Semiconductor device

1 . A semiconductor device comprising:

a first group III-V nitride semiconductor layer formed above a substrate and having a two-dimensional electron gas layer;

a second group III-V nitride semiconductor layer formed on said first group III-V nitride semiconductor layer and having a larger band gap than said first group Ill-V nitride semiconductor layer;

at least one ohmic electrode formed to have a base portion penetrating said second group III-V nitride semiconductor layer and reaching a portion of said first group III-V nitride semiconductor layer disposed beneath said two-dimensional electron gas layer; and

an impurity doped layer formed in portions of said first group III-V nitride semiconductor layer and said second group III-V nitride semiconductor layer in contact with said ohmic electrode, and doped with an impurity having conductivity.

2 . The semiconductor device of claim 1 ,

wherein said second group III-V nitride semiconductor layer has a multilayered structure in which a plurality of group III-V nitride semiconductor layers are stacked.

3 . The semiconductor device of claim 1 ,

wherein said ohmic electrode is two in number spaced from each other, and

a gate electrode is formed above said second group Ill-V nitride semiconductor layer to be disposed between said two ohmic electrodes.

4 . The semiconductor device of claim 1 , further comprising a third group III-V nitride semiconductor layer formed on said second group III-V nitride semiconductor layer,

wherein said ohmic electrode is formed to have at least a portion thereof penetrating said third group III-V nitride semiconductor layer.

5 . The semiconductor device of claim 4 ,

wherein said third group III-V nitride semiconductor layer has a multilayered structure in which a plurality of group III-V nitride semiconductor layers are stacked.

6 . The semiconductor device of claim 4 ,

wherein said ohmic electrode is two in number spaced from each other, and

a gate electrode is formed above said second group III-V nitride semiconductor layer to be disposed between said two ohmic electrodes.

7 . The semiconductor device of claim 6 ,

wherein said third group III-V nitride semiconductor layer has, in a region disposed between said two ohmic electrodes, a gate recess for exposing said second group III-V nitride semiconductor layer therein, and

said gate electrode is formed in said gate recess.

8 . The semiconductor device of claim 6 , further comprising a fourth group III-V nitride semiconductor layer having a p-type conductivity and formed between said gate electrode and said third group III-V nitride semiconductor layer,

wherein said gate electrode is in ohmic contact with said fourth group III-V nitride semiconductor layer.

9 . The semiconductor device of claim 1 , further comprising an anode electrode formed above said second group III-V nitride semiconductor layer in a position different from said ohmic electrode and in Schottky contact with said second group III-V nitride semiconductor layer.

10 . The semiconductor device of claim 1 ,

wherein said ohmic electrode is formed by filling an opening penetrating said second group III-V nitride semiconductor layer and reaching a portion of said first group Ill-V nitride semiconductor layer disposed beneath said two-dimensional electron gas layer, and

a wall of said opening is inclined to have a larger width in a higher portion thereof.

11 . The semiconductor device of claim 1 ,

wherein said impurity having the conductivity is silicon.

12 . The semiconductor device of claim 1 ,

wherein said base portion of said ohmic electrode is formed down to a portion of said first group III-V nitride semiconductor layer deeper than said two-dimensional electron gas layer by 10 nm or more.

13 . The semiconductor device of claim 1 ,

wherein said ohmic electrode has an overhang portion overhanging a top face of said second group III-V nitride semiconductor layer, and

said overhang portion has a length of 1 μm or less.