# Patent

## Title
Thin film transistor

## Abstract

A thin film transistor includes a source electrode, a drain electrode, a semiconducting layer, and a gate electrode. The drain electrode is spaced from the source electrode. The semiconducting layer is connected to the source electrode and the drain electrode. The gate electrode is insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer. The semiconducting layer includes a carbon nanotube film, a plurality of carbon nanotubes in the carbon nanotube film oriented along a direction from the source electrode to the drain electrode.

## Background
RELATED APPLICATIONS
This application is related to commonly-assigned applications entitled, “METHOD FOR MAKING THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,245, filed Apr. 2, 2009; “METHOD FOR MAKING THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,331, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,329, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,310, filed Apr. 2, 2009; “THIN FILM TRANSISTOR PANEL”, U.S. patent application Ser. No. 12/384,309, filed Apr. 2, 2009; “THIN FILM TRANSISTOR PANEL”, U.S. patent application Ser. No. 12/384,244, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,281, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No 12/384,299, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,292, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,293, filed Apr. 2, 2009; “METHOD FOR MAKING THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,241, filed Apr. 2, 2009; “THIN FILM TRANSISTOR”, U.S. patent application Ser. No. 12/384,238, filed Apr. 2, 2009. The disclosures of the above-identified applications are incorporated herein by reference.
BACKGROUND
1. Technical Field
The present disclosure relates to thin film transistors and, particularly, to a carbon nanotube based thin film transistor.
2. Discussion of Related Art
A typical thin film transistor (TFT) is made of a substrate, a gate electrode, an insulation layer, a drain electrode, a source electrode, and a semiconducting layer. The thin film transistor performs a switching operation. In use, the thin film transistor modulates carriers in an interface between the insulation layer and the semiconducting layer from an accumulation state to a depletion state, with voltage applied to the gate electrode. Thus, the thin film transistor can change the amount of the current passing between the drain electrode and the source electrode. In practical use, a high carrier mobility affect of the material of the semiconducting layer of the thin film transistor is desired.
Previously, the material of the semiconducting layer has been amorphous silicone (a-Si), poly-silicone (p-Si), or organic semiconducting material. The carrier mobility of an a-Si thin film transistor is relatively lower than a p-Si thin film transistor. However, the method for making the p-Si thin film transistor is complicated and has a high cost. The organic thin film transistor is flexible but has low carrier mobility.
Carbon nanotubes (CNTs) are a novel carbonaceous material and have received a great deal of interest since the early 1990s. Carbon nanotubes have interesting and potentially useful heat conducting, electrical conducting, and mechanical properties. Further, there are two kinds of carbon nanotubes: metallic carbon nanotubes and semiconducting carbon nanotubes determined by the arrangement of the carbon atoms therein. The carrier mobility of semiconducting carbon nanotubes along a length direction thereof can reach about 1000 to 1500 cm2V−1s−1. Thus, a thin film transistor adopting carbon nanotubes as a semiconducting layer has been produced.
A conventional carbon nanotube based thin film transistor is generally made by the method of printing the mixture of carbon nanotubes and a polymer on a substrate to form a semiconducting layer. There are some problems in the thin film transistor adopting carbon nanotubes as a semiconducting layer. Firstly, the carbon nanotubes are prone to aggregate in the mixture. Thus, the carbon nanotubes cannot be uniformly dispersed in the carbon nanotube layer. Secondly, the organic solvent is hard to eliminate from the carbon nanotube layer. Thus, impurities exist in the carbon nanotube layer. Thirdly, the carbon nanotubes in the carbon nanotube layer lack high carrier mobility and cannot be well used in the thin film transistor. Additionally, the carbon nanotube layer formed by the printing method is inflexible. Accordingly, the thin film transistor is inflexible.
What is needed, therefore, is providing a thin film transistor that has high carrier mobility and is flexible.



BRIEF DESCRIPTION OF THE DRAWINGS
Many aspects of the present thin film transistor can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present thin film transistor.
FIG. 1 is a cross sectional view of a thin film transistor in accordance with a first embodiment.
FIG. 2 shows a Scanning Electron Microscope (SEM) image of a carbon nanotube film containing semiconducting carbon nanotubes used in the thin film transistor of FIG. 1.
FIG. 2A is a local structural schematic of the carbon nanotube film of FIG. 2.
FIG. 2B is a schematic view of one embodiment of a method for making the carbon nanotube film in FIG. 2.
FIG. 3 is a schematic view of the thin film transistor of FIG. 1 connected to a circuit.
FIG. 4 is a cross sectional view of a thin film transistor in accordance with a second embodiment.



Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of the present thin film transistor, in at least one form, and such exemplifications are not to be construed as limiting the scope of the disclosure in any manner.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
References will now be made to the drawings to describe, in detail, embodiments of the present thin film transistor.
Referring to FIG. 1, a thin film transistor 10 is provided in a first embodiment, and has a top gate structure. The thin film transistor 10 includes a semiconducting layer 140, a source electrode 151, a drain electrode 152, an insulating layer 130, and a gate electrode 120. The thin film transistor 10 is located on an insulating substrate 110.
The semiconducting layer 140 is located on the insulating substrate 110. The source electrode 151 and the drain electrode 152 are connected to the semiconducting layer 140 and spaced from each other a certain distance. The insulating layer 130 is located between the semiconducting layer 140 and the gate electrode 120. The insulating layer 130 is located at least on the semiconducting layer 140, or covers at least part of the semiconducting layer 140, the source electrode 151, and the drain electrode 152. The gate electrode 120 is located on the insulating layer 130. The gate electrode 120 is located on the semiconducting layer 140 and insulated from the semiconducting layer 140, the source electrode 151, and the drain electrode 152 by the insulating layer 130. A channel 156 is formed in the semiconducting layer 140 at a region between the source electrode 151 and the drain electrode 152.
The source electrode 151 and the drain electrode 152 can be located on the semiconducting layer 140 or on the insulating substrate 110. More specifically, the source electrode 151 and the drain electrode 152 can be located on a top surface of the semiconducting layer 140, and on a same side of the semiconducting layer 140 as the gate electrode 120. In other embodiments, the source electrode 151 and the drain electrode 152 can be located on the insulating substrate 110 and covered by the semiconducting layer 140. The source electrode 151 and the drain electrode 152 are located on different sides of the semiconducting layer 140. In other embodiments, the source electrode 151 and the drain electrode 152 can be formed on the insulating substrate 110, and coplanar with the semiconducting layer 140.
The insulating substrate 110 is provided for supporting the thin film transistor 10. The material of the insulating substrate 110 can be the same as a substrate of a printed circuit board (PCB), and can be selected from rigid materials (e.g., p-type or n-type silicon, silicon with an silicon dioxide layer formed thereon, crystal, crystal with a oxide layer formed thereon), or flexible materials (e.g., plastic or resin). In the present embodiment, the material of the insulating substrate is glass. The shape and size of the insulating substrate 110 is arbitrary. A plurality of thin film transistors 10 can be located on one insulating substrate 110 to form a thin film transistor panel.
The material of the semiconducting layer 140 can be selected from a group consisting of amorphous silicone (a-Si), poly-silicone (p-Si), organic semiconducting material, or semiconducting carbon nanotubes. In the present embodiment, the semiconducting layer 140 is a semiconducting carbon nanotube layer. The semiconducting carbon nanotube layer includes a plurality of single-walled carbon nanotubes, double-walled carbon nanotubes, or combination thereof. A diameter of the single-walled carbon nanotubes is in the range from about 0.5 nanometers to about 50 nanometers. A diameter of the double-walled carbon nanotubes is in the range from about 1.0 nanometer to about 50 nanometers. In the present embodiment, the diameter of the semiconducting carbon nanotubes is less than 10 nanometers.
Referring to FIG. 2 and FIG. 2A, the semiconducting carbon nanotube layer comprises one carbon nanotube film 160. The carbon nanotube film 160 is formed by a plurality of carbon nanotubes 165 primarily oriented along a same direction in each film and parallel to a surface of the carbon nanotube film 160. In one embodiment, the ordered carbon nanotube film 160 includes a plurality of successive and oriented carbon nanotubes 165 joined end to end by van der Waals attractive force. At least a portion of the carbon nanotubes 165 in the semiconducting layer 140 are aligned along a direction from the source electrode 151 to the drain electrode 152, to form a channel 156 from the source electrode 151 to the drain electrode 152. In the present embodiment, all the carbon nanotubes 165 in the semiconducting layer 140 are aligned along the direction from the source electrode 151 to the drain electrode 152.
Referring to FIG. 2B, the carbon nanotube film 160 is pulled out from a super-aligned carbon nanotube array 162. The carbon nanotube film 160 includes a plurality of successively oriented carbon nanotube segments 163 joined end-to-end by van der Waals attractive force. Furthermore, each carbon nanotube segment 163 has a plurality of the carbon nanotubes 165. The carbon nanotubes 165 have substantially the same length and are parallel to each other. Adjacent carbon nanotubes 165 are combined together by van der Waals attractive force. Because carbon nanotubes 165 have better electrical conductivity along a length direction, the ordered carbon nanotube film 160 have higher carrier mobility along the aligned direction than the disorder carbon nanotube film in the prior art. A thickness of the carbon nanotube film 160 is in the range from about 0.5 nanometers to about 100 microns.
A length of the semiconducting layer 140 can be in the range from about 1 micron to about 100 microns. A width of the semiconducting layer 140 ranges from about 1 micron to about 1 millimeter. A thickness of the semiconducting layer 140 ranges from about 0.5 nanometers to about 100 microns. A length of the channel 156 can range from about 1 micron to about 100 microns. A width of the channel 156 ranges from about 1 micron to about 1 millimeter. In the present embodiment, the length of the semiconducting layer 140 is about 50 microns, the width of the semiconducting layer 140 is about 300 microns, the thickness of the semiconducting layer 140 is about 1 micron, the length of the channel 156 is about 40 microns, and the width of the channel 156 is about 300 microns. In one embodiment, the semiconducting layer 140 includes one carbon nanotube film 160. All the carbon nanotubes 165 in the carbon nanotube film are oriented along a direction from the source electrode 151 to the drain electrode 152.
The material of the source electrode 151, the drain electrode 152 and the gate electrode 120 has a good conductive property, and can be selected from a group consisting of pure metals, metal alloys, indium tin oxide (ITO), antimony tin oxide (ATO), silver paste, conductive polymer, metallic carbon nanotubes and combinations thereof. The pure metals and metal alloys can be selected from a group consisting of aluminum, copper, tungsten, molybdenum, gold, cesium, palladium and combinations thereof. A thickness of the source electrode 151, the drain electrode 152 and the gate electrode 120 is about 0.5 nanometers to about 100 microns. A distance between the source electrode 151 and the drain electrode 152 is about 1 to about 100 microns.
In one embodiment, when the source electrode 151 and the drain electrode 152 are made of pure metals, metal alloys, indium tin oxide (ITO), or antimony tin oxide (ATO), a conducting layer can be formed by a depositing, sputtering, evaporating method, and etched to form the source electrode 151 and the drain electrode 152. In another embodiment, the source electrode 151 and the drain electrode 152 made of silver paste or conductive polymer can be formed directly by a print method.
The material of the insulating layer 130 can be a rigid material such as silicon nitride (Si3N4) or silicon dioxide (SiO2), or a flexible material such as polyethylene terephthalate (PET), benzocyclobutenes (BCB), or acrylic resins. A thickness of the insulating layer 130 can be in the range from about 5 nanometers to about 100 microns. In the present embodiment, the insulating layer 130 is Si3N4.
Referring to FIG. 3, in use, the source electrode 151 is grounded. A voltage Vds is applied to the drain electrode 152. Another voltage Vg is applied to the gate electrode 120. The voltage Vg forms an electric field in the channel 156 of the semiconducting layer 140. Accordingly, carriers exist in the channel 156 nearing the gate electrode 120. As Vg increases, current can flow from the source electrode 151 to the drain electrode 152 through the channel 156, thus the thin film transistor 10 is in an ON state, the source electrode 151 and the drain electrode 152 are electrically connected. When the semiconducting layer 140 is made of semiconducting carbon nanotube, and the carbon nanotubes are aligned along a direction from the source electrode 151 to the drain electrode 152, the high carrier mobility of the carbon nanotubes along the length direction thereof is very useful in the thin film transistor 10. The carrier mobility of the thin film transistor 10 in the present embodiment is higher than 10 cm2/V−1s−1 (e.g., 10 to 1500 cm2/V−1s−1), and the on/off current ratio is in the range from about 1.0×102 to about 1.0×106.
Referring to FIG. 4, a thin film transistor 20 is provided in a second embodiment and has a bottom gate structure. The thin film transistor 20 includes a gate electrode 220, an insulating layer 230, a semiconducting layer 240, a source electrode 251, and a drain electrode 252. The thin film transistor 20 is located on an insulating substrate 210.
The structure of the thin film transistor 20 in the second embodiment is similar to the thin film transistor 10 in the first embodiment. The difference is that, in the second embodiment, the gate electrode 220 is located on the insulating substrate 210. The insulating layer 230 covers the gate electrode 220. The semiconducting layer 240 is located on the insulating layer 230, and insulated from the gate electrode 220 by the insulating layer 230. The source electrode 251 and the drain electrode 252 are spaced apart from each other and connected to the semiconducting layer 240. The source electrode 251 and the drain electrode 252 are insulated from the gate electrode 220 by the insulating layer 230. A channel 256 is formed in the semiconducting layer 240 at a region between the source electrode 251 and the drain electrode 252.
The source electrode 251 and the drain electrode 252 can be located on the semiconducting layer 240 or on the insulating layer 230. More specifically, the source electrode 251 and the drain electrode 252 can be located on a top surface of the semiconducting layer 240, and at the same side of the semiconducting layer 240 with the gate electrode 220. In other embodiments, the source electrode 251 and the drain electrode 252 can be located on the insulating layer 230 and covered by the semiconducting layer 240. The source electrode 251 and the drain electrode 252 are located on different sides of the semiconducting layer 240. In other embodiments, the source electrode 251 and the drain electrode 252 can be formed on the insulating layer 230, and coplanar with the semiconducting layer 240.
The thin film transistors provided in the present embodiments have the following superior properties. Firstly, the semiconducting carbon nanotube layer is tough and flexible. Thus, thin film transistors using semiconducting carbon nanotube layers as semiconducting layer are durably flexible. Secondly, the semiconducting carbon nanotube layer is durable at high temperatures. Thirdly, the carbon nanotubes in the carbon nanotube film are aligned along a same direction and joined end to end by Van der Waals attractive force. Thus, in the semiconducting layer of the thin film transistor, the carbon nanotubes can be easily arranged to align along a direction from source electrode to drain electrode. Accordingly, the carrier mobility of the thin film transistor can be improved.
It is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the invention but do not restrict the scope of the invention.

## Claims

1. A thin film transistor comprising:
a source electrode;
a drain electrode spaced from the source electrode;
a semiconducting layer connected to the source electrode and the drain electrode;
an insulating layer; and
a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by the insulating layer;
wherein the semiconducting layer comprises a carbon nanotube film, and the carbon nanotube film comprises a plurality of carbon nanotubes primarily oriented along a same direction and joined end to end by van der Waals attractive force, and the plurality of carbon nanotubes are oriented along a direction from the source electrode to the drain electrode.

2. The thin film transistor as claimed in claim 1, wherein the plurality of carbon nanotubes are semiconducting carbon nanotubes.

3. The thin film transistor as claimed in claim 1, wherein the plurality of carbon nanotubes are single-walled carbon nanotubes, double-walled carbon nanotubes, or any combination thereof.

4. The thin film transistor as claimed in claim 1, wherein a diameter of the plurality of carbon nanotubes is less than 10 nanometers.

5. The thin film transistor as claimed in claim 1, wherein the carbon nanotube film comprises a plurality of successively oriented carbon nanotube segments joined end-to-end by van der Waals attractive force therebetween, the carbon nanotube segments consist of the plurality of the carbon nanotubes, the carbon nanotubes have the substantially same length, adjacent carbon nanotubes are combined by van der Waals attractive force therebetween.

6. The thin film transistor as claimed in claim 1, wherein a thickness of the carbon nanotube film ranges from about 0.5 nanometer to about 100 microns.

7. The thin film transistor as claimed in claim 1, wherein a material of the insulating layer is selected from a group consisting of silicon nitride, silicon dioxide, polyethylene terephthalate, benzocyclobutenes, and acrylic resins.

8. The thin film transistor as claimed in claim 1, wherein a material of the source electrode, drain electrode, or gate electrode is selected from a group consisting of pure metals, metal alloys, indium tin oxide, antimony tin oxide, silver paste, conductive polymer, metallic carbon nanotubes and combinations thereof.

9. The thin film transistor as claimed in claim 8, wherein a material of the source electrode, drain electrode, or gate electrode is selected from the group consisting of aluminum, copper, tungsten, molybdenum, gold, cesium, palladium and the combination thereof.

10. The thin film transistor as claimed in claim 1, wherein the insulating layer is located between the semiconducting layer and the gate electrode.

11. The thin film transistor of claim 1, wherein the semiconducting layer is located on an insulating substrate, the source electrode and the drain electrode are located on a surface of the semiconducting layer, the insulating layer is located on the semiconducting layer, and the gate electrode is located on the insulating layer.

12. The thin film transistor of claim 11, wherein a material of the insulating substrate is selected from the group consisting of silicon nitride, silicon dioxide, polyethylene terephthalate, benzocyclobutenes, acrylic resins and combinations thereof.

13. The thin film transistor of claim 11, wherein the source electrode and the drain electrode are located on a same side of the semiconducting layer.

14. The thin film transistor of claim 11, wherein the source electrode and the drain electrode are located on different sides of the semiconducting layer.

15. The thin film transistor of claim 1, wherein the gate electrode is located on an insulating substrate, the insulating layer is located on the gate electrode, the semiconducting layer is located on the insulating layer, the source electrode and the drain electrode are located on a surface of the semiconducting layer.

16. The thin film transistor of claim 1, further comprising a channel formed in the semiconducting layer at a region between the source electrode and the drain electrode, a length of the channel ranging from 1 micron to 100 microns, and a width of the channel ranging from about 1 micron to about 1 millimeter.

17. The thin film transistor of claim 16, wherein the channel comprises one or more of the plurality of carbon nanotubes.

18. The thin film transistor of claim 1, wherein the carrier mobility of the thin film transistor ranges from about 10 to about 1500 cm2/V−1s−1, and the on/off current ratio ranges from about 1.0×102 to about 1.0×106.

19. A thin film transistor comprising:
a source electrode;
a drain electrode spaced from the source electrode;
a semiconducting layer connected to the source electrode and the drain electrode;
an insulating layer; and
a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by the insulating layer;
wherein the semiconducting layer comprises a plurality of semiconducting carbon nanotubes joined end-to-end by van der Waals attractive force therebetween, and the plurality of semiconducting carbon nanotubes are oriented along a direction from the source electrode to the drain electrode.

20. A thin film transistor comprising:
a source electrode;
a drain electrode spaced from the source electrode;
a semiconducting layer connected to the source electrode and the drain electrode;
an insulating layer; and
a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by the insulating layer;
wherein the semiconducting layer comprises a plurality of semiconducting carbon nanotubes directly joined end-to-end by van der Waals attractive force therebetween, and the plurality of semiconducting carbon nanotubes are oriented along a direction from the source electrode to the drain electrode.