Semiconductor device and manufacturing method thereof

1. A semiconductor device comprising:

an oxide semiconductor layer including a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer;

a gate insulating film in contact with the oxide semiconductor layer; and

a gate electrode overlapping with the oxide semiconductor layer with the gate insulating film provided between the gate electrode and the oxide semiconductor layer,

wherein a first energy gap of the first oxide semiconductor layer is greater than or equal to 2.7 eV and less than or equal to 4.9 eV,

wherein a third energy gap of the third oxide semiconductor layer is larger than a second energy gap of the second oxide semiconductor layer and an energy is continuously changed in a vicinity of an interface between the second oxide semiconductor layer and the third oxide semiconductor layer in a band structure, and

wherein the third oxide semiconductor layer has a thickness of less than 5 nm.

2. The semiconductor device according to claim 1 ,

wherein the oxide semiconductor layer is in contact with an insulating film containing silicon on a side facing the gate insulating film and comprises a channel region which is apart from the insulating film.

3. The semiconductor device according to claim 1 ,

wherein the gate insulating film is provided over the gate electrode and the oxide semiconductor layer is provided over the gate insulating film.

4. The semiconductor device according to claim 1 ,

wherein the oxide semiconductor layer contains indium and at least one of aluminum, gallium, yttrium, zirconium, and tin.

5. The semiconductor device according to claim 1 ,

wherein the oxide semiconductor layer is one of a c-axis aligned crystalline oxide semiconductor layer, a polycrystalline oxide semiconductor layer, a microcrystalline oxide semiconductor layer, and an amorphous oxide semiconductor layer.

6. The semiconductor device according to claim 1 ,

wherein the second energy gap of is greater than or equal to 2.5 eV and less than or equal to 4.2 eV.

7. The semiconductor device according to claim 1 ,

wherein the first oxide semiconductor layer has a thickness greater than or equal to 5 nm and less than or equal to 100 nm, and

wherein the second oxide semiconductor layer has a thickness greater than or equal to 5 nm and less than or equal to 200 nm.

8. The semiconductor device according to claim 1 ,

wherein a carrier density of the second oxide semiconductor layer is lower than or equal to 1×10 17 /cm 3 .

9. A semiconductor device comprising:

an oxide semiconductor layer including a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer, each of the first oxide semiconductor layer and the third oxide semiconductor layer containing indium and an element M, wherein M represents aluminum, gallium, yttrium, zirconium, or tin;

a gate insulating film in contact with the oxide semiconductor layer; and

a gate electrode overlapping with the oxide semiconductor layer with the gate insulating film provided between the gate electrode and the oxide semiconductor layer,

wherein each of the first oxide semiconductor layer and the third oxide semiconductor layer contains the element M at a higher proportion than indium, wherein a third energy gap of the third oxide semiconductor layer is larger than a second energy gap of the second oxide semiconductor layer and an energy is continuously changed in a vicinity of an interface between the second oxide semiconductor layer and the third oxide semiconductor layer in a band structure, and

wherein the third oxide semiconductor layer has a thickness of less than 5 nm.

10. The semiconductor device according to claim 9 ,

wherein the oxide semiconductor layer is in contact with an insulating film containing silicon on a side facing the gate insulating film and comprises a region in which a channel is formed in a region apart from the insulating film.

11. The semiconductor device according to claim 9 ,

wherein the gate insulating film is provided over the gate electrode and the oxide semiconductor layer is provided over the gate insulating film.

12. The semiconductor device according to claim 9 ,

wherein the oxide semiconductor layer is one of a c-axis aligned crystalline oxide semiconductor layer, a polycrystalline oxide semiconductor layer, a microcrystalline oxide semiconductor layer, an amorphous oxide semiconductor layer.

13. The semiconductor device according to claim 9 ,

wherein amount of the element M in the first oxide semiconductor layer in an atomic ratio is 1.5 times or more as much as amount of indium in an atomic ratio.

14. The semiconductor device according to claim 9 ,

wherein the first oxide semiconductor layer contains the element M at a higher proportion than the second oxide semiconductor layer.

15. The semiconductor device according to claim 9 ,

wherein the third oxide semiconductor layer contains the element M at a higher proportion than the second oxide semiconductor layer.

16. The semiconductor device according to claim 9 ,

wherein the first oxide semiconductor layer has a thickness greater than or equal to 5 nm and less than or equal to 100 nm, and

wherein the second oxide semiconductor layer has a thickness greater than or equal to 5 nm and less than or equal to 200 nm.

17. The semiconductor device according to claim 9 ,

wherein a carrier density of the second oxide semiconductor layer is lower than or equal to 1×1017/cm 3 .