WO2004019427A1

WO2004019427A1 - Organic field-effect transistor and method of manufacturing same

Info

Publication number: WO2004019427A1
Application number: PCT/JP2003/005750
Authority: WO
Inventors: Hidenori Okuzaki
Original assignee: Yamanashi Tlo Co., Ltd
Priority date: 2002-08-22
Filing date: 2003-05-08
Publication date: 2004-03-04

Abstract

An organic field-effect transistor having a simple structure and employing an organic conductor includes a single organic insulating substrate on which an organic conducting layer is formed in such a manner that portions that act as a source, channel and drain are :rendered continuous. An insulating layer is formed on the organic conducting layer, with the exception of at :Least part of the portions thereof that act as the source and drain, so as to cover the portion that acts as the channel. An organic conducting layer that acts a gate is formed on the insulating layer so as to overlay the portion of the organic conducting layer that acts as the channel.

Description

DESCRIPTION

ORGANIC FIELD-EFFECT TRANSISTOR AND METHOD OF MANUFACTURING SAME

Technical Field

This invention relates to an organic field-effect transistor, an organic electronic device that represents the organic field-effect transistor and other devices in more general terms, and methods of manufacturing the organic field-effect transistor and organic electronic device .

Background Art

The development of electronic devices using organic materials has resulted in the new field of light-weight, flexible, inexpensive plastic electronics. Various organic electronic devices have been proposed. For example, see H.E. Katz and Z. Bao, "The Physical

Chemistry of Organic Field-Effect Transistors", J. Phys . Chem. , 104, 671 (2000); C.J. Drury, CM. Mutsaers, CM. Hart, M. Matters and D.M. deLeeuw, "Low-cost all-polymer integrated circuits", Appl . Phys. Lett., 73, 108 (1998); and H. Sirringhaus, N. Tessler and R.H. Friend,

"Integrated Optoelectronic Devices Based on Conjugated Polymers", Science, 280, 1741 (1998), etc.

The organic electronic devices introduced in these references use, in part, inorganic semiconductor materials such as silicon, or employ organic semiconductor materials even in all-organic devices. With the former, conventional silicon semiconductor manufacturing processes must be employed in part and, as a consequence, the features of organic electronic devices and their methods of manufacture cannot be exploited fully. The latter involves a comparatively high operating voltage. Disclosure of Invention

Accordingly, an object of the present invention is to provide an organic field-effect transistor or organic electronic device having a new structure that employs an organic conducting material .

Another object of the present invention is to provide an all-organic-type organic field-effect transistor or organic electronic device that employs an organic conducting material . A further object of the present invention is to provide an organic field-effect transistor or organic electronic device having a comparatively low operating voltage.

Yet another object of the present invention is to provide a comparatively simple method of manufacturing the above-mentioned organic field-effect transistor or organic electronic device.

An organic field-effect transistor (FET) according to the present invention has a source, a channel and a drain consisting of a single organic conducting material and being continuous within an organic conductor (first organic conductor) ; a conductor (second conductor) acting as a gate being provided on one surface of the organic conductor via an insulator; a region in which the conductor overlaps the organic conductor serving as a channel region; and one of the organic conductor and conductor being provided on a single substrate.

In this specification, the term "field effect" is used in its broadest sense to mean that a current which flows into a channel between a source and a drain (a current path between portions or regions corresponding to a source and a drain) is controlled by an electric field produced by a voltage applied to a gate (or to a portion or region corresponding to a gate) (where "control" includes the formation or extinction of a current path) . Depending upon the case, the source, drain, channel and gate may be expressed as a source region or source portion, drain region or drain portion, channel region or channel portion and gate region and gate portion, respectively.

The organic FET may be expressed differently, namely as an organic FET comprising: an organic conductor (first organic conductor) in which a source region, a channel region and a drain region are connected seamlessly therein so as to form a current path from the source region to the drain region via the channel region; a conductor (second conductor) acting as a gate to which is applied a voltage that controls conductivity of the channel region of the organic conductor; an insulator provided between the organic conductor and the conductor; and a substrate for supporting one of the organic conductor and the conductor; wherein the insulator is spread over an area that covers at least the channel region on one surface of the organic conductor, and the conductor resides in an area in which it overlaps the channel region and does not overlap the source region and the drain region. Expressed more generally, the present invention provides an organic electronic device. The organic electronic device comprises: an organic conductor (first organic conductor) in which a first region, a second region and a third region are connected seamlessly therein so as to form a current path from the first region to the third region via the second region (channel) ; a conductor (second conductor) to which is applied a voltage that controls conductivity of the second region of the organic conductor; an insulator provided between the organic conductor and the conductor; and a substrate for supporting one of the organic conductor and the conductor; wherein the insulator is spread over an area that covers at least the second region on one surface of the organic conductor, and the conductor resides in an area in which it overlaps the second region and does not overlap the first region and the third region.

Most generally, the organic conductor is obtained by doping an organic semiconductor with a dopant (a substance exhibiting an electron acceptor property or electronic donor property) . Types of organic semiconductors include polythiophene, polypyrrole, polyaniline, polyacetylene, polyphenylene, polyfuran, polyselenophene, polyisothianaphthene, polyphenylene sulfide, polyphenylenevinylene, polythienylenevinylene, polynaphthalene, polyanthracene, polypyrene, polyazulene, phthalocyanine, pentacene, melocyanine and derivatives thereof. Dopants include iodine, perchloric acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boron tetrafluoride, arsenic pentafluoride, hexafluorophosphate, alkyl sulfonate, perfluoroalkyl sulfonate, polyacrylic acid and polystyrene sulfonate, etc. These organic semiconductors and dopants can be combined freely.

Examples of the organic conductor (first organic conductor) are an organic conducting layer and an organic conducting film. The conductor (second organic conductor) may be an organic or inorganic material so long as the material is electrically conductive.

Similarly, the insulator may be an organic insulator or an inorganic insulator. It should be understood that the insulator includes an air layer.

The conductor (second conductor) and insulator also include those in the form of a film or layer.

The substrate may be formed from either an organic or inorganic material, and an insulator or conductor (or semiconductor) is used depending upon the form or type of the organic field-effect transistor or organic electronic device.

By way of example, the substrate is an insulator in a case where the organic conductor (first organic conductor) is supported on the substrate. Of course, an insulating layer can be formed on a conductive substrate and an organic conducting layer may be provided on the insulating layer. The organic conductor itself can be used as the substrate (i.e., the organic conductor may serve the dual purposes of an organic conductor and substrate) .

The substrate may be a conductor or an insulator in a case where the conductor (second conductor) is supported on the substrate. Thus the conductor (second conductor) can serve the dual purposes of a conductor and substrate. An insulating layer may be formed on the conductive substrate and a conductor (second conductor) may be provided on the insulating layer.

Most generally, the organic conductor (first organic conductor) , insulator and conductor (second conductor) are formed in layer or film form and are built up on the substrate. In a case where the insulator is an organic insulator, the conductor (second conductor) is an organic conductor (second organic conductor) and the substrate is an organic substrate, the organic field- effect transistor or organic electronic device according to the present invention is implemented as an all- organic type .

The transistor or device is of the normally-on and normally-off type. In the normally-on type, a current flows into the channel (second region) of the organic conductor (first organic conductor) in a state in which voltage is not applied to the conductor (second conductor) (gate) . In the nor ally-off type, the current does not flow in the above-mentioned state. In either case, the current (conductivity) that flows into the channel (second region) can be controlled by the voltage applied to the conductor. The organic field-effect transistor or organic electronic device according to the present invention acts as a switching element or amplifying element. The present invention is characterized in that the source (first region), channel (second region) and drain (third region) are connected seamlessly within the organic conductor (first organic conductor) . The extent of the region of the channel (second region) is defined by the extent of the spread of the conductor (second conductor) provided via the insulator, and the regions connected to both sides of the region ^Jof the channel (second region) in the organic conductor are portions that act as the source (first region) and drain (third region) . It will suffice if the source (first region) and drain (third region) have a width needed to effect an electrical (electronic) connection to other electronic or electrical circuits (inclusive of simple wiring and electrical connections) .

Thus, according to the present invention, the source (first region), channel (second region) and drain (third region) can be formed seamlessly by a single organic conductor. As a result, the structure is simple and manufacture easy because the organic conductor, insulator and conductor are supported on a single substrate.

In one embodiment, the organic field-effect transistor or organic electronic device has a low operating voltage and exhibits a switching function (the ON/OFF ratio of which is 100 or greater) when a voltage applied to the conductor acting as the gate falls within the range of -5 V to 5 V. Typically, if special configurations are excluded, methods of manufacture are classified into two types. The first type is a method of building up an organic conducting layer, insulating layer and conducting layer on a substrate in the order mentioned. The second type is a method of building up a conducting layer, insulating layer and organic conducting layer on a substrate in the order mentioned.

The first manufacturing method is defined in concrete terms as follows: The first manufacturing method comprises the steps of: forming an organic conducting layer on a single insulating substrate in such a manner that portions acting as a source, channel and drain are rendered continuous; forming an insulating layer on the organic conducting layer so as to cover at least the portion acting as the channel (excluding at least part of the portions acting as the source and drain) ; and forming a conducting layer acting as a gate on the insulating layer so as to overlap the portion of the organic conducting layer serving as the channel (so as not to overlap the portions serving as the source and drain) .

It is possible to manufacture an organic field- effect transistor by implementing at least three patterning processes, namely patterning of an organic conducting layer, patterning of an insulating layer and patterning of a conducting layer.

If the first manufacturing method is expressed generally as a method of manufacturing an organic electronic device, the method comprises the steps of: forming an organic conducting layer on a single insulating substrate in such a manner that portions acting as a first region, second region and third region are rendered continuous; forming an insulating layer on the organic conducting layer with the exception of at least part of the first region and third region; and forming a conducting layer on the insulating layer so as to overlap the second region of the organic conducting layer.

The second method of manufacturing an organic electronic device comprises the steps of : forming a conducting layer on an insulating portion of a single substrate; forming an insulating layer so as to cover at least a portion of the conducting layer; and forming a second region that overlaps the portion of the conducting layer covered by the insulating layer, as well as a first region and a third region connected to the second region on both sides thereof so as not to overlap the conducting layer and, moreover, in a state in which they are insulated from the substrate.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

Brief Description of Drawings

Fig. 1 is a plan view illustrating a partially broken-away view of an organic FET according to a first embodiment of the present invention; Fig. 2 is a sectional view taken along line II-II in Fig. 1;

Fig. 3 is a graph illustrating a drain voltage vs. drain current characteristic of the organic FET;

Fig. 4 is a graph illustrating the relationship between the gate voltage of the organic FET and the square root of drain current;

Fig. 5 is a graph illustrating a change in the ON/OFF ratio of the organic FET with a change in gate voltage; Fig. 6, which is a plan view illustrating a process for manufacturing the organic FET, shows the patterning of a mask for forming a first organic conducting layer;

Fig. 7, which is a plan view illustrating a process for manufacturing the organic FET, shows a phase of the process where the first organic conducting layer has been formed;

Fig. 8, which is a plan view illustrating a process for manufacturing the organic FET, shows the patterning of a mask for forming an insulating layer; Fig. 9, which is a plan view illustrating a process for manufacturing the organic FET, shows the patterning of a mask for forming a second organic conducting layer; and

Fig. 10 is a sectional view corresponding to Fig. 2 and illustrating another embodiment of an organic FET according to the present invention. Best Mode for Carrying Out the Invention

Preferred embodiments of the present invention will now be described in detail with reference to the drawings . Figs. 1 and 2 illustrate the structure of an organic field-effect transistor (referred to simply as an "organic FET" below) according to a preferred embodiment of the present invention.

As shown in Figs. 1 and 2, a first organic conducting layer 20 is formed on an organic substrate 10. The organic conducting layer 20 has a source (source region) 21, a drain (drain region) 23 and a slender channel (channel region) 22 between the source and drain, The source 21, channel 22 and drain 23 are connected seamlessly. (The lines indicating the boundaries of the regions 21, 22 and 23 do not appear in Fig. 2.) The source 21 and drain 23 are spaced away from each other except for the portion where they are interconnected by the channel 22. An insulating layer 30 is formed on the organic conducting layer 20. In this embodiment, the insulating layer 30 is provided in the shape of a band that includes a zone covering the channel 22 and portions of the source 21 and drain 23 extending trapezoidally to the side of the channel 22. However, it will suffice if the insulating layer 30 covers the upper portion of at least the channel 22.

A second organic conducting layer 40 acting as a gate is formed on the insulating layer 30. The second organic conducting layer 40 is formed in the shape of long, slender band having a width that covers the entire length of the channel 22. Conversely, the area of the first organic conducting layer 20 overlapped by the second organic conducting layer 40 is the channel 22. The source 21 and drain 23 of the first organic conducting layer 20, and the second organic conducting layer 40, are used as terminals for effecting a connection to a power supply and other electrical or electronic circuits, etc. The connection to the power supply and other electrical or electronic circuits is achieved by a conductive adhesive, integration or simply by clips, etc. In the drawings, the first organic conducting layer

20 is formed up to the edges of the substrate 10.

However, it goes without saying that the first organic conducting layer 20 need not be formed on part of the substrate 10 or on the edges thereof. Further, though the insulating layer 30 and second organic conducting layer 40 also are formed in the shape of a band extending from one edge of the substrate 10 to the opposite edge thereof, these need not reach the edges. Specifically, the organic FET is such that the substrate 10 is PET [poly (ethylene terephthalate)] film having a thickness of 100 um, the first organic conducting layer 20 is poly(3 , 4-ethylene dioxythiophene) doped with poly (4-styrenesul onate) (referred to below as "PEDOT/PSS") having a thickness of 25 to 40 run, the insulating layer 30 is poly (4-vinylphenol) (referred to as "PVP" below) having a thickness of 400 n , and the second organic conducting layer 40 is PEDOT/PSS having a thickness of 200 to 500 nm. The width and length of the channel 22 are 0.23 mm and 1.03 mm, respectively. The organic FET according to this embodiment is an all-organic FET in which the substrate 10, first organic conducting layer 20, insulating layer 30 and second organic conducting layer 40 all consist of organic material. Fig. 3 is a graph illustrating a drain voltage vs. drain current characteristic of the above-described organic FET. In a case where a gate voltage (gate-source voltage) is not applied (V_G = 0 V) , the drain current increases substantially linearly with an increase in the drain voltage. A conducting channel is formed as the channel 22 of the first organic conducting layer 20. This organic FET is of the normally-on type.

When the gate voltage is raised, the drain current decreases. It is believed that this is due to holes within the conductive channel 22 recombining with electrons induced by the gate voltage (electric field) .

When the gate voltage exceeds 1.5 V, drain current no longer flows. That is, the organic FET assumes the OFF state. The organic FET exhibits a depression-type response when the gate voltage is in the positive range.

As shown in Fig. 4, which is a plot of the value of the square root of drain current I_D versus gate voltage V_G, the drain current I_D is zero when the gate voltage V_G is 1.5 V. The threshold value (voltage) is 1.5 V.

Conversely, if the gate voltage is increased in the negative direction, the drain current increases and exhibits an enhancement-type response. It is believed that this is due to holes induced within the channel 22. Fig. 5 illustrates the result of measuring the ON/OFF ratio, which is the ratio of the drain current in the ON state to the drain current in the OFF state. It will be understood that the ON/OFF ratio attains a value of 1000 when the gate voltage is 2 V, so that the switching function is fully achieved.

Figs. 6 to 9 illustrate a process for manufacturing the organic FET described above.

As shown in Fig. 6, a mask 51 is patterned on the PET film substrate 10 in an area thereof that excludes a region in which the first organic conducting layer 20 is to be formed. Though various well-known patterning methods can be used, the simplest method is to print the mask 51 using a laser printer. Next, the substrate 10 is coated with a PEDOT/PSS solution. Bar coating is satisfactory as the coating method.

This is followed by removing the mask 51, whereby the first organic conducting layer 20 having the patterns of the regions for the source 21, channel 22 and drain 23 is formed on the substrate 10, as illustrated in Fig. 7. The toner of the laser printer is removed by ultrasonic cleaning in toluene. As shown in Fig. 8, a mask 52 is patterned on the first organic conducting layer 20 with the exception of the area in which the insulating layer 30 is to be formed, then a coating of PVP is applied from an isopropanol solution. The mask 52 is then removed.

Finally, as shown in Fig. 9, a mask 53 is formed except in the area in which the second organic conducting layer 40 is to be formed, then a coating of PEDOT/PSS solution is applied. The mask 53 is then removed.

The masks 52, 53 (and mask 51 as well) can also be patterned using a conventional method such as photolithography or vacuum deposition, or patterning may be achieved simply by placing or affixing patterning paper or patterning tape. The coating can be implemented by spin coating or spray coating, etc., in addition to the above-mentioned bar coating.

Fig. 10 illustrates another example of the structure of an organic FET according to the present invention. Here a second organic conducting layer 40A acting as a gate is formed on an insulating substrate 10A, and an organic insulating layer 30A is formed on the insulating substrate 10A and the second organic conducting layer 40A with the exception of a portion of the second organic conducting layer 40A that is to serve as a connection terminal. A first organic conducting layer 20A is formed on the organic insulating layer 30A. The first organic conducting layer 20A has a source and drain formed in areas where they will not overlap the second organic conducting layer 40A, and a channel formed in an area where it will overlap the second organic conducting layer 40A. Alternatively, an insulating layer can be formed on an electrically conductive substrate, and the second organic conducting layer 40A can be formed on this insulating layer. The first organic conducting layer 20A is held in a state in which it is insulated from the substrate by the above- mentioned insulating layer or the organic insulating layer 30A .

As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims .

Claims

1. An organic field-effect transistor (FET) having a source, a channel and a drain consisting of a single organic conducting material, said source, channel and drain being continuous within an organic conductor; wherein a conductor acting as a gate is provided on one surface of said organic conductor via an insulator, and a region of said organic conductor in which said conductor overlaps said organic conductor serves as a channel region; one of said organic conductor and said conductor being provided on a single substrate.

2. The organic field-effect transistor according to claim 1, wherein said conductor is an organic conductor and said insulator is an organic insulator.

3. The organic field-effect transistor according to claim 1 or 2 , wherein said organic conductor is an organic conducting layer, said insulator is an insulating layer, and said conductor is a conducting layer; said organic conducting layer, said insulating layer and said conducting layer being built up on a single substrate.

4. The organic field-effect transistor according to any one of claims 1 to 3 , wherein said substrate is formed from an organic material .

5. The organic field-effect transistor according to any one of claims 1 to 4, wherein said transistor has a switching function when a gate voltage applied to said conductor falls within a range of -5 V to 5 V.

6. An organic field-effect transistor comprising: an organic conductor in which a source region, a channel region and a drain region are connected seamlessly therein so as to form a current path from the source region to the drain region via the channel region; a conductor acting as a gate to which is applied a voltage that controls conductivity of said channel region of said organic conductor; an insulator provided between said organic conductor and said conductor; and a substrate for supporting one of said organic conductor and said conductor; wherein said insulator is spread over an area that covers at least said channel region on one surface of said organic conductor, and said conductor resides in an area in which it overlaps said channel region and does not overlap said source region and said drain region.

7. An organic electronic device, comprising: an organic conductor in which a first region, a second region and a third region are connected seamlessly therein so as to form a current path from the first region to the third region via the second region; a conductor to which is applied a voltage that controls conductivity of said second region of said organic conductor; an insulator provided between said organic conductor and said conductor; and a substrate for supporting one of said organic conductor and said conductor; wherein said insulator is spread over an area that covers at least said second region on one surface of said organic conductor, and said conductor resides in an area in which it overlaps said second region and does not overlap said first region and said third region.

8. A method of manufacturing an organic field-effect transistor comprising the steps of: forming an organic conducting layer on a single insulating substrate in such a manner that portions of said layer acting as a source, channel and drain are rendered continuous; forming an insulating layer on said organic conducting layer with the exception of at least part of the portions acting as the source and drain; and forming a conducting layer acting as a gate on said insulating layer so as to overlap the portion of said organic conducting layer serving as said channel .

9. A method of manufacturing an organic electronic device comprising the steps of: forming an organic conducting layer on a single insulating substrate in such a manner that portions of said layer acting as a first region, second region and third region are rendered continuous ; forming an insulating layer on said organic conducting layer with the exception of at least part of the first region and third region; and forming a conducting layer on said insulating layer so as to overlap said second region of said organic conducting layer .

10. A method of manufacturing an organic electronic device comprising the steps of: forming a conducting layer on an insulating portion of a single substrate; forming an insulating layer so as to cover at least a portion of said conducting layer; and forming a second region that overlaps the portion of said conducting layer covered by said insulating layer, as well as a first region and a third region connected to said second region on both sides thereof so as not to overlap said conducting layer and, moreover, in a state in which they are insulated from said substrate.