Diode with heterojunction of nitride semiconductor layers

1. A diode comprising:

a nitride semiconductor layer structure formed on a principal surface of a substrate, having a channel, and including an undoped nitride layer or an n-type nitride layer; and

cathode and anode electrodes formed on the nitride semiconductor layer structure so as to be spaced apart from each other, wherein

the anode electrode includes:

a p-type nitride semiconductor layer formed on the nitride semiconductor layer structure;

a first metal layer formed on an upper surface of the p-type nitride semiconductor layer, and being in ohmic contact with the p-type nitride semiconductor layer;

a second metal layer being in contact with the first metal layer, being opposite to the cathode electrode with respect to the p-type nitride semiconductor layer, and being in ohmic contact with the channel, and

the second metal layer being in direct contact with the channel.

2. The diode of claim 1 , wherein

the nitride semiconductor layer structure includes a plurality of first nitride semiconductor layers, and a plurality of second nitride semiconductor layers which are each formed on one of the first nitride semiconductor layers, and each have a larger band gap than the first nitride semiconductor layers, and

the first nitride semiconductor layers and the second nitride semiconductor layers are alternately formed.

3. The diode of claim 2 , wherein

the nitride semiconductor layer structure has a recess which does not reach an interface between a lowest one of the first nitride semiconductor layers, and a lowest one of the second nitride semiconductor layers, and

the p-type nitride semiconductor layer is formed to fill the recess.

4. The diode of claim 3 , wherein the recess reaches the lowest second nitride semiconductor layer.

5. The diode of claim 2 , wherein

the nitride semiconductor layer structure has an anode recess passing through the second nitride semiconductor layer, and extending below an interface between the first and second nitride semiconductor layers.

6. The diode of claim 5 , wherein a location at which the second metal layer is in contact with the channel is substantially aligned with a location of an end of the p-type nitride semiconductor layer opposite to the cathode electrode.

7. The diode of claim 2 , wherein

the second nitride semiconductor layer has a recess which does not reach the first nitride semiconductor layer, and

the p-type nitride semiconductor layer is formed to fill the recess.

8. The diode of claim 1 , wherein the p-type nitride semiconductor layer is made of Al x Ga 1-x N, where 0≦x<1.

9. The diode of claim 1 , wherein

the first metal layer is made of a material containing nickel or palladium, and

the second metal layer is made of a material containing aluminum or titanium.

10. The diode of claim 1 , wherein electrons travel through the channel in a direction parallel to the principal surface of the substrate.

11. The diode of claim 1 , further comprising:

a protection film covering a region of an upper surface of the nitride semiconductor layer structure between the cathode and anode electrodes.

12. The diode of claim 1 , wherein the first metal layer is wholly disposed on an upper surface of the p-type nitride semiconductor layer.

13. The diode of claim 1 , wherein the second metal layer is in direct contact with an upper surface and a side surface of the first metal layer, and in direct contact with a side surface of the p-type nitride semiconductor layer.

14. The diode of claim 1 , wherein the second metal layer is in direct contact with a side surface of the channel.