Cascode circuit employing a depletion-mode, GaN-based FET

1. A circuit, comprising:

input drain, source and gate nodes;

a group III nitride depletion mode FET having a source, drain and gate, wherein the gate of the depletion mode FET is coupled to a potential that maintains the depletion mode FET in its on-state;

an enhancement mode FET having a source, drain and gate, wherein the source of the depletion mode FET is serially coupled to the drain of the enhancement mode FET; and

wherein the drain of the depletion mode FET serves as the input drain node, the source of the enhancement mode FET serves as the input source node and the gate of the enhancement mode FET serves as the input gate node, and

wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer;

a flash layer disposed on the second active layer; and

a source, gate and drain contact disposed on the flash layer.

2. The circuit of claim 1 , wherein the first active layer comprises GaN and the second active layer comprises a group III nitride semiconductor material.

3. The semiconductor device according to claim 2 , wherein the second active layer comprises Al X Ga 1-X N, wherein 0<X<1.

4. The semiconductor device according to claim 2 , wherein the second active layer is selected from the group consisting of AlGaN, AlInN, and AlInGaN.

5. The semiconductor device according to claim 1 further comprising a nucleation layer disposed between the substrate and the first active layer.

6. The semiconductor device according to claim 1 wherein the flash layer comprises metallic Al.

7. The semiconductor device according to claim 1 wherein the flash layer comprises metallic Ga.

8. The semiconductor device according to claim 1 wherein the flash layer is an annealed flash layer forming a native oxide layer.

9. The semiconductor device of claim 1 wherein the second active layer and the flash layer include first and second recesses formed therein and the source and drain contacts are disposed in the first and second recesses, respectively.

10. The circuit of claim 1 wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer;

an AlN layer formed over the second active layer; and

a source, gate and drain contact disposed over the AlN layer.

11. The circuit of claim 1 wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer, wherein the second active layer includes first and second recesses formed therein;

a source and drain contact disposed in the first and second recesses, respectively; and

a gate electrode disposed over the second active layer.

12. A circuit, comprising:

input drain, source and gate nodes;

a group III nitride depletion mode FET;

an enhancement mode FET arranged in series with the depletion mode FET and

wherein a first terminal of the depletion mode FET serves as the input drain node, and second and third terminals of the enhancement mode FET serve as the source and gate nodes, respectively,

wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer;

a flash layer disposed on the second active layer; and

a source, gate and drain contact disposed on the flash layer.

13. The circuit of claim 12 wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer;

an AlN layer formed over the second active layer; and

a source, gate and drain contact disposed over the AlN layer.

14. The circuit of claim 12 wherein the group III nitride depletion mode FET comprises:

a substrate;

a first active layer disposed over the substrate;

a second active layer disposed on the first active layer, the second active layer having a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer, wherein the second active layer includes first and second recesses formed therein;

a source and drain contact disposed in the first and second recesses, respectively; and

a gate electrode disposed over the second active layer.

15. The circuit of claim 12 , wherein the first active layer comprises GaN and the second active layer comprises a group III nitride semiconductor material.

16. The semiconductor device according to claim 12 wherein the flash layer comprises metallic Al.