JPH0797640B2 - Thin film transistor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor, and more particularly to a thin film transistor in which instability of thin film transistor characteristics due to an interface between a gate insulating film and a semiconductor layer is improved.

2. Description of the Related Art A thin film transistor is a so-called field effect transistor in which the electric conductivity of a semiconductor between a source electrode and a drain electrode is controlled by a voltage applied to a third electrode (gate electrode) provided via an insulating layer in contact with the semiconductor. Known as. Conventional thin film transistors continue to be researched for the purpose of switching arrays such as image sensors or liquid crystal or EL display devices in that it is easy to form a switching array over a large area, or because the cost is low because the material is inexpensive. Has been. The most important point of such a thin film transistor is that it operates stably over a long period of time without fluctuations in device characteristics.

It is considered that the change in the characteristics of the thin film transistor with time is due to a charge trap capable of trapping electrons in the semiconductor thin film, the interface between the semiconductor thin film and the insulating layer, or the insulating layer. Among them, the number of charge traps existing in the insulator layer is larger than that of other charge traps, and since the conductivity in the insulator layer is low, a long relaxation time is usually required. It is considered to be the main cause of long-term aging. If there are many charge traps inside the insulator layer or if the leakage current of the insulator layer is large, electrons moving in the channel formed at the interface between the semiconductor layer and the insulator layer are drawn into the insulator layer. , Trapped in charge traps, the effective gate voltage changes, the drain voltage changes, and the drain current changes. From the above points, in order to realize an element having stable transistor characteristics,
It is desirable to use a thin film having a small number of charge traps and a small leak current as the insulator layer.

Conventionally, as the insulator layer of the above-mentioned thin film transistor, Al ₂ O ₃ , Ta ₂ O formed by an electron beam evaporation method or a sputtering method is used.
Thin films such as ₅ , SiO ₂ , and Si ₃ N ₄ were used.

Problems to be Solved by the Invention Al ₂ O ₃ and Ta ₂ O ₅ or composite insulating films thereof have a high relative dielectric constant, but generally have poor adhesion to other materials and are physically damaged during the manufacturing process. It had the drawback that it was easy to peel off and the yield was poor. Further, for these reasons, there is a problem that many charge traps are generated at the interface and the change over time is large. Further, an insulating film containing silicon as a main component such as SiO ₂ or Si ₃ N ₄ has a defect that the relative dielectric constant is generally small and the mutual conductance of the thin film transistor cannot be increased.

In addition, in an insulating film obtained by an electron beam evaporation method, a vapor deposition substance is heated to a high temperature in a high vacuum, so thermal dissociation occurs during vapor deposition, and a composition ratio often deviates from a stoichiometric composition. In the case of oxides, oxygen defects are created,
It not only becomes a charge trap but also causes an increase in leak current. For the above reasons, the conventional thin film transistor has a large change with time, a small mutual conductance, and a poor reproducibility.

Therefore, an object of the present invention is to solve the above problems and to provide a thin film transistor which has stable characteristics for a long period of time and can be manufactured with good reproducibility.

Means for Solving the Problems To achieve the above object, the present invention provides a thin film transistor, wherein the gate insulating film comprises a first gate insulating film formed of a complex oxide sputtered thin film containing at least aluminum and tantalum as main components. It is characterized in that it is formed between the first gate insulating film and the semiconductor layer, and is composed of a multilayer film including at least a second gate insulating film formed of an insulating material sputtered thin film containing silicon as a main component.

Effect According to the present invention, an insulating thin film containing silicon as a main component formed by sputtering is used as the gate insulating film in contact with the semiconductor layer, and these films have little deviation from the stoichiometric composition, Therefore, there are few charge traps due to defects, and the semiconductor thin film and the first gate insulating film, which is a composite oxide sputtered thin film mainly composed of aluminum and tantalum, have excellent adhesiveness to thin films in contact with both sides, and near the interface. Since defects are less likely to be generated,
The electrons flowing in the semiconductor channel are less likely to be trapped. Further, since the growth of defects such as pinholes is cut off at the interface in the middle because of the multi-layer structure, the leak current becomes very small. Further, since the complex oxide sputtered thin film containing aluminum and tantalum as a main component has a large relative permittivity, the capacitance is increased as a whole, and the thin film transistor has a large mutual conductance.

The thin film transistor of the present invention by the operation described above,
Since the structure of the gate insulating film has few charge traps at the interface with the semiconductor layer, has a high relative permittivity as a whole, and has a small leak current, the change over time is small.

EXAMPLES Examples of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional view showing an embodiment of the thin film transistor of the present invention.

A having a film thickness of about 100 nm on an insulating substrate 1 such as glass
a first gate electrode 2 made of l and having a film thickness of about 300 nm on the insulating substrate 1 including the gate electrode 2 and made of a composite oxide film of Al and Ta formed by a high frequency magnetron sputtering method. Gate insulating film 3 and further on it
The second gate insulating film 4 made of SiO ₂ and having a film thickness of about 30 nm and similarly formed by the high frequency magnetron sputtering method, and the semiconductor layer 5 made of CdSe having a film thickness of about 50 nm on the second gate insulating film 4. It is composed of a source electrode 6 and a drain electrode 7 which are made of Al and have a film thickness of about 100 nm with a predetermined interval of several to several tens of microns.

In order to investigate the effect of the thin film transistor of the present invention, the thicknesses of the first gate insulating film 3 and the second gate insulating film 4 in FIG.
(II) and (III) were prototyped. Here, (I) is the thin film transistor of the present invention shown above. The channel length of each sample was 10 μm, and the channel width was 50 μm.

FIG. 2 shows the drain current when the gate voltage is changed. As is clear from the figure, the thin film transistor (I) of the present invention has substantially the same electrical characteristics as the thin film transistor (II) because the second gate insulating film SiO ₂ is sufficiently thin, and has a large relative dielectric constant. It shows that the features of the complex oxide sputtered thin film of (ε _r -15) Al and Ta are not impaired. On the other hand, in the thin film transistor of (III), it is found that a large gate voltage is required to obtain a large drain current because the relative permittivity of SiO ₂ is low (ε _{r to} 3.5).

FIG. 3 shows the leak current when the source electrode 6 and the drain electrode 7 are used as common electrodes in FIG. 1 and a voltage is applied between them and the gate electrode 2. It can be seen that the thin film transistor (I) of the present invention has a sufficiently smaller leak current than the thin film transistor (II) or (III).
It is considered that this is because the multilayer structure has a high probability that defects such as pinholes generated during sputtering are cut off at the interface between the first gate insulating film and the second gate insulating film. When the leak current is small, the number of electrons injected into the charge traps in the insulating film also decreases, so that the change over time in thin film transistor characteristics can be reduced.

FIG. 4 shows changes over time in the drain current for the thin film transistors (I), (II) and (III).
The thin film transistor (I) of the present invention has very little change over time. This means that in the vicinity of the interface between the semiconductor layer and the gate insulating film, the adhesiveness is very good, so that the occurrence of defects is small and the number of charge traps, which is the main cause of the change over time, is very small. Further, it is considered that the number of electrons injected into the charge trap itself is small because the leak current is small as described above.

It can be seen that SiO ₂ produced by the resistance heating method or the electron beam method as the second gate insulating film in FIG. 1 does not provide the above characteristics, and the use of a sputtered thin film reduces charge trapping.

Since the thickness of the second gate insulating film has a small relative dielectric constant, it is preferably 50 nm or less in order to obtain a large transconductance.

Further, the effect when SiO ₂ is used as the second gate insulating film is as described above, but when Si ₃ N ₄ is used, similar characteristics are obtained.

In the examples, the case where CdSe is used as the semiconductor layer has been described, but it has been found that the effect of the present invention is great also in the case of CdS, CdTe or a solid solution thereof.

EFFECTS OF THE INVENTION As is clear from the above description, in the thin film transistor of the present invention, there are few charge traps at the interface between the gate insulating film and the semiconductor layer, and since the leak current is small, injection of electrons into the charge traps itself occurs. Since it is difficult, the electrical characteristics and stability of the thin film transistor can be greatly improved, and it can be widely used for driving various display devices.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the thin film transistor of the present invention, FIG. 2 is a view showing electric characteristics of the thin film transistor,
FIG. 3 is a diagram showing a leak current of a gate insulating film, and FIG. 4 is a diagram showing a change with time in thin film transistor characteristics. 1 ... Insulating substrate, 2 ... Gate electrode, 3 ... First gate insulating film (composite oxide sputtered thin film of Al and Ta), 4
...... Second gate insulating film (Si insulator sputtered thin film),
5 ... Semiconductor layer, 6 ... Source electrode, 7 ... Drain electrode.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Atsushi Abe 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP 59-182572 (JP, A)

Claims (5)

[Claims] 1. An at least drain electrode on an insulating substrate,
A first gate insulating film composed of a gate electrode, a source electrode, a semiconductor layer and a gate insulating film, wherein the gate insulating film is a composite oxide sputtered thin film containing at least aluminum and tantalum as main components, and the first gate insulating film. A thin film transistor, comprising: a film, the first gate insulating film, and the semiconductor layer, and a multi-layered film including at least a second gate insulating film formed of an insulator sputtered thin film containing silicon as a main component. 2. The thin film transistor according to claim 1, wherein the thickness of the second gate insulating film is 50 nm or less. 3. The thin film transistor according to claim 1, wherein the second gate insulating film is made of silicon oxide. 4. The thin film transistor according to claim 1, wherein the second gate insulating film is made of silicon nitride. 5. The thin film transistor according to claim 1, wherein the semiconductor layer is CdS, CdSe, CdTe and a solid solution thereof.