Transistor for preventing current collapse and having improved leakage current characteristics and method for fabricating the same

1. A transistor comprising:

a substrate;

a first semiconductor layer formed over the substrate;

a second semiconductor layer provided on the first semiconductor layer, having a channel formed by heterojunction at the interface with the first semiconductor layer, and having a first region and a second region;

a first p-type semiconductor layer formed on a region of the second semiconductor layer other than the first region and the second region;

a second p-type semiconductor layer formed on the first p-type semiconductor layer;

a gate electrode formed on the second p-type semiconductor layer; and

a source electrode and a drain electrode formed on or above the first semiconductor layer and at both sides of the gate electrode as seen from a horizontal plane, respectively,

wherein the first region is provided between the gate electrode and the drain electrode as seen from a horizontal plane, and the second region is provided between the gate electrode and the source electrode as seen from a horizontal plane.

2. The transistor of claim 1 , wherein the first p-type semiconductor layer is provided so that a first groove having a bottom composed of the first region is interposed between the layer and the drain electrode in order not to come into contact with the drain electrode, and also so that a second groove having a bottom composed of the second region is interposed between the layer and the source electrode in order not to come into contact with the source electrode.

3. The transistor of claim 1 ,

wherein the first p-type semiconductor layer includes: a third groove having a bottom composed of the first region; and a fourth groove having a bottom composed of the second region, and

the first p-type semiconductor layer is separated by the third and fourth grooves into a first portion in contact with the source electrode, a second portion including a region thereof located below the gate electrode, and a third portion in contact with the drain electrode.

4. The transistor of claim 1 , wherein the second p-type semiconductor layer has a convex shape in which a portion thereof located below the gate electrode has a greater thickness than the other portion thereof.

5. The transistor of claim 1 , wherein the first p-type semiconductor layer has a convex shape in which a portion thereof in contact with the second p-type semiconductor layer has a greater thickness than the other portion thereof.

6. The transistor of claim 1 , wherein the first p-type semiconductor layer is made of Al x Ga 1-x (0≦x≦1), and the second p-type semiconductor layer is made of Al y Ga 1-y N (0≦y≦1).

7. The transistor of claim 1 , wherein the first p-type semiconductor layer is made of Al x Ga 1-x N (0≦x≦1), the second p-type semiconductor layer is made of Al y Ga 1-y N (0≦y≦1), and the layers satisfy x≧y.

8. The transistor of claim 1 , wherein the first region has a width in the gate length direction of 0.7 to 4.5 μm inclusive.

9. The transistor of claim 1 , wherein in the state where a voltage at which the transistor turns into an off-state is applied to the gate electrode, a drain current flowing between the drain electrode and the source electrode has a value of 10−8 A/mm or lower.

10. The transistor of claim 1 , wherein the source electrode and the drain electrode are in contact with the interface between the first semiconductor layer and the second semiconductor layer.

11. The transistor of claim 1 , wherein the heterojunction is a junction composed of Al z Ga 1-z N (0≦z≦1) and GaN.

12. The transistor of claim 1 , wherein a first threshold voltage V th1 below the gate electrode and a second threshold voltage V th2 of a transistor formed as a virtual transistor in a region below the side of the gate electrode satisfy V th1 -V th2 ≧2.5 V.

13. The transistor of claim 1 ,

wherein the distance between the gate electrode and the source electrode is smaller than the distance between the gate electrode and the drain electrode, and

the distance between the second p-type semiconductor layer and the source electrode is smaller than the distance between the second p-type semiconductor layer and the drain electrode.

14. The transistor of claim 1 , wherein the levels of the first and second regions of the second semiconductor layer are equal to or lower than the level of the region of the second semiconductor layer other than the first and second regions.