Tunnel barrier schottky

1. A diode comprising:

a semiconductor substrate;

a cathode metal layer contacting a bottom of the substrate;

a semiconductor drift layer on the substrate;

a graded aluminum gallium nitride (AlGaN) semiconductor barrier layer on the drift layer and having a larger bandgap than the drift layer, the barrier layer having a top surface and a bottom surface between the drift layer and the top surface, the barrier layer having an increasing aluminum composition from the bottom surface to the top surface; and

an anode metal layer directly contacting the top surface of the barrier layer,

wherein the barrier layer comprises an ion implantation region at a peripheral portion of the barrier layer and a non-ion implanted region at a central portion of the barrier layer, and

the anode metal layer completely covers the non-ion implanted region of the barrier layer, and overlaps the ion implantation region at the peripheral portion of the barrier layer.

2. The diode of claim 1 , wherein the aluminum composition of the barrier layer is linearly graded from the bottom surface to the top surface.

3. The diode of claim 2 , wherein the aluminum composition of the barrier layer linearly increases from about 0% at the bottom surface to about 25% at the top surface.

4. The diode of claim 1 , wherein the barrier layer has a thickness of about 5 nanometers (nm).

5. A semiconductor structure comprising:

a semiconductor substrate;

a semiconductor drift layer on the substrate; and

a nitride semiconductor barrier layer on the drift layer and having a larger bandgap than the drift layer, the barrier layer having a top surface and a bottom surface between the drift layer and the top surface,

wherein a composition of the barrier layer is graded so that the bandgap of the barrier layer increases from a portion of the barrier layer near the bottom surface to a portion of the barrier layer near the top surface,

wherein the barrier layer comprises an ion implantation region at a peripheral portion of the barrier layer and a non-ion implanted region at a central portion of the barrier layer, and

the structure further comprises an anode metal layer contacting a top surface of the barrier layer, completely covering the non-ion implanted region of the barrier layer, and overlapping the ion implantation region at the peripheral portion of the barrier layer.

6. The structure of claim 5 , further comprising:

a cathode metal layer contacting a bottom of the substrate.

7. The structure of claim 5 , wherein the substrate and the drift layer comprise a III-nitride semiconductor.

8. The structure of claim 5 , wherein

the barrier layer comprises aluminum gallium nitride (AlGaN), and

a composition of aluminum in the barrier layer increases from the bottom surface to the top surface.

9. The structure of claim 8 , wherein the barrier layer has a thickness between 2.5 nanometers (nm) and 10 nm.

10. The structure of claim 5 , wherein the barrier layer contacts the drift layer.

11. A semiconductor structure comprising:

a semiconductor substrate;

a semiconductor drift layer on the substrate; and

a nitride semiconductor barrier layer on the drift layer and having a larger bandgap than the drift layer, the barrier layer having a top surface and a bottom surface between the drift layer and the top surface,

wherein a composition of the barrier layer is graded so that the bandgap of the barrier layer increases from a portion of the barrier layer near the bottom surface to a portion of the barrier layer near the top surface, and

wherein the drift layer comprises a plurality of semiconductor drift layers.

12. The structure of claim 11 , wherein the plurality of semiconductor drift layers comprises a first drift layer on the substrate and a second drift layer on the first drift layer, the first drift layer having a higher doping concentration than the second layer.