Semiconductor device using a nitride semiconductor

1. A semiconductor device, comprising:

a first semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layer represented by a composition formula Al y Ga 1-y N (0≦y≦1,x<y) and is formed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1) and is selectively formed above the second semiconductor layer;

a gate insulator formed on the third semiconductor layer;

a gate electrode formed on the gate insulator;

a source electrode electrically connected to the second semiconductor layer;

a drain electrode electrically connected to the second semiconductor layer; and

a fourth semiconductor layer represented by a composition formula Al z Ga 1-z N (1≦z≦1) and is formed so as to be inserted between the second semiconductor layer and the third semiconductor layer.

2. The semiconductor device according to claim 1 ,

wherein the sheet impurity concentration of the third semiconductor layer is equal to or more than the sheet impurity concentration of the second semiconductor layer.

3. A semiconductor device, comprising:

a first semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layer represented by a composition formula Al y Ga 1-y N (0≦y≦1,x<y) and is formed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1) and is selectively formed above the second semiconductor layer;

a gate insulator formed on the third semiconductor layer;

a gate electrode formed on the gate insulator;

a source electrode electrically connected to the second semiconductor layer;

a drain electrode electrically connected to the second semiconductor layer;

an insulator formed on the gate electrode;

a first field plate electrode which is formed on the insulator so as to cover the gate electrode and is connected to the source electrode; and

a second filed plate electrode which is formed on the insulator and is connected to the drain electrode.

4. The semiconductor device according to claim 3 ,

wherein the distance between the drain electrode and the third semiconductor layer is longer than the distance between the source electrode and the third semiconductor layer.

5. A semiconductor device, comprising:

a first semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layer represented by a composition formula Al y Ga 1-y N (0≦y≦1,x<y) and is formed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1) and is selectively formed above the second semiconductor layer;

a gate insulator formed on the third semiconductor layer;

a gate electrode formed on the gate insulator;

a source electrode electrically connected to the second semiconductor layer;

a drain electrode electrically connected to the second semiconductor layer;

a fourth semiconductor layer formed on a surface of the first semiconductor layer opposite to a surface facing the second semiconductor layer,

wherein the fourth semiconductor layer is connected to the source electrode.

6. The semiconductor device according to claim 5 ,

wherein the distance between the drain electrode and the third semiconductor layer is longer than the distance between the source electrode and the third semiconductor layer.

7. A semiconductor device, comprising:

a first semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layer represented by a composition formula Al y Ga 1-y N (0≦y≦1,x<y) and is formed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1) and is selectively formed above the second semiconductor layer;

a gate insulator formed on the third semiconductor layer;

a gate electrode formed on the gate insulator;

a source electrode electrically connected to the second semiconductor layer;

a drain electrode electrically connected to the second semiconductor layer;

wherein the gate electrode and the source electrode are formed in stripe shapes parallel to each other; and

wherein the third semiconductor layer has a first stripe portion and a second stripe portion, the first stripe portion being formed in a first direction which is the longitudinal direction of the gate electrode, and the second stripe portion being formed so as to extend in a second direction orthogonal to the first direction from the first stripe portion and to contact the source electrode.

8. The semiconductor device according to claim 7 ,

wherein the first stripe portion is discontinuously formed so that each segment is arranged at a predetermined interval.

9. The semiconductor device according to claim 7 ,

wherein the third semiconductor layer has a plurality of the second stripe portion arranged at a predetermined pitch, and

the width of the second stripe portion is equal to or less than the pitch.

10. The semiconductor device according to claim 7 ,

wherein the distance between the drain electrode and the third semiconductor layer is longer than the distance between the source electrode and the third semiconductor layer.