Semiconductor device
The oxide semiconductor film has the top and bottom surface portions each provided with a metal oxide film containing a constituent similar to that of the oxide semiconductor film. An insulating film containing a different constituent from the metal oxide film and the oxide semiconductor film is further formed in contact with a surface of the metal oxide film, which is opposite to the surface in contact with the oxide semiconductor film. The oxide semiconductor film used for the active layer of the transistor is an oxide semiconductor film highly purified to be electrically i-type (intrinsic) by removing impurities such as hydrogen, moisture, a hydroxyl group, and hydride from the oxide semiconductor and supplying oxygen which is a major constituent of the oxide semiconductor and is simultaneously reduced in a step of removing impurities.
1. A semiconductor device comprising:
a first gate electrode;
a first gate insulating film over the first gate electrode;
a first metal oxide film over and in contact with the first gate insulating film;
an oxide semiconductor film over and in contact with the first metal oxide film;
a source electrode and a drain electrode in contact with the oxide semiconductor film;
a second gate insulating film over the source electrode and the drain electrode; and
a second gate electrode over the second gate insulating film,
wherein the first metal oxide film comprises one or more constituent metal elements of the oxide semiconductor film, and
wherein a side surface of each of the first metal oxide film and the oxide semiconductor film is tapered and in contact with the source electrode or the drain electrode.
2. The semiconductor device according to claim 1 , wherein at least part of a top surface of the oxide semiconductor film is in contact with the source electrode and the drain electrode.
3. The semiconductor device according to claim 1 , wherein the first metal oxide film has a larger energy gap than the oxide semiconductor film.
4. The semiconductor device according to claim 1 , wherein an energy at a bottom of a conduction band of the first metal oxide film is higher than an energy at a bottom of a conduction band of the oxide semiconductor film.
5. The semiconductor device according to claim 1 , wherein the oxide semiconductor film comprises In, Ga, and Zn.
6. The semiconductor device according to claim 1 , wherein the first metal oxide film comprises gallium.
7. A semiconductor device comprising:
a first gate electrode;
a first gate insulating film over the first gate electrode;
a first metal oxide film over and in contact with the first gate insulating film;
an oxide semiconductor film over and in contact with the first metal oxide film;
a source electrode and a drain electrode in contact with the oxide semiconductor film;
a second metal oxide film over the source electrode and the drain electrode;
a second gate insulating film over the second metal oxide film; and
a second gate electrode over the second gate insulating film,
wherein the first metal oxide film comprises one or more constituent metal elements of the oxide semiconductor film, and
wherein a side surface of each of the first metal oxide film and the oxide semiconductor film is tapered and in contact with the source electrode or the drain electrode.
8. The semiconductor device according to claim 7 , wherein at least part of a top surface of the oxide semiconductor film is in contact with the source electrode and the drain electrode.
9. The semiconductor device according to claim 7 , wherein the first metal oxide film has a larger energy gap than the oxide semiconductor film.
10. The semiconductor device according to claim 7 , wherein an energy at a bottom of a conduction band of the first metal oxide film is higher than an energy at a bottom of a conduction band of the oxide semiconductor film.
11. The semiconductor device according to claim 7 , wherein the oxide semiconductor film comprises In, Ga, and Zn.
12. The semiconductor device according to claim 7 , wherein the first metal oxide film comprises gallium.