Nitride semiconductor device, Doherty amplifier and drain voltage controlled amplifier

1. A nitride semiconductor device, comprising:

a substrate;

a stacked semiconductor structure formed over said substrate and including a electron channel layer of an undoped nitride semiconductor and an electron supplying layer of an n-type nitride semiconductor formed epitaxially over said electron channel layer, said n-type nitride semiconductor having an electron affinity smaller than an electron affinity of said undoped nitride semiconductor and a two-dimensional electron gas being formed in said electron channel layer along an interface to said electron supplying layer;

a gate electrode formed over said stacked semiconductor structure in correspondence to a channel region; and

source and drain electrodes formed over said stacked semiconductor structure in ohmic contact therewith respectively at a first side and a second side of said gate electrode,

said stacked semiconductor structure including, between said substrate and said electron channel layer, an n-type conductive layer and a barrier layer containing Al formed consecutively and epitaxially, and an n-type band control layer between said n-type conductive layer and said barrier layer.

2. The nitride semiconductor device as claimed in claim 1 , wherein said n-type conductive layer and said electron channel layer contain Ga and N as constituent elements thereof, while said barrier layer contains Ga, N and Al as constituent elements thereof.

3. The nitride semiconductor device as claimed in claim 1 , wherein said barrier layer increases an Al composition thereof continuously from a side thereof contacting to said electron supplying layer to a side thereof contacting said n-type conductive layer.

4. The nitride semiconductor device as claimed in claim 3 , wherein said Al composition, represented in terms of an Al compositional parameter x representing a composition of said barrier layer as Al x Ga 1-x N, falls in a range of 0.03-0.07 at an interface to said conductive layer and falls in a range of 0.0-0.3 at an interface to said electron channel layer.

5. The nitride semiconductor device as claimed in claim 4 , wherein said barrier layer has a film thickness in the range of 2-50 nm.

6. The nitride semiconductor device as claimed in claim 1 , wherein said barrier layer is doped to n-type at a part thereof adjacent to an interface to said n-type conductive layer.

7. The nitride semiconductor device as claimed in claim 1 , wherein said n-type conductive layer is doped with an n-type dopant with a concentration level in the range of 1×10 17 -5×10 18 cm −3 .

8. The nitride semiconductor device as claimed in claim 1 , wherein said n-type conductive layer is doped to n-type at a part thereof adjacent to an interface to said barrier layer.

9. The nitride semiconductor device as claimed in claim 1 , wherein said n-type conductive layer has a film thickness of 5-100 nm.

10. A Doherty amplifier comprising a carrier amplifier and a peak amplifier, each of said carrier amplifier and peak amplifier comprising a nitride semiconductor device, said nitride semiconductor device comprising:

a substrate;

a stacked semiconductor structure formed over said substrate and including a electron channel layer of an undoped nitride semiconductor and an electron supplying layer of an n-type nitride semiconductor formed epitaxially over said electron channel layer, said n-type nitride semiconductor having an electron affinity smaller than an electron affinity of said undoped nitride semiconductor and a two-dimensional electron gas being formed in said electron channel layer along an interface to said electron supplying layer;

a gate electrode formed over said stacked semiconductor structure in correspondence to a channel region; and

source and drain electrodes formed over said stacked semiconductor structure in ohmic contact therewith respectively at a first side and a second side of said gate electrode,

said stacked semiconductor structure including, between said substrate and said electron channel layer, an n-type conductive layer and a barrier layer containing Al formed consecutively and epitaxially, and an n-type band control layer between said n-type conductive layer and said barrier layer.

11. A drain voltage controlled amplifier comprising a main amplifier amplifying an incoming signal and a control amplifier controlling a drain voltage in response to a level of said incoming signal,

said main amplifier comprising a nitride semiconductor device, said nitride semiconductor device comprising:

a substrate;

a stacked semiconductor structure formed over said substrate and including a electron channel layer of an undoped nitride semiconductor and an electron supplying layer of an n-type nitride semiconductor formed epitaxially over said electron channel layer, said n-type nitride semiconductor having an electron affinity smaller than an electron affinity of said undoped nitride semiconductor and a two-dimensional electron gas being formed in said electron channel layer along an interface to said electron supplying layer;

a gate electrode formed over said stacked semiconductor structure in correspondence to a channel region; and

source and drain electrodes formed over said stacked semiconductor structure in ohmic contact therewith respectively at a first side and a second side of said gate electrode,

said stacked semiconductor structure including, between said substrate and said electron channel layer, an n-type conductive layer and a barrier layer containing Al formed consecutively and epitaxially, and an n-type band control layer between said n-type conductive layer and said barrier layer.