Semiconductor device and method for fabricating the same

1. A semiconductor device, comprising:

a first nitride semiconductor layer having at least one projection on an upper surface thereof,

a second nitride semiconductor layer formed on a top surface of the projection of the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer;

a first electrode formed on the second nitride semiconductor layer so as to overhang like a canopy and functioning as one of a source and a drain; and

a second electrode formed to the side of the projection on the first nitride semiconductor layer and functioning as a gate.

2. The semiconductor device of claim 1 , wherein a conductive film made of the same material as the second electrode is formed on the first electrode.

3. The semiconductor device of claim 1 , wherein the second nitride semiconductor layer is made of a quaternary mixed crystal of InAlGaN.

4. The semiconductor device of claim 3 , wherein the quaternary mixed crystal has a composition that allows the quaternary mixed crystal to achieve lattice matching with the first nitride semiconductor layer.

5. The semiconductor device of claim 3 , wherein an amount of polarization occurring in the quaternary mixed crystal is greater than or equal to an amount of polarization occurring in the first nitride semiconductor layer.

6. The semiconductor device of claim 1 , wherein a high-resistance region is formed in part of the first nitride semiconductor layer located under the second electrode.

7. The semiconductor device of claim 6 , wherein the high-resistance region is formed in an entire part of the first nitride semiconductor layer located under a peripheral portion of the second electrode.

8. The semiconductor device of claim 1 , wherein a high-resistance region is formed in part of the first nitride semiconductor layer below the first electrode.

9. The semiconductor device of claim 1 , wherein part of the second nitride semiconductor layer below the first electrode is selectively removed.

10. The semiconductor device of claim 1 , further comprising an insulating film formed in a region between the first electrode and the second nitride semiconductor layer.

11. The semiconductor device of claim 1 , wherein each side of a perimeter of the first electrode is a line segment in a <1-100> or <11-20> direction of a zone axis of the second nitride semiconductor layer.

12. The semiconductor device of claim 1 , wherein part of the first electrode is made of tungsten silicide (WSi).

13. The semiconductor device of claim 1 , wherein a region around the first electrode is filled with a resin material made of benzocyclobutene.

14. The semiconductor device of claim 1 , wherein surfaces of the first and second nitride semiconductor layers are non-polarized surfaces in which the number of nitrogen atoms and the number of group III metal atoms are equal to each other.

15. The semiconductor device of claim 1 , wherein surfaces of the first and second nitride semiconductor layers are nitrogen surfaces containing only nitrogen atoms, and the plane orientation thereof is (000-1).

16. The semiconductor device of claim 1 , further comprising a conductive substrate made of gallium nitride and functioning as an underlying layer for the first nitride semiconductor layer; and

a third electrode formed on a surface of the conductive substrate away from the first nitride semiconductor layer and functioning as the other of the source and the drain.