Semiconductor device and manufacturing method thereof

1. A semiconductor device comprising:

a substrate;

an n-type nitride semiconductor layer or an undoped nitride semiconductor layer over the substrate; and

a source electrode and a drain electrode being in direct physical contact with and being in ohmic contact with said n-type nitride semiconductor layer or said undoped nitride semiconductor layer;

wherein each of said source electrode and said drain electrode comprises:

a tantalum layer formed on said n-type nitride semiconductor layer or said undoped nitride semiconductor layer;

an aluminum layer formed on said tantalum layer and made of aluminum only; and

a metal layer formed on said aluminum layer and made of any one material of tantalum, nickel and palladium;

further comprising a compound layer of aluminum and any one material of tantalum, nickel and palladium between said aluminum layer and said metal layer; and

a gate electrode comprising a nickel layer and a gold layer formed on the nickel layer.

2. The semiconductor device according to claim 1 , wherein said n-type nitride semiconductor layer or said undoped nitride semiconductor layer is a III-V group nitride compound semiconductor layer.

3. The semiconductor device according to claim 1 , further comprising an Al x Ga 1-x N (0≦x≦1) layer as said n-type nitride semiconductor layer or said undoped nitride semiconductor layer, and being a field effect transistor having a gate electrode.

4. The semiconductor device according to claim 3 ,

further comprising:

a GaN electron transit layer formed below said Al x Ga 1-x N (0≦x≦1) electron supply layer; and

a GaN layer formed between said gate electrode and said Al x Ga 1-x N (0≦x≦1) electron supply layer;

wherein said Al x Ga 1-x N (0≦x≦1) layer is an Al x Ga 1-x N (0≦x≦1) electron supply layer.

5. The semiconductor device according to claim 4 , wherein said GaN layer is doped with n-type impurity materials of 1×10 17 cm −3 or more.

6. The semiconductor device according to claim 4 , wherein said GaN layer is formed below said source electrode and said drain electrode,

wherein said GaN layer is thinner in thickness at a portion below said source electrode and said drain electrode than at a portion below said gate electrode.

7. The semiconductor device according to claim 4 , wherein said Al x Ga 1-x N (0≦x≦1) electron supply layer is thinner in thickness at a portion below said source electrode and said drain electrode than at a portion below said gate electrode.

8. The semiconductor device according to claim 1 , wherein said substrate is a silicon carbide substrate having a resistivity of 1×10 6 Ω·cm or more.

9. The semiconductor device according to claim 1 , wherein said substrate is a conductive substrate having a resistivity of 1×10 5 Ω·cm or less.

10. A semiconductor device comprising:

a substrate;

an n-type nitride semiconductor layer or an undoped nitride semiconductor layer over the substrate; and

a source electrode and a drain electrode being in direct physical contact with and being in ohmic contact with said n-type nitride semiconductor layer or said undoped nitride semiconductor layer;

wherein each of said source electrode and said drain electrode comprises:

a first tantalum layer formed on said n-type nitride semiconductor layer or said undoped nitride semiconductor layer;

an aluminum layer formed on said first tantalum layer and made of aluminum only; and

a second tantalum layer formed on said aluminum layer;

further comprising a compound layer of aluminum and tantalum between said aluminum layer and said second tantalum layer; and

a gate electrode comprising a nickel layer and a gold layer formed on the nickel layer.