amorphous silicon line plasma OVD method, optical OVP method, etc., a good semiconductor film with a localized level density in the forbidden band of 10"/d LV or less can be obtained. It is suitable as a semiconductor film used in the invention.It is known that microcrystalline or amorphous silicon can change its conductivity type to % type or Pffl by doping it with impurities such as phosphorus or boron. Furthermore, it is known that the doped O film has properties close to that of an intrinsic conductor.

Next, the operation of the thin film transistor of the present invention will be explained. FIG. 1(6) shows a band diagram when a positive voltage is applied to the gate electrode of the thin film transistor of FIG. 1(C) to turn it on. Electrons induced in the conduction band of the semiconductor layer 3 are confined in a very thin region having a thickness of less than xsoffi. Therefore, the movement of electrons in the thickness direction of the semiconductor layer 3 is quantized, and the electrons in the conduction band of the semiconductor layer 3 behave as a two-dimensional epsilon gas. The density of states of the two-dimensional electron gas is O at the conduction band edge, and takes a finite non-zero value from the point above the negative energy ΔE, and has a large free electron density compared to the case where it is not converted into a two-dimensional electron gas. get. Furthermore, the larger the energy difference from the conductor for electrons flowing through the conduction band in an amorphous material, the less the influence from the potential due to irregular atomic arrangement will be, and the greater the mobility ()10 ai
/V, Bee). In this way, the transistor φ) in FIG. 1, which has a large electron concentration and mobility, can flow a large current and therefore operates at high speed. The conditions for obtaining a two-dimensional electron gas near room temperature are as shown in Figure 1 (0), △E + Δ, Q, 3s''l or more, and the thickness of the semiconductor layer 3 is 1so'
This is the case where it is as thin as i- or less. As described above, the thin film transistor having the structure of the channel portion according to the present invention can control the conductance of the channel by applying a gate voltage, can flow a large on-current, and operates at high speed. Examples of the present invention will be shown below with specific structures.

FIG. 2 is a diagram showing a cross-sectional structure of a channel portion of an embodiment of a 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., and 8 is an amorphous material doped with n-type. A first semiconductor layer made of silicon or the like and having a thickness of 150A or less, 9.10, 100 amorphous silicon/layers for source, drain and source contacts, ii and 12, respectively. The source and drain electrodes 13 are a second non-doped amorphous silicon layer with a thickness of 100 OA or more, and 14 is a protective insulating layer made of silicon dioxide, silicon nitride, or the like. The first and second amorphous silicon layers can be formed by plasma OVD, optical OVD, or the like, and may contain hydrogen, fluorine, or the like. Moreover, instead of amorphous, microcrystalline silicon (formed by plasma OVD method or the like) may be used as the first and second semiconductor layers.

FIG. 3 is a diagram showing a cross-sectional structure of another embodiment of the thin film transistor of the present invention. In FIG. 3, 15 is an insulating substrate such as glass, 16 is a gate electrode, 17 is a gate insulating film, 1
8 is made of amorphous silicon and has a thickness of 150A or less (the first
1.9.20 is an rt-t'n source drain electrode, 21 is a non-doped or P-type amorphous silicon layer with a thickness of 1000 mm or more, and n is a protective insulating layer.

In this embodiment, the source and the drain are on the same plane as the channel formed at the interface between the semiconductor layer 18 and the insulating layer 17;

In the drain part, the resistance of the 20th semiconductor layer, which appears in the case of FIG. 3, can be eliminated, and a transistor with higher performance can be realized. When the semiconductor layer 21 is of P type, it can be considered as the case in FIG. 1 (-) where ΔEC is larger.

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.
It has made it possible to create active matrix display devices that have a display section and drive circuit on the same substrate, and which are compact and highly reliable with inexpensive circuit connections, as well as devices that have sensors and drive circuits on the same substrate. .

[Brief explanation of drawings]

FIG. 1(α) and FIG. 1(6) are diagrams showing the band structure of the embodiment of the present invention, and FIGS. 2 and 3 are diagrams showing the cross-sectional structure of the embodiment of the present invention. 10. Gate electrode, 21. Gate insulating layer, 3°40. semiconductor layer, 50. Glass substrate, 61. gate electrode, 70.
Gate insulating film, 80. semiconductor layer, g, 1o,, n soil layer,
110. source electrode, 128. Drain electrode, 13. ,
Semiconductor layer s 14--insulating layer, 150. glass substrate, 1
60. gate electrode, 170. gate insulating layer, 18;
, n soil layer, 190. Source electrode, addition. Dray/electrode, 21, , semiconductor layer, B1. insulation layer. Figure 1 (a) Figure 1 (ip) l etc.

Claims (3)

[Claims] (1) Consists of a gate electrode, gate insulating film, semiconductor layer, source electrode, and drain electrode, and the semiconductor layer has a thickness of 150A.
A first semiconductor layer doped with impurities under J9J and a second semiconductor layer having a thickness of 100 OA or more and having an impurity concentration lower than or opposite to that of the first semiconductor layer are laminated.
formed on an insulating substrate, characterized by
Field effect thin film transistor. (2) The thin film transistor according to claim 1, wherein the first semiconductor layer and the second semiconductor layer are made of amorphous silicon. (3) The thin film transistor according to claim 1, wherein the first semiconductor layer and the second semiconductor layer are made of microcrystalline silicon.