JPS6376378A - Field-effect transistor - Google Patents

Abstract

PURPOSE:To obtain a stable operation and reduce a leakage of electricity so that an electric current between a source electrode and a drain electrode can be drastically changed by a gate voltage by causing a semiconductor layer to form an organic thin film having a specific thickness which is composed of pi-conjugated polymer, when conductivity of the above semiconductor layer serving as a current path between the source and drain electrodes is controlled by the gate voltage through an insulating thin film. CONSTITUTION:A metal film 2 that functions as a gate electrode, an insulating thin film 3, an organic thin film that is composed of pi-conjugated polymer and has a thickness of 1000 Angstrom or less, thereby performing the task as a semiconductor layer 4, as well as the metal film 6 that functions as respective electrodes of source and drain are formed on a substrate. In view of the ease of the film formation and composition, pi-conjugated polymer having a five-membered heterocyclic ring is in wide use. Among them in particular, it is preferable to have pi-conjugated polymer exhibited by I and II expressions and it is still more desirable for it to use polythiophene and poly (3-methylthiophene) from the practical point of view. Thus, the above measure makes it possible to obtain a stable operation and reduce a leakage of electricity and furthermore change drastically an electric current between source and drain electrodes by means of a gate voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a field effect transistor (hereinafter abbreviated as FET element) using an organic semiconductor.

[Conventional technology]

The chemical structure of π-conjugated polymers consists of a conjugated double bond or a conjugated triple bond, and a band consisting of a valence band and a conduction band formed by overlapping π-electron orbits, and a forbidden band separating these bands. It is thought that it has a structure. The forbidden band width varies depending on the material, but is in the range of 1.5 to 4 eV for most π-conjugated polymers. For this reason, the π-conjugated polymer itself is an insulator. However, deuterons can be removed from the valence band (oxidation) by chemical methods, electrochemical methods, physical methods, etc.

Alternatively, injecting (reducing) electrons into the conduction band (hereinafter referred to as
It is simply explained that carriers that carry charges are generated through doping (called doping). As a result, by controlling the amount of doping, the conductivity can be varied over a wide range from the insulator region to the metal region. When doping is an oxidation reaction, the polymer obtained becomes p-type, and when doping is a reduction reaction, it becomes n-type. This is similar to the case of impurity addition in inorganic semiconductors.

For this reason, a semiconductor element using a π-conjugated polymer as a semiconductor material can be manufactured.

Specifically, a Schottky-type junction element using polyacetylene f Journal of Applied Physics (J
, Appl, Phys, Volume 62, Page 369.

Published in 1981, Japanese Patent Application Publication No. 147486/1986, etc.)
, a Schottky type junction element using a polypyrrole-based conjugated polymer (Japanese Unexamined Patent Publication No. 59-68760, etc.) is known. In addition, a heterojunction device combining the inorganic semiconductor n-Cd5 and p-type polyacetylene has been reported, Appl, Phys, Vol. 51, No. 42.
52 pages, published in 1980). As a junction element combining π-conjugated polymers, a pn homojunction element using p-type and n-type polyacetylene is known (Applied Physics Letters (Appl, Phys.
, Lett, Volume 38, Page 18, published in 1978). In addition, a heterojunction device consisting of polyacetylene and poly(N-methylpyrrole) has been reported (J,
Appl, Phys, Volume 58, Page 1279.

(Published in 1986).

On the other hand, FET using π-conjugated polymer as a semiconductor layer
The element is polyacetylene (J, Appl.

Volume 54, page 3256, published in 1988), Poly(N
-Methylpyrrole) (Polymer Preprints, Japan
), Vol. 34, No. 4, p. 917, published in 1985) and polythiophene (Polymer Preprints, Japan).
n) Vol. 35, No. 3, p. 609, published in 1986) is known.

FIG. 6 is a cross-sectional view of a conventional FET element using polyacetylene.

In the figure, (1) is the glass substrate, (2) is the aluminum film that will be the gate electrode, (3) is the prisiloxane film that will be the insulating thin film, and (4) is the polyacetylene film that will be the semiconductor layer (thickness: approx. 290OA), (5) and (6) are gold films serving as a source electrode and a drain electrode, respectively.

Next, the operation will be explained. When a voltage is applied between the source electrode (5) and the drain electrode (6), the polyacetylene film (
4), a current flows between the source electrode (5) and the drain electrode (6). At this time, a polyacetylene film (4) is provided on the glass substrate (1) and is formed by an insulating thin film (3).
When a voltage is applied to the gate electrode (2) separated from the polyacetylene film (4), the electrical conductivity of the polyacetylene film (4) can be changed by the electric field effect, and the current between the source and drain can therefore be controlled as shown in Figure 6. can. (J, Ap
pl, Phys, Volume 54, Page 3255, 198
Figure 6 shows the gate voltage ov of a conventional FET element.
, -av and av are characteristic diagrams showing changes in source-drain current (μA) depending on the source-drain voltage, and in the figure, (1), ! 2) and (3)
) are the above characteristics at gate voltages -av, ov, and 8v, respectively, and the horizontal axis is the source-drain voltage (to).
, the vertical axis is the source-drain current (μA). This change is caused by the polyacetylene film (4) adjacent to the insulating thin film (3).
It is thought that this is because the width of the depletion layer in ) changes depending on the voltage applied to the gate electrode (2), and the effective hole channel cross-sectional area changes. However, this FET
The actual situation is that the stability of the element itself is extremely poor, because polyacetylene itself rapidly deteriorates due to oxygen and moisture in the air, rather than problems with element characteristics.

FIG. 7 shows a cross-sectional view of an FET element whose semiconductor layer is poly(N-methylpyrrole) or polythiophene. In the figure, (3) is silicon oxide which becomes an insulating thin film, (4)
) is a poly(N-methylpyrrole) film or polythiophene film that serves as a semiconductor layer, (5) and (6) are gold films that serve as the source and drain electrodes, respectively, and (7) is a p-type film that serves as the substrate and gate electrode. It's silicon. Even in this case, the current (conductive dust) flowing between the source electrode (5) and the drain electrode (6) through the semiconductor layer (4) can be controlled by the voltage applied to the gate electrode.

Figure 3 shows Polymer Bripurin Japan (Polymer Bripurin Japan)
mer Preprints, Japan Vol. 35, No. 3, p. 609, published in 1986), a conventional FET element using polythiophene as a semiconductor layer,
It is a characteristic diagram showing the change in source-drain current (mA) depending on the source-drain voltage (T) at gate voltages -50, -40, -80, -20, -10 Ov, and (3) or 8) is each gate voltage -50V, -
40V, -80V.

In the characteristics at -20V, -10V, and GV, the horizontal axis is the source-Fl/in voltage (total), and the vertical axis is the source-drain current (mA).

[Problem that the invention seeks to solve]

However, in FET devices using these polyacetylene, poly(N-methylpyrrole), and polythiophene as semiconductor layers, the conductive dust between the source and drain cannot be changed significantly by the voltage applied from the gate, and from a practical point of view. , improvements in characteristics were required. In particular, so-called leaf current, in which the source-drain current increases as the source-drain voltage increases when the gate voltage is Ov, has been a problem when these devices are used as switching devices.

That is, the ratio of the source-drain current (switching ratio) between when the gate voltage is applied and when the voltage is applied becomes low, which is a big problem in practice.

This invention was made to solve these problems, and is a field-effect type that operates stably, reduces leakage current, and can significantly change the source-drain current depending on the gate voltage. The purpose is to obtain transistors.

[Means for solving problems]

The field-effect transistor of the present invention is one in which conductive dust in a semiconductor layer, which is a current path between a source electrode and a drain electrode, is controlled by a gate voltage through an insulating thin film, wherein the semiconductor layer is composed of a π-conjugated polymer. thickness consisting of 10
It is characterized by being an organic thin film with a thickness of 00 Å or less.

[Effect]

It is thought that the only part necessary for transistor operation is the semiconductor layer between the source and drain electrodes and near the gate insulating film, and the remaining semiconductor layer simply acts as a resistor. As a result, a leakage current constantly flows through the resistor in addition to the current that can be controlled by the gate voltage. Therefore, in the present invention, an attempt is made to remove the excess portion for transistor operation by reducing the thickness of the entire semiconductor layer.

〔Example〕

FIG. 1 shows a sectional view of an FET element according to an embodiment of the present invention. In the figure, (1) is the substrate, and (2) is the substrate (1).
The gate provided on the top [metal film serving as a pole, (3) is an insulating thin film, (4) is an organic thin film made of a π-conjugated polymer with a thickness of 1000 mm or less and serving as a semiconductor layer, (5) and ( 6) are metal films that act as source and drain electrodes, respectively.

Here, the materials used in this invention include those described below.

Any insulating material can be used for the substrate (1), and specifically, various insulating plastics such as glass, alumina sintered body, polyimide film, and polyester film can be used.

The metal film (2) serving as the gate electrode and the metal films (5) and (6) serving as the source and drain are gold,
platinum.

Metals such as chromium, palladium, aluminum, isodium, isodium tin oxide, isodium/tin oxide (I
TO) etc. are commonly used.

Of course, the material is not limited to these materials, and two or more of these materials may be used for the gate electrode. Here, methods for providing the metal film include vapor deposition, spacing, and plating.

There are methods such as CVD growth.

In the FET element of one embodiment of the present invention shown in FIG. 1, p-type silicon or n-type silicon can be used for both the gate electrode (2) and the substrate (1).

In this case, the substrate (1) can be omitted.

Further, in this case, it is practically preferable that the specific volume resistivity of p-type silicon or n-type silicon is smaller than that of the π-conjugated polymer used as the semiconductor layer. In addition, game) [
A conductive organic polymer may be used as the electrode. Further, depending on the purpose of use, it is also possible to use a metal plate such as a stainless steel plate or a copper plate to serve both as the gate electrode (2) and the substrate (1).

Further, as the insulating thin film (3), any inorganic or organic material can be used as long as it is insulating, and silicon oxide (SiOr) and silicon nitride are generally used.

Aluminum oxide, polyethylene, polyvinylcarbazole, polyphenylene sulfide, polyparaxylene, etc. are used. The method for manufacturing these insulating films is CV
There are D method, plasma CVD method, vapor deposition method, spin coating method, cluster ion beam deposition method, etc., and any of them can be used.

Furthermore, the LB single molecule accumulation method can also be used.

In addition, p-type silicon or n-type silicon is used as the gate electrode (2).
When used as the substrate (1), the insulating thin film (3)
As the material, a silicon oxide film obtained by a silicon thermal oxidation method or the like is preferably used.

The π-conjugated polymer used in this invention can be any π-conjugated polymer, specifically polypyrrole and poly(N-substituted pyrrole).

Bori (2,4-disubstituted pyrrole), polythiophene, poly(3-substituted thiophene), poly(8,4-disubstituted thiophene), polyaniline, polyazulene, polypyrene,
Polycarbazole, poly(N-n substituted carbazole), polyselenophene, polyfuran, polybenzothiophene,
Examples include poly(phenylenevinylene), polybenzofuran, poly(halapenylene), polyindole, polyisothiophene, polypyridazine, polydiacetylenes, graphite polymers, etc., but are not limited to these, of course. However, from the viewpoint of FET characteristics, film formability, and ease of synthesis, π-conjugated polymers having a five-membered heterocyclic ring are preferred; one type of R1 and R
2 is -H, -CH,, -0CR,, -C2H, and -
one of the 0C2H5 groups, n is an integer), and those represented by the general formula (wherein RI and R2 are -H, -CH,, -0CR,
, -C2H.

and -QC2H, one of the groups, R3 is -H, -CH
3゜Type, n is an integer. ) are particularly preferred, and polythiophene and poly(8-methyloffene) are often used from a practical standpoint.

Note that these π-conjugated polymers are extremely excellent materials from the viewpoint of stability and characteristics of FET devices.

The method for producing organic thin films made of these π-conjugated polymers includes spin coating and vapor deposition of π-conjugated polymers obtained by ordinary polymer synthesis methods.

Those provided by dipping method, methods obtained by introducing monomer gas into a place where a catalyst has been applied in advance, and CV
D method, photo-CVD method, chemical oxidation polymerization method, electrochemical polymerization method, etc., but are not limited to these methods.

Alternatively, it is also possible to use the LB method in which a monomer is developed on a subphase of water or glycerin to form a monomolecular film or a cumulative film, and the film is deposited on a substrate. At this time, an organic thin film made of the π-conjugated polymer can be obtained by polymerizing the polymer before depositing it on the substrate or by polymerizing it after the deposition. However, an electrochemical polymerization method is preferably used from the viewpoint of film formability, ease of production, and the like.

π-conjugated polymers can be used without doping treatment.
Although its conductivity is low, it generally has properties as a p-type semiconductor. However, doping treatment is often performed to improve the characteristics of FET devices. Methods for this doping include chemical methods and physical methods (Kogyo Zasei, Vol. 34, No. 4, p. 55, published in 1986). The former includes (1) doping from the gas phase and (if) doping from the liquid phase.

(1) [There are methods such as vapor chemical doping and photo-initiated doping, and the latter includes ion implantation, both of which can be used. However, an electrochemical doping method is preferably used from the viewpoint of operability and controllability of the doping amount. Moreover, in electrochemical doping, π−
When a conjugated polymer is obtained by electrochemical polymerization, it has the advantage that the amount of doping can be controlled using the same equipment after polymerization.

For example, in order to form an organic thin film made of a π-conjugated polymer having a thickness of xoooX or less using an electrolytic polymerization method, a monomer corresponding to the π-conjugated polymer and a supporting electrolyte are mixed with an organic solvent, water, or water. Dissolve in a mixed solvent with an organic solvent to make a reaction solution, and prepare the F of one example of this invention shown in Fig. 1 above.
In the production of an ET device, at least one of the source electrode (5) and the drain electrode (6) is used as a working electrode, and a current is caused to occur between it and a counter electrode such as platinum to cause a polymerization reaction on the working electrode and its vicinity. π-conjugated polymer is deposited to form a source electrode (5) and a drain electrode (6).
) is connected with a π-conjugated polymer, and the precipitated organic thin film made of the π-conjugated polymer is thoroughly washed and then dried. Controlling the thickness of an organic thin film made of a π-conjugated polymer by electrochemical polymerization can be achieved relatively easily by controlling the total amount of coulombs flowing during synthesis. When organic thin films made of π-conjugated polymers are obtained by electrochemical polymerization, most of them are oxidatively polymerized and are doped with supporting electrolyte anions. , the amount of doping may be adjusted, and in some cases, most of the doping may be removed. Polythiophene and poly(8-methylthiophene) films obtained by electrochemical polymerization with a thickness of 1000λ or less have particularly excellent properties as semiconductor layers of FET devices, so this synthesis method is preferably used.

Note that the semiconductor layer according to the embodiment of the present invention obtained as described above must have a thickness of 1 oooA or less. That is, if it exceeds 100 GA, the characteristics of the FET element will deteriorate, which is not good.

Now, the organic solvent used in the electrochemical polymerization method may be any solvent that can dissolve the supporting 9 solute and the above monomers, such as acetonitrile, nitrobenzene,
Benzonitrile, nitromethane, N,N-dimethylformamide (DMF) dimethyl sulfoxide (DMSO)
, dichloromethane.

Polar solvents such as tetrahydrofuran, ethyl alcohol, and methyl alcohol water are used alone or as a mixed solvent of two or more. The supporting electrolyte is a substance that has high oxidation potential and reduction potential, does not itself undergo oxidation or reduction reactions during electrolytic polymerization, and imparts conductivity to the solution by dissolving it in a solvent. Tetraalkylammonium chlorate salts, tetraalkylammonium tetrafluoroborate salts, tetraalkylammonium hexafluorophosphate salts.

Tetraalkylammonium paratoluenesulfonate salt, sodium hydroxide, and the like are used, but two or more of them may of course be used in combination.

The above has described the case where an organic thin film made of a π-conjugated polymer is produced by electrochemical polymerization in the FET device shown in FIG. 1, which is an embodiment of the present invention.
Depending on the structure of the T element, it may be better to fabricate the FET element using a film forming method other than the electrochemical method. FE of an embodiment of the present invention thus obtained
The T element is useful as a switching element or a driving circuit for a large area liquid crystal display element.

EXAMPLES Hereinafter, the present invention will be explained in detail with reference to Examples, but of course the present invention is not limited to these Examples.

Example 1 6S/c! Silicon oxide films with a thickness of 8000 A were provided by thermal oxidation on both sides of an 880 μm thick n-type silicon plate (3.0 crn x 8.0 cIrL) having a conductivity of rL. Next, use a positive resist on one side to form patterns that will become the source and drain electrodes (each with an effective area of 0.2
tl'1FLX0.4a; Gap to become a channel: 5 μm) was drawn, and then a chromium film of 20 OA was formed by vacuum evaporation.

Furthermore, after providing a gold film 300A thereon, the resist was removed to form a source electrode and a drain electrode. Leads were provided to the source and drain electrodes using silver paste, and the contact portions were fixed using epoxy resin.

2,2'-dithiophene (0,15,F) and tetraethelammonium perchlorate (o,ssi) were dissolved in r5rnl of acetonitrile, and this was used as a reaction solution.

The above source and drain electrodes on the silicon plate were used as working electrodes, and the platinum plate (1ffiX 2 (m
), and 5EC (saturated calomel t electrode) was used as a reference electrode, and these were immersed in the reaction solution. A constant current C is applied between the working electrode as the anode and the counter electrode in a nitrogen gas atmosphere.
10 μA/m 2 ) was applied for 8 minutes to completely cover the working electrode, that is, the source and drain electrodes and the silicon oxide between the two electrodes, with a polythiophene thin film having a thickness of about 50 OA.

Next, the potential of the working electrode is set to SEC with a potentiostat.
After electrochemically dedoping the polythiophene in the p-type doping state by setting the voltage to +○ and 4V for 4 hours, the polythiophene in the p-type doping state was washed twice with acetonitrile and dried under reduced pressure. shall be.

With sandpaper, remove a portion of the silicon oxide on the other side of the silicon plate that is not covered with the polythiophene prepared in this way.
, 5Cnt), make ohmic contact with the n-type silicon using indium-gallium, take out the lead from this, fix the contact part with epoxy resin, and connect the n-type silicon through this lead wire.
The molded silicon acts as a gate electrode.

As described above, an FET element according to an embodiment of the present invention having the structure shown in FIG. 1 was fabricated. In this example, (1) and (2) in FIG. 1 are composed of n-type silicon, which serves as the substrate and gate electrode, (3) is silicon oxide, which acts as an insulating thin film, and (4) is polythiophene, which is a semiconductor layer. Films (5) and (6) are the source tg electrode and drain electrode, respectively, which are made of a chromium film covered with a gold film.

Example 2 After producing a polythiophene film with a thickness of approximately 600λ in the same manner as in Example 1, the potential of the working electrode was set to OV with respect to SCH using a potentiostat for 4 hours, and polythiophene in a p-type doped state was electrochemically After performing dedoping, the sample was washed twice with acetonitrile and dried under reduced pressure, and this was designated as sample 2.

Comparative Example 1 When synthesizing polythiophene, a constant current (100 μA
A) was flowed for 8 minutes to reduce the thickness of the polythiophene film to approximately 14
A FET element was prepared in the same manner as in Example 1 except that the OA was changed to 0OA, and this was used as Comparative Sample 1.

Comparative Example 2 When synthesizing polythiophene, a constant current (100 μA
/7) for 10 minutes to reduce the thickness of the polythiophene film by approximately 1.
A FET element was manufactured in the same manner as in Example 2 except that the current was 80OA, and this was designated as Comparative Sample 2.

Figure 2 is a source for comparing this invention and the conventional method.
FIG. 3 is a characteristic diagram showing a change in source-drain current (to) depending on gate voltage (to) at a drain-to-drain voltage of -50V. In the figure, the horizontal axis represents the gate voltage, and the vertical axis represents the source-drain current. In the figure, (II-1) is the characteristic of Sample 2, and (II-2> is the characteristic of Comparative Sample 2).

As is clear from Fig. 2, in sample 2, which is a semiconductor layer and has a film thickness of about 500^, the source-drain current (leakage current) that flows when the gate voltage is Ov depends on the film thickness. This is significantly reduced compared to Comparative Sample 2, which has a polythiophene film, which is about 1400'A. As a result, the source-drain current, which can be modulated by the gate voltage, could be varied by about 8 orders of magnitude.

FIGS. 3(a) and 3(b) are characteristic diagrams showing changes in source-drain current depending on source-drain voltage for Sample 1 and Comparative Sample 1, respectively. In the figure, (9) and side are each using sample 1, gate voltage -60V, -
50V, -40V, -80V, -20V, -10V
(7) Characteristics of time, αQ or goods) are each comparative sample 1
Using gate voltage -50V, -40V% - [)V,
These are the characteristics at -20V and -10V (7). In the figure, the horizontal axis is the source-drain voltage, and the vertical axis is the source-drain current.

Figure 4 shows the source for comparing this invention and the conventional one.
FIG. 3 is a characteristic diagram showing changes in source-drain current due to gate voltage when the drain-to-drain voltage is 180V. In the figure (1-
1') is the characteristic of sample 1, (I-2) is the characteristic of comparative sample 1, the horizontal axis is the source-drain voltage (total), and the vertical axis is the source-drain current.

As is clear from Figures 3 and 4 above, Sample 1, which has polythiophene with a thickness of about 50 OA as a semiconductor layer, has significantly reduced leakage current compared to Comparative Sample 1, which has polythiophene with a thickness of about 180 OA in its semiconductor layer. In addition, the source-drain current could be changed by changing the gate voltage.

Further, the samples obtained in Examples 1 and 2 operated stably even after being left in the air for one month.

〔Effect of the invention〕

As explained above, the present invention is a device in which the conductivity of a semiconductor layer, which is a current path between a source electrode and a drain electrode, is controlled by a gate voltage via an insulating thin film, in which the semiconductor layer is made of a π-conjugated polymer. thickness of 1000Å
By using the following organic thin films, it is possible to operate stably and reduce leakage current, and as a result, the field effect that allows the source-drain current to be changed significantly depending on the gate voltage. type transistor can be obtained.

[Brief Description of the Drawings] Fig. 1 is a cross-sectional view of an FET element according to an embodiment of the present invention, and Figs. 2 and 4 are cross-sectional views of the FET device according to the gate voltage (7) for comparing the present invention and the conventional device. Between YU style (3)
FIGS. 3(a) and 3(b) are characteristic diagrams showing changes in source-drain current due to source-drain voltage of an FET element according to an embodiment of the present invention and a comparative sample, respectively. Figure 5 is a cross-sectional view of a conventional FET element, and Figure 6 is a cross-sectional view of a conventional FET element.
The figure is a characteristic diagram showing the change in source-drain current depending on the source-drain voltage of a conventional FET element, Figure 7 is a cross-sectional view of a conventional FET element, and Figure 3 is a characteristic diagram showing the source-drain current change of a conventional FET element. FIG. 3 is a characteristic diagram showing changes in source-drain current due to SiJ pressure. In the figure, (2) is the gate [pole], (3) is the insulating film,
(4) is a semiconductor layer, (5) is a source electrode, and (6) is a drain electrode. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (7)

[Claims] (1) In a device in which the conductivity of a semiconductor layer, which is a current path between a source electrode and a drain electrode, is controlled by a gate voltage via an insulating thin film, the thickness of the semiconductor layer made of a π-conjugated polymer. A field effect transistor characterized by being an organic thin film of 1000 Å or less. (2) The field effect transistor according to claim 1, wherein the π-conjugated polymer has a five-membered heterocyclic ring. (3) A π-conjugated polymer having a five-membered heterocyclic ring has a general formula ▲ Numerical formula, chemical formula, table, etc. ▼ (In the formula, X is one of S and O atoms, R_1 and R_2 are -H , -CH, -OCH, -C_2H_5 and -OC_2H_5 groups, n being an integer, the field type transistor according to claim 2. (4) A π-conjugated polymer having a five-membered heterocyclic ring has a general formula ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (In the formula, R_1 and R_2 are -H, -CH_■, -O
One of the groups CH_■, -C_2H_5 and -OC_2H_5, R_■ is -H, -CH_■, -C_2H_5
, -C_■H_7, ▲There are mathematical formulas, chemical formulas, tables, etc.▼
and ▲ a mathematical formula, a chemical formula, a table, etc. ▼ one of the groups, n is an integer) The field-effect transistor according to claim 2. (5) The field effect transistor according to claim 3, wherein the π-conjugated polymer having a five-membered hetero ring is polythiophene. (6) A π-conjugated polymer having a five-membered heterocyclic ring is poly(3
-methylthiophene) according to claim 3. (7) A field effect transistor according to any one of claims 1 to 6, wherein the organic thin film is obtained by an electrochemical polymerization method.