In recent years, a thin-film transistor has been developed, in which a carbon nanomaterial such as a carbon nanotube is used in a semiconductor layer (see Patent Literature 1 and Patent Literature 2, for example).
The carbon nanotube has a structure in which a graphene sheet is rolled up in a cylinder shape and generally has a straw shape. The carbon nanotube is classified into a single-walled carbon nanotube (SWCNT) formed of a single tube, a double-walled carbon nanotube (DWCNT) in which two carbon nanotubes which have different diameters are laminated, and a multi-walled carbon nanotube (MWCNT) in which a large number of tubes which have different diameters are laminated, and applied researches have been developed so as to bring out their characteristics of each structure. In the SWCNT, for example, there is a structure that exhibits semiconductor characteristics due to rolling up manner of the graphene sheet, high mobility is expected, and thus the researches on the SWCHT are energetically carried out for an application to a thin film transistor (TFT) (see Non-Patent Literatures 1-4, for example). Non-Patent Literature 3 reports that the TFT using the carbon nanotube has a high property equal to or greater than silicon. Since the DWCNT and MWCNT show high electrical conductivity, the researches on them are carried out for applications to an electrode material, conductive material for wiring, antistatic film and transparent electrode.
When the carbon nanotube or mixture of the carbon nanotubes is used as a semiconductor material, one carbon nanotube, several carbon nanotubes or a large number of carbon nanotubes are dispersed to make the TFT (see Patent Literature 3, for example). When a small number of the carbon nanotubes are used, since many of the carbon nanotubes generally have a length of about 1 μm or less, producing the TFT needs micromachining, and it is necessary to make a channel length between a source electrode and a drain electrode on a submicron scale. According to Patent Literature 3, for example, a method of manufacturing a carbon nanotube semiconductor device comprises: dropping a solution containing a carbon nanotube with conductor property and a carbon nanotube with semiconductor property onto a first electrode, a second electrode, and a region between the first electrode and the second electrode overlapped with a third electrode through the insulating film while an alternating current voltage is applied between the first electrode and the second electrode which are located over an insulating film over a third electrode; controlling the carbon nanotubes in a predetermined alignment direction; and applying a direct current voltage between the first electrode and the second electrode to remove the carbon nanotube with conductor property, wherein the first electrode is connected with the second electrode through the carbon nanotube with semiconductor property in the carbon nanotube semiconductor device. A distance between the second electrode and the third electrode is 5 μm to 50 μm.
On the other hand, when a large number of the carbon nanotubes are used, since a network of the carbon nanotubes is used as a channel, the channel length can be made longer, and the TFT can be easily manufactured (see Non-Patent Literature 5, for example). In order to make the thin film by dispersing a large number of the carbon nanotubes, the thin film can be easily made (see Non-Patent Literatures 6-9, for example) if a solution or dispersion liquid of the carbon nanotubes is used.
In the TFT, flexibility can be given to the entire film by forming the thin film of the carbon nanotubes as the semiconductor layer on a resin substrate in a process using the solution or dispersion liquid. If an applying or printing process is applied, a manufacturing cost can be decreased.
In the TFT using the semiconductor layer including the carbon nanotubes, the efficiency, especially field-effect mobility, can be enhanced, the TFT can be operated at a higher speed than an organic TFT including no carbon nanotube, and thus the scope of an applied electronic device can be expanded greatly.
A field effect transistor described in Patent Literature 1 comprises: a semiconductor layer through which carriers injected from a source region travel toward a drain region, the semiconductor layer being formed from a composite material comprising an organic semiconductor material and nanotubes.
A method of producing a thin film transistor, described in Patent Literature 2, comprising: a gate electrode formation step that forms a gate electrode on a substrate; a gate insulating layer formation step that forms a gate insulating layer on the substrate in such a manner as to cover the gate electrode formed in the gate electrode formation step; a source/drain electrodes formation step that forms a source electrode and a drain electrode on the gate insulating layer; and a semiconductor layer formation step that applies an aqueous solution for semiconductor layer formation which is an aqueous solution comprising at least single wall carbon nanotubes and a surfactant between the source electrode and the drain electrode formed in the source/drain electrodes formation step by a coating process to form a semiconductor layer comprising the single wall carbon nanotube.    Patent Literature 1: WO2005/008784    Patent Literature 2: JPA2008-235880    Patent Literature 3: JPA2004-71654    Non-Patent Literature 1: S. J. Tans et al., Room-temperature transistor based on a single carbon nanotube, NATURE, (1998) vol. 393, p. 49    Non-Patent Literature 2: R. Martel et al., Single- and multi-wall carbon nanotube field-effect transistors, Appl. Phys. Lett., (1998) vol. 73, No. 17, p. 2447    Non-Patent Literature 3: S. Wind et al., Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes, Appl. Phys. Lett., (2002) vol. 80, No. 20, p. 3817    Non-Patent Literature 4: K. Xiao et al., High-mobility thin-film transistors based on aligned carbon nanotubes, Appl. Phys. Lett., (2003) vol. 83, No. 1, p. 150    Non-Patent Literature 5: S. Kumar et al., Performance of carbon nanotube-dispersed thin-film transistors, Appl. Phys. Lett., (2006) vol. 89, p. 143501    Non-Patent Literature 6: N. Saran et al., Fabrication and Characterization of thin films of single-walled carbon nanotube bundles on flexible plastic substrates, J. Am. Chem. Soc., (2004) vol. 126, p. 4462    Non-Patent Literature 7: Z. Wu et al., Transparent, conductive carbon nanotube films, SCIENCE, (2004) No. 305, p. 1273    Non-Patent Literature 8: M. Zhang et al., Strong, transparent, multifunctional, carbon nanotube sheets, SCIENCE, (2005) No. 309, p. 1215    Non-Patent Literature 9: Y. Zhou et al., A method of printing carbon nanotube thin films, Appl. Phys. Lett., (2006) vol. 88, p. 123109
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