JP2725591B2 - Field-effect transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect transistor using a .pi.-conjugated polymer for a semiconductor layer, and more particularly to a .pi.-conjugated polymer in which a semiconductor layer contains a substituent having a large dipole moment. The present invention relates to a field-effect transistor having a large on / off ratio, characterized in that the main component is a combination.

[0002]

2. Description of the Related Art π-conjugated polymers such as polythiophene, oligothiophene and polythienylenevinylene exhibit semiconducting properties, and have flexibility not found in inorganic materials such as silicon and gallium arsenide. In addition, a π-conjugated polymer that is soluble in an organic solvent can be synthesized by introducing a substituent, and a thin film can be formed by a simple method such as a spin coating method or a dipping method (immersion method). Under such circumstances, a rectifying element using a π-conjugated polymer and a field effect transistor have been experimentally manufactured.

As a conventional example using a π-conjugated polymer as a semiconductor layer of a field-effect transistor, there is known a method in which a π-conjugated polymer thin film is formed by electrolytic polymerization (see, for example, JP-A-62-085467). ), A solution of a π-conjugated polymer or a solution of a π-conjugated polymer precursor (for example, JP-A-5-110069). FIG. 3 shows an example of the element structure of these conventional π-conjugated polymer field-effect transistors.

A study has been made to apply a field effect transistor using a π-conjugated polymer to a pixel driving element of an active matrix type liquid crystal display.
In this case, the on / off ratio is high, that is, the off current (current flowing between the source and drain when the gate voltage is 0 V) is small, and the on current (when the voltage is applied to the gate electrode, between the source and drain) A large flowing current) is required for improving the contrast and increasing the response speed.

The conventional π-conjugated polymer field-effect transistor has a low on-current and a small on / off ratio.
Various attempts have been made to improve the on-current and the on / off ratio.

In Japanese Patent Application Laid-Open No. H5-110069, by applying a new material to a semiconductor layer, a considerably high 1 × 10 ⁻¹ is used as an example of using a π-conjugated polymer as a semiconductor layer of a field effect transistor. The value of the field-effect mobility of cm ² / V · s is obtained, and a sufficient on-current is obtained as in an amorphous silicon thin film transistor. However, in this example, the off-state current (current flowing between the source and the drain when the gate voltage is 0 V) is also larger than that of the others, which does not lead to an improvement in the on / off ratio.

In order to give priority to improving the on / off ratio, there is an example of a π-conjugated polymer field-effect transistor which realizes an on / off ratio of 5 digits by reducing an off current (Applied. Physics Letter,
(Applied Physics Letter),
62, 1794, 1993). But in this case,
The field-effect mobility is limited to 2 × 10 ⁻⁴ cm ² / V · s, and a large on-current cannot be obtained. As described above, it has conventionally been difficult to simultaneously reduce the off-current and increase the on-current.

A field effect transistor using an inorganic semiconductor typified by a silicon-based material, which is now widely used,
Alternatively, the operation principle of the above-described conventional π-conjugated polymer field-effect transistor is based on the following mechanism. That is, when a voltage is applied to the gate electrode, a conduction channel is formed near the interface with the insulating layer in the semiconductor layer due to an electric field effect from the gate electrode. When a voltage is applied between the source and the drain, the conduction channel passes through the conduction channel. It is a mechanism that current flows. On the other hand, even when a conduction channel is not formed near the interface, if a voltage is applied between the source and the drain, a region other than the vicinity of the semiconductor layer / gate insulating film interface (bulk) unless the semiconductor layer is a perfect insulator. A small amount of current flows between the source and the drain via This is the cause of the off current. When the conductivity of the entire semiconductor layer is increased by doping or improving the carrier mobility of the entire semiconductor layer, the on-current increases but the off-current flowing through the bulk also increases.
Therefore, conventionally, a material having low conductivity had to be used for the semiconductor layer.

[0009]

As described above, conventionally, in a field effect transistor using a π-conjugated polymer for a semiconductor layer, the on-current can be improved and the off-current can be reduced at the same time. It was difficult.

The present invention has been made to solve this problem, and has as its object to provide a field effect transistor having a large on / off ratio.

[0011]

Means for Solving the Problems The present inventors use a thin film of a π-conjugated polymer having a substituent having a large dipole moment as a semiconductor layer of a field-effect transistor, so that a source-drain gap can be obtained. The present inventors have found that the electric conductivity can be largely changed by the gate voltage, and have reached the present invention.

That is, in the field-effect transistor according to the present invention, the semiconductor layer comprises -CHO, -COCH ₃ , -N
A π-conjugated polymer containing a substituent of either O ₂ or —CN as a main component , wherein the π-conjugated polymer is
Is doped and is confined when an external electric field is applied.
Compounds that change their conformation and consequently decrease their conductivity
Object, by applying a voltage to the gate electrode
Reduce the conductivity of the semiconductor layer and reduce the source and drain electrodes
It is characterized by controlling the current between the two .

The gate electrode, source electrode and drain electrode are not particularly limited as long as they are conductive materials.
Gold, silver, nickel, chromium, copper, iron, tin, antimony lead, tantalum, indium, aluminum, zinc, magnesium, their alloys, conductive metal oxides such as indium and tin oxide, or doping Inorganic and organic semiconductors with improved conductivity, such as silicon single crystal, polysilicon, amorphous silicon, germanium, graphite, polyacetylene, polyparaphenylene, polythiophene, polypyrrole, polyaniline, polythienylenevinylene, polyparaphenylenevinylene, and the like. Can be As the source electrode and the drain electrode, those having low electric resistance at the contact surface with the semiconductor layer are preferable among the above.

The insulating film is not particularly limited in material, but preferably has a high dielectric constant and a low conductivity. Examples include silicon oxide, silicon nitride, aluminum oxide, titanium oxide, tantalum oxide, polyethylene, polyimide, and the like.

The π-conjugated polymer as a main component of the semiconductor layer includes -CHO, -COCH ₃ , -NO ₂ , -C
There is no particular limitation as long as it contains any substituent of N. Examples include polyacetylene, polyparaphenylene, polythiophene, polypyrrole, polyaniline, polythienylenevinylene, polyparaphenylenevinylene,
Poly pyridazine, Porinafuchiren, polyazulene, the polyisothianaphthene, etc., -CHO, -COCH _3, -N
O ₂ or a substituent having any one of —CN introduced therein, and oligomers of the above compounds may be mentioned.
In particular, those in which monomer units are alternately bonded in opposite directions, such as a heteropentacyclic π-conjugated polymer, are preferred. This is because in such materials, the polymer is subjected to a torsional force when an external electric field is applied by a gate voltage.
Examples include polythiophene, oligothiophene, polypyrrole, oligopyrrole, etc.
O, —COCH ₃ , —NO ₂ , and —CN are introduced.

In the π-conjugated polymer used in the present invention, the position and the number of substituents of any of —CHO, —COCH ₃ , —NO ₂ and —CN are not particularly limited. That, .pi.-conjugated backbone directly, or any arbitrary position of the substituents attached to the .pi.-conjugated skeleton -CHO, -COC
It is sufficient that at least one of H ₃ , —NO ₂ , and —CN is bonded, but the polymer is introduced at a position where the polymer is easily subjected to a twisting force when an external electric field is applied by a gate voltage. Is desirable.

The degree of polymerization of the π-conjugated polymer is not particularly limited. However, if the degree of polymerization is small, the glass transition temperature becomes low and the heat resistance becomes poor.

In the present invention, it is preferable that a proper electrical conductivity is provided between the source and drain electrodes when no voltage is applied to the gate electrode. It is preferable to apply an appropriate conductivity so as to show appropriate conductivity.

The semiconductor layer may have any structure as long as the above-mentioned π-conjugated polymer is the main component. For example, the compound may be dispersed in a polymer binder, or a high molecular weight compound having the compound in a side chain may be used. A configuration such as a molecule may be used.

The method for forming the semiconductor layer is not particularly limited, and examples thereof include electrolytic polymerization, vapor deposition, CVD, LB (Langmuir-Blodgett), spin casting, dipping, dropping, letterpress or intaglio. A printing method, an ink-jet method and the like can be mentioned.

The structure of the field effect transistor in the present invention is not limited to a thin film type, but may be a three-dimensional type such as a cylindrical type.

[0022]

The operation principle of the field effect transistor of the present invention will be described below.

In a π-conjugated polymer, if the main chain skeleton is bent or twisted, the π-conjugation is broken at that point, so that the effect of the conformation (positional relationship of the constituent atoms) on the electrical properties is affected. It is known to be large.

When no voltage is applied to the gate electrode, FIG.
As shown in (a), the π-electron conjugation of the polymer chain allows carriers to flow through the channel by applying a lateral electric field (generated by source-drain voltage). When a voltage is applied to the gate electrode, an electric field is generated in a direction perpendicular to the gate electrode surface as shown in FIG. 1B, and a force is applied to the substituent having a dipole moment. Then, the conformation of the polymer chain changes, and the π-electron conjugation is broken. If the π-electron conjugation is lost, the conductivity of the polymer is significantly reduced, so that no current flows between the source and drain electrodes.

As described above, the electric conductivity between the source and the drain can be changed by the gate voltage to a greater extent than in the prior art, so that an operation with a high on / off ratio becomes possible.

[0026]

EXAMPLES Examples will be shown below, but the present invention is not limited to the examples.

(Embodiment 1) FIG.
1 shows a structure of a conjugated polymer field-effect transistor. First, chromium was deposited on a non-alkali glass substrate 9 to form a gate electrode 10. Next, after depositing a 5000 angstrom silicon nitride film as the gate insulating film 11 by plasma CVD, chromium and gold were deposited in this order, and the source electrode 12 and the drain electrode 13 were formed by ordinary photolithography. Subsequently, the semiconductor layer 14 made of poly [3- (cyanoethyl) thiophene] was formed by a normal mask method and a vacuum evaporation method. further,
10 ^-3 mol / of nitrosonium tetrafluoroborate
By immersing the whole substrate in 1 l of nitromethane solution,
Doping was performed. After doping, perform vacuum drying,
Unwanted residual solvent was removed.

By the above procedure, a π-conjugated polymer field-effect transistor having a channel length of 20 μm, a channel width of 2 mm and a semiconductor layer thickness of about 1 μm was obtained. This transistor had an on / off ratio of about 6 digits.

(Example 2) A π-conjugated polymer field-effect type according to Example 1 except that poly [3- (cyanoethyl) thiophene] in Example 1 was changed to poly (3-nitrothiophene). The transistor was completed. This transistor had an on / off ratio of about four digits.

Example 3 In the formation and doping process of the semiconductor layer in Example 1, poly [o- (cyanoethyl) aniline] was used as a π-conjugated polymer material.
After depositing this, π was applied in the same manner as in Example 1 except that doping was performed by exposing it to an atmosphere containing hydrogen chloride.
-A conjugated polymer field effect transistor was completed.
This transistor had an on / off ratio of about 5 digits.

Example 4 Using a π-conjugated polymer and poly [o- (cyanoethyl) aniline] with p-toluenesulfonic acid as a dopant, a substrate was immersed in a 2 wt% xylene solution of this compound. Then, a π-conjugated polymer field-effect transistor was completed according to Example 1, except that the semiconductor layer was formed by slowly pulling up. This transistor had an on / off ratio of about 6 digits.

[0032]

As described above, according to the present invention, a π-conjugated polymer field effect transistor having a large on / off ratio can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a part of a channel portion of a field-effect transistor of the present invention.

FIG. 2 is a sectional view of a field-effect transistor showing one embodiment of the present invention.

FIG. 3 is a cross-sectional view of an example of a conventional π-conjugated polymer field-effect transistor.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 gate electrode 3 gate insulating film 4 semiconductor layer 5 π-conjugated main chain portion 6 dipole substituent 7 polymer chain in semiconductor layer 8 electric field according to gate voltage 9, 15 substrate 10, 20 gate electrode 11 , 19 Gate insulating film 12, 17 Source electrode 13, 18 Drain electrode 14, 16 Semiconductor layer

Claims (2)

(57) [Claims] 1. A gate electrode, a gate insulating film, and a semiconductor layer.
And a source electrode and a drain for flowing a current through the semiconductor layer.
In the field effect type transistor having a in-electrode, the semiconductor layer, -CHO, -COCH _3, -N
A π-conjugated polymer containing a substituent of either O ₂ or —CN as a main component , wherein the π-conjugated polymer is
Is doped and is confined when an external electric field is applied.
Compounds that change their conformation and consequently decrease their conductivity
Object, by applying a voltage to the gate electrode
Reduce the conductivity of the semiconductor layer and reduce the source and drain electrodes
A field-effect transistor characterized by controlling a current between the transistors. 2. A gate electrode, a gate insulating film, and a semiconductor layer.
And a source electrode and a drain for flowing a current through the semiconductor layer.
Field effect transistor with in-electrode
Te, wherein the semiconductor layer is, -CHO, -COCH _3, -N
Heterocyclic 5-membered cyclic π containing any substituent of O ₂ and —CN
A conjugated polymer as a main component,
Is the 5-membered π-conjugated polymer doped?
When an external electric field is applied, the
Is a compound that reduces the electrical conductivity of the gate electrode.
By applying pressure, the conductivity of the semiconductor layer is reduced,
Controls current between source electrode and drain electrode
Field effect transistor.