Semiconductor device

1. A semiconductor device, comprising:

a substrate;

a first nitride semiconductor layer formed over the substrate;

a second nitride semiconductor layer formed over the first nitride semiconductor layer;

a source electrode and a drain electrode for a field effect transistor formed over the second nitride semiconductor layer while being separated from each other;

a third nitride semiconductor layer formed over the second nitride semiconductor layer sandwiched between the source electrode and the drain electrode while being separated from each of the source electrode and the drain electrode;

a fourth nitride semiconductor layer formed over the third nitride semiconductor layer; and

a gate electrode for the field effect transistor formed over the fourth nitride semiconductor layer,

wherein the band gap of the second nitride semiconductor layer is greater than the band gap of the first nitride semiconductor layer,

wherein the third nitride semiconductor layer contains aluminum and is an n type semiconductor layer,

wherein the fourth nitride semiconductor layer is a p type semiconductor layer,

wherein the second nitride semiconductor layer contains aluminum at a composition ratio smaller than the aluminum composition ratio of the third nitride semiconductor layer or contains no aluminum,

wherein the second nitride semiconductor layer contains a first n type impurity,

wherein the third nitride semiconductor layer contains a second n type impurity,

wherein the concentration of the second n type impurity of the third nitride semiconductor layer is greater than the concentration of the first n type impurity of the second nitride semiconductor layer, and

wherein the third nitride semiconductor layer is wider than the fourth nitride semiconductor layer.

2. The semiconductor device according to claim 1 ,

wherein the fourth nitride semiconductor layer contains magnesium or carbon.

3. The semiconductor device according to claim 1 ,

wherein the fourth nitride semiconductor layer comprises:

a fifth nitride semiconductor layer formed over the third nitride semiconductor layer; and

a sixth nitride semiconductor layer formed over the fifth nitride semiconductor layer,

wherein the fifth nitride semiconductor layer contains carbon, and

wherein the sixth nitride semiconductor layer contains magnesium.

4. The semiconductor device according to claim 1 ,

wherein the fourth nitride semiconductor layer contains magnesium and carbon,

wherein the magnesium concentration of the fourth nitride semiconductor layer decreases from the interface between the fourth nitride semiconductor layer and the gate electrode to the interface between the fourth nitride semiconductor layer and the third nitride semiconductor layer, and

wherein the carbon concentration of the fourth nitride semiconductor layer increases from the interface between the fourth nitride semiconductor layer and the gate electrode to the interface between the fourth nitride semiconductor layer and the third nitride semiconductor layer.

5. The semiconductor device according to claim 1 ,

wherein the fourth nitride semiconductor layer contains aluminum.

6. The semiconductor device according to claim 1 , further comprising:

a seventh nitride semiconductor layer formed over the third nitride semiconductor layer sandwiched between the fourth nitride semiconductor layer and the source electrode or over the third nitride semiconductor layer sandwiched between the fourth nitride semiconductor layer and the drain electrode,

wherein the seventh nitride semiconductor layer has a p type semiconductor layer which is the same layer as the fourth nitride semiconductor layer, and

wherein the seventh nitride semiconductor layer has a thickness smaller than the thickness of the fourth nitride semiconductor layer.

7. The semiconductor device according to claim 1 ,

wherein the first nitride semiconductor layer has indium gallium nitride or gallium nitride,

wherein the second nitride semiconductor layer has aluminum gallium nitride,

wherein the third nitride semiconductor layer has aluminum gallium nitride,

wherein the fourth nitride semiconductor layer has gallium nitride, and

wherein the second nitride semiconductor layer has aluminum at a composition ratio smaller than the aluminum composition ratio of the third nitride semiconductor layer.

8. The semiconductor device according to claim 1 ,

wherein the first nitride semiconductor layer has indium gallium nitride,

wherein the second nitride semiconductor layer has gallium nitride,

wherein the third nitride semiconductor layer has aluminum gallium nitride, and

wherein the fourth nitride semiconductor layer has gallium nitride.

9. The semiconductor device according to claim 1 , further comprising:

a semiconductor layer formed over the third nitride semiconductor layer adjacent to a side of the fourth nitride semiconductor layer closer to the source electrode, and physically sandwiched between the fourth nitride semiconductor layer and the source electrode or formed over the third nitride semiconductor layer adjacent to another side of the fourth nitride semiconductor layer closer to the drain electrode, and physically sandwiched between the fourth nitride semiconductor layer and the drain electrode,

wherein the semiconductor layer is physically separated from the fourth nitride semiconductor layer.