Transistor and method for manufacturing same

1. A transistor comprising:

a substrate;

an underlying layer formed on the substrate; and

a first layer made of a nitride semiconductor formed on the underlying layer, wherein:

the underlying layer is a multilayer structure including a plurality of stacked nitride semiconductor layers,

the underlying layer includes an AlGaN layer doped with at least one of cobalt, nickel, ruthenium, osmium, rhodium, or iridium, as a first transition metal and a superlattice layer doped with at least one of cobalt, nickel, ruthenium, osmium, rhodium, or iridium, as a second transition metal, and

the superlattice layer is an insulating layer.

2. The transistor of claim 1 , wherein

the superlattice layer is disposed on the AlGaN layer.

3. The transistor of claim 1 , wherein

the superlattice layer includes an AlN layer and a GaN layer, and is a most strongly polarized layer in the multilayered structure.

4. The transistor of claim 1 , wherein

a layer in contact with an under surface of the superlattice layer contains the second transition metal within at least 0.1 μm from the under surface.

5. The transistor of claim 1 , wherein

a sub-layer of the superlattice layer which has a first lattice constant contains the second transition metal,

a sub-layer of the superlattice layer which has a second lattice constant is free of the second transition metal, and

the second lattice constant is larger than the first lattice constant.

6. The transistor of claim 1 , wherein

the multilayer structure has an interface formed by two of the nitride semiconductor layers having different polarization properties, and

one of the two layers forming the interface is the AlGaN layer.

7. The transistor of claim 1 , wherein

the AlGaN layer is a most strongly polarized layer in the multilayer structure.

8. The transistor of claim 1 , wherein

the AlGaN layer has a higher carbon content than the other layers in the multilayer structure.

9. The transistor of claim 1 , wherein

a layer provided above the AlGaN layer is free of the first transition metal.

10. The transistor of claim 1 , wherein

the first transition metal is introduced into the AlGaN layer from a metallocene compound,

the metallocene compound has a structure sandwiching the first transition metal between two pairs of cyclopentadienyl groups including an alkyl side chain, and

the alkyl side chain includes two or more carbon atoms.

11. The transistor of claim 1 , wherein

the first layer is composed of an operation layer.

12. The transistor of claim 1 , further comprising:

a gate electrode, a source electrode and a drain electrode disposed on the first layer respectively,

wherein the gate electrode is disposed between the source electrode and the drain electrode.

13. The transistor of claim 12 , further comprising:

a protective film which partially covers both an upper surface of the source electrode and an upper surface of the drain electrode.

14. The transistor of claim 12 , further comprising:

a protective film which partially covers an upper surface of the gate electrode.

15. The transistor of claim 12 , wherein:

the first layer includes a channel layer disposed on the underlying layer, a cap layer disposed on the channel layer and a p-type layer disposed on the cap layer,

the gate electrode is disposed on the p-type layer, and

the source electrode and the drain electrode are disposed on the cap layer.

16. The transistor of claim 1 , wherein

the first transition metal is the same as the second transition metal.

17. The transistor of claim 1 , wherein

the AlGaN layer is an insulating layer.

18. The transistor of claim 1 , wherein

the superlattice layer does not emit light.