Thin film transistor, semiconductor device and electronic device
A thin film transistor includes, as a buffer layer, an amorphous semiconductor layer having nitrogen or an NH group between a gate insulating layer and source and drain regions and at least on the source and drain regions side. As compared to a thin film transistor in which an amorphous semiconductor is included in a channel formation region, on-current of a thin film transistor can be increased. In addition, as compared to a thin film transistor in which a microcrystalline semiconductor is included in a channel formation region, off-current of a thin film transistor can be reduced.
1. A thin film transistor comprising:
a gate electrode over a substrate having an insulating surface;
a gate insulating layer which covers the gate electrode;
a first semiconductor layer which is in contact with the gate insulating layer;
a second semiconductor layer which is stacked over the first semiconductor layer; and
impurity semiconductor layers forming a source region and a drain region which are in contact with part of the second semiconductor layer,
wherein the second semiconductor layer comprises an amorphous semiconductor layer having an NH group or an NH 2 group,
wherein different semiconductor atoms included in the second semiconductor layer are cross-linked with the NH group,
wherein the second semiconductor layer comprises a concave portion between the impurity semiconductor layers, and
wherein a nitrogen concentration in the second semiconductor layer gradually decreases toward the impurity semiconductor layers.
2. The thin film transistor according to claim 1 , wherein different dangling bonds in semiconductor atoms included in the second semiconductor layer are terminated with the NH 2 group.
3. The thin film transistor according to claim 1 , wherein the first semiconductor layer is a microcrystalline semiconductor layer.
4. The thin film transistor according to claim 1 , wherein the first semiconductor layer is a dispersed microcrystalline semiconductor layer or a net-like microcrystalline semiconductor layer.
5. The thin film transistor according to claim 1 , wherein an oxygen concentration of the second semiconductor layer which is measured by secondary ion mass spectrometry is less than or equal to 5×10 18 cm −3 .
6. The thin film transistor according to claim 1 or claim 9 , wherein an NH group is included in the impurity semiconductor layers forming the source region and the drain region.
7. The thin film transistor according to claim 1 , wherein a concentration of nitrogen in the second semiconductor layer is greater than or equal to 1×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .
8. The thin film transistor according to claim 1 , wherein a concentration of nitrogen in the second semiconductor layer is greater than or equal to 2×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .
9. A thin film transistor comprising:
a gate electrode over a substrate having an insulating surface;
a gate insulating layer which is in contact with the gate electrode;
a semiconductor layer which is in contact with the gate insulating layer;
impurity semiconductor layers forming a source region and a drain region; and
a buffer layer which is formed between the semiconductor layer and the impurity semiconductor layers,
wherein the buffer layer comprises an amorphous semiconductor layer having an NH group or an NH 2 group,
wherein different semiconductor atoms included in the buffer layer are cross-linked with the NH group,
wherein the buffer layer comprises a concave portion between the impurity semiconductor layers, and
wherein a nitrogen concentration in the buffer layer gradually decreases toward the impurity semiconductor layers.
10. The thin film transistor according to claim 9 , wherein different dangling bonds in semiconductor atoms included in the buffer layer are terminated with the NH 2 group.
11. The thin film transistor according to claim 9 , wherein the semiconductor layer is a microcrystalline semiconductor layer.
12. The thin film transistor according to claim 9 , wherein the semiconductor layer is a dispersed microcrystalline semiconductor layer or a net-like microcrystalline semiconductor layer.
13. The thin film transistor according to claim 9 , wherein an oxygen concentration of the buffer layer which is measured by secondary ion mass spectrometry is less than or equal to 5×10 18 cm −3 .
14. The thin film transistor according to claim 9 , wherein a concentration of nitrogen in the buffer layer is greater than or equal to 1×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .
15. The thin film transistor according to claim 9 , wherein a concentration of nitrogen in the buffer layer is greater than or equal to 2×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .
16. A semiconductor device comprising:
a thin film transistor comprising:
a gate electrode over a substrate having an insulating surface;
a gate insulating layer which covers the gate electrode;
a first semiconductor layer which is in contact with the gate insulating layer;
a second semiconductor layer which is stacked over the first semiconductor layer; and
impurity semiconductor layers forming a source region and a drain region which are in contact with part of the second semiconductor layer,
wherein the second semiconductor layer comprises an amorphous semiconductor layer having an NH group or an NH 2 group,
wherein different semiconductor atoms included in the second semiconductor layer are cross-linked with the NH group,
wherein the second semiconductor layer comprises a concave portion between the impurity semiconductor layers, and
wherein a nitrogen concentration in the second semiconductor layer gradually decreases toward the impurity semiconductor layers.
17. An electronic device having the semiconductor device according to claim 16 ,
wherein the electronic device is selected from the group consisting of a television device, a cellular phone, a portable computer, and a desk lamp.
18. A semiconductor device according to claim 16 , wherein a concentration of nitrogen in the second semiconductor layer is greater than or equal to 1×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .
19. A semiconductor device according to claim 16 , wherein a concentration of nitrogen in the second semiconductor layer is greater than or equal to 2×10 20 cm −3 and less than or equal to 1×10 21 cm −3 .