Semiconductor device and method for fabricating the same

1. A semiconductor device, comprising:

a fin-shaped structure on a substrate, wherein the fin-shaped structure comprises:

a first semiconductor layer on the substrate;

a second semiconductor layer on the first semiconductor layer;

an insulating layer on the second semiconductor layer;

a third semiconductor layer on and directly contacting the insulating layer, wherein the first semiconductor layer and the third semiconductor layer comprise III-V semiconductors and a sidewall of the third semiconductor layer is aligned with a sidewall of the insulating layer; and

a shallow trench isolation (STI) around the first semiconductor layer, the second semiconductor layer, the insulating layer, and the third semiconductor layer, wherein a top surface of the third semiconductor layer is higher than a top surface of the STI.

2. The semiconductor device of claim 1 , further comprising a gate electrode on the STI and the fin-shaped structure.

3. The semiconductor device of claim 1 , wherein the second semiconductor layer comprises aluminum gallium nitride (AlGaN).

4. The semiconductor device of claim 1 , wherein the first semiconductor layer and the third semiconductor layer comprise a same material.

5. The semiconductor device of claim 4 , wherein the first semiconductor layer and the third semiconductor layer comprise gallium nitride (GaN).

6. The semiconductor device of claim 1 , wherein the first semiconductor layer and the substrate comprise a same material.

7. The semiconductor device of claim 6 , wherein the first semiconductor layer and the substrate comprise gallium nitride (GaN).

8. The semiconductor device of claim 1 , wherein the insulating layer comprises metal oxide.

9. The semiconductor device of claim 8 , wherein the insulating layer comprises aluminum oxide (Al 2 O 3 ).