It is an object to provide a thin film transistor with high speed operation, in which a large amount of current can flow when the thin film transistor is on and off-state current is extremely reduced when the thin film transistor is off. The thin film transistor is a vertical thin film transistor in which a channel formation region is formed using an oxide semiconductor film in which hydrogen is contained in an oxide semiconductor at a concentration of lower than or equal to 5×10 19 /cm 3 , preferably lower than or equal to 5×10 18 /cm 3 , more preferably lower than or equal to 5×10 17 /cm 3 , hydrogen or an OH group contained in the oxide semiconductor is/are removed, and carrier concentration is lower than or equal to 5×10 14 /cm 3 , preferably lower than or equal to 5×10 12 /cm 3 .
1. A transistor comprising:
a first electrode over a substrate, wherein end portions of the first electrode have a tapered shape;
an oxide semiconductor film on and in contact with the first electrode, wherein end portions of the oxide semiconductor film have a tapered shape;
a second electrode on and in contact with the oxide semiconductor film, wherein end portions of the second electrode have a tapered shape;
a gate insulating film on at least side surfaces of the oxide semiconductor film; and
a third electrode having a ring shape, the third electrode adjacent to the side surfaces of the oxide semiconductor film with the gate insulating film interposed therebetween.
2. The transistor according to claim 1 ,
wherein the first electrode functions as one of a source electrode and a drain electrode,
wherein the second electrode functions as the other of the source electrode and the drain electrode, and
wherein the third electrode functions as a gate electrode.
3. The transistor according to claim 1 , wherein carrier concentration of the oxide semiconductor film is lower than or equal to 5×10 14 /cm 3 .
4. The transistor according to claim 1 , wherein hydrogen concentration of the oxide semiconductor film is lower than or equal to 5×10 19 /cm 3 .
5. A display device comprising the transistor according to claim 1 , wherein the display device is incorporated into one selected from the group consisting of a portable game machine, a digital camera, a television set, a computer, a mobile phone and a device including electronic paper.
6. A transistor comprising:
a first electrode over a substrate;
an oxide semiconductor film on and in contact with the first electrode;
a second electrode on and in contact with the oxide semiconductor film;
a gate insulating film on at least side surfaces of the oxide semiconductor film;
a third electrode having a ring shape, the third electrode adjacent to the side surfaces of the oxide semiconductor film with the gate insulating film interposed therebetween;
an interlayer insulating film over the third electrode,
an opening in the gate insulating film and the interlayer insulating film, and
a wiring electrically connected to the second electrode via the opening.
7. The transistor according to claim 6 ,
wherein the first electrode functions as one of a source electrode and a drain electrode,
wherein the second electrode functions as the other of the source electrode and the drain electrode, and
wherein the third electrode functions as a gate electrode.
8. The transistor according to claim 6 , wherein carrier concentration of the oxide semiconductor film is lower than or equal to 5×10 14 /cm 3 .
9. The transistor according to claim 6 , wherein hydrogen concentration of the oxide semiconductor film is lower than or equal to 5×10 19 /cm 3 .
10. The transistor according to claim 6 , wherein the interlayer insulating film comprises a material selected from the group consisting of a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film and an aluminum nitride oxide film.
11. An display device comprising the transistor according to claim 6 , wherein the display device is incorporated into one selected from the group consisting of a portable game machine, a digital camera, a television set, a computer, a mobile phone and a device including electronic paper.
12. A transistor comprising:
a first electrode over a substrate, wherein end portions of the first electrode have a tapered shape;
an island-shaped oxide semiconductor film on and in contact with the first electrode, wherein the island-shaped oxide semiconductor film includes a crystal grain, and end portions of the island-shaped oxide semiconductor film have a tapered shape;
a second electrode on and in contact with the island-shaped oxide semiconductor film, wherein end portions of the second electrode have a tapered shape;
a gate insulating film on at least side surfaces of the island-shaped oxide semiconductor film; and
a third electrode covering the side surfaces of the island-shaped oxide semiconductor film with the gate insulating film interposed therebetween.
13. The transistor according to claim 12 ,
wherein the first electrode functions as one of a source electrode and a drain electrode,
wherein the second electrode functions as the other of the source electrode and the drain electrode, and
wherein the third electrode functions as a gate electrode.
14. The transistor according to claim 12 , wherein carrier concentration of the island-shaped oxide semiconductor film is lower than or equal to 5×10 14 /cm 3 .
15. The transistor according to claim 12 , wherein hydrogen concentration of the island-shaped oxide semiconductor film is lower than or equal to 5×10 19 /cm 3 .
16. A display device comprising the transistor according to claim 12 , wherein the display device is incorporated into one selected from the group consisting of a portable game machine, a digital camera, a television set, a computer, a mobile phone and a device including electronic paper.