Compound semiconductor device, power supply unit, and amplifier

1. A compound semiconductor device comprising:

a substrate;

an electron transit layer formed on the substrate and made of nitride semiconductor doped with an impurity that forms a trap level;

a barrier layer formed on the electron transit layer; and

a source electrode, a drain electrode, and a gate electrode formed over the electron transit layer at a distance from one another, wherein

the electron transit layer includes:

a first conductivity type region;

a second conductivity type region located over the first conductivity region, where the second conductivity type region having an electron concentration higher than an electron concentration of the first conductivity type region; and

a third conductivity type region located over the second conductivity type region, where the third conductivity type region having an electron concentration lower than a concentration of the impurity and being in contact with an upper surface of the electron transit layer.

2. The compound semiconductor device according to claim 1 , wherein the first conductivity type region is any one of a region having the electron concentration equal to or lower than 1×10 16 cm −3 and a region having a concentration of the impurity equal to or greater than 5×10 15 cm −3 .

3. The compound semiconductor device according to claim 1 , wherein the electron transit layer includes:

a lower portion; and

an upper portion formed on the lower portion and having a concentration of the impurity lower than a concentration of the impurity in the lower portion.

4. The compound semiconductor device according to claim 1 , wherein a concentration profile of the impurity includes:

a first profile located close to the substrate; and

a second profile located close to the barrier layer and having a peak concentration lower than a peak concentration of the first profile.

5. The compound semiconductor device according to claim 4 , further comprising:

a buffer layer formed between the substrate and the electron transit layer and doped with the impurity, wherein

a peak of the first profile is located at a lower surface of the buffer layer.

6. The compound semiconductor device according to claim 1 , further comprising:

a buffer layer formed on the substrate and doped with the impurity; and

a compound semiconductor layer formed on the buffer layer and having a conduction band higher than a conduction band of the electron transit layer, wherein

the electron transit layer is formed on the compound semiconductor layer.

7. The compound semiconductor device according to claim 6 , wherein the buffer layer includes:

a lower portion; and

an upper portion formed on the lower portion and having a concentration of the impurity higher than a concentration of the impurity in the lower portion.

8. The compound semiconductor device according to claim 1 , further comprising:

a recess formed in the electron transit layer; and

a contact layer of compound semiconductor formed in the recess, wherein

the source electrode is formed on the contact layer.

9. The compound semiconductor device according to claim 1 , wherein a gate length of the gate electrode is equal to or shorter than 120 nm.

10. A power supply unit comprising:

a compound semiconductor device including

a substrate,

an electron transit layer formed on the substrate and made of nitride semiconductor doped with an impurity that forms a trap level,

a barrier layer formed on the electron transit layer, and

a source electrode, a drain electrode, and a gate electrode formed over the electron transit layer at a distance form one another, wherein

the electron transit layer includes:

a first conductivity type region;

a second conductivity type region located over the first conductivity type region, where the second conductivity type region having an electron concentration higher than an electron concentration of the first conductivity type region; and

a third conductivity type region located over the second conductivity type region, where the third conductivity type region having an electron concentration lower than a concentration of the impurity and being in contact with an upper surface of the electron transit layer.

11. An amplifier comprising:

a compound semiconductor device including

a substrate,

an electron transit layer formed on the substrate and made of nitride semiconductor doped with an impurity that forms a trap level,

a barrier layer formed on the electron transit layer, and

a source electrode, a drain electrode, and a gate electrode formed over the electron transit layer at a distance form one another, wherein

the electron transit layer includes:

a first conductivity type region,

a second conductivity type region located over the first conductivity type region, where the second conductivity type region having an electron concentration higher than an electron concentration of the first conductivity type region; and

a third conductivity type region located over the second conductivity type region, where the third conductivity type region having an electron concentration lower than a concentration of the impurity and being in contact with an upper surface of the electron transit layer.