Heterojunction bipolar transistor

1. A heterojunction bipolar transistor, comprising:

a subcollector layer of a first conductivity type;

a first collector layer formed on the subcollector layer, the first collector layer containing an impurity of the first conductivity type;

a third collector layer formed on the first collector layer, the third collector layer containing an impurity of the first conductivity type at a higher concentration than the first collector layer;

a second collector layer formed on the third collector layer, the second collector layer containing an impurity of the first conductivity type at a lower concentration than the first collector layer;

a base layer of a second conductivity type formed on the second collector layer;

an emitter layer of the first conductivity type formed on the base layer, the emitter layer containing a semiconductor with a wider bandgap than the base layer; and

an emitter cap layer of the first conductivity type formed on the emitter layer,

wherein the first collector layer, the second collector layer and the third collector layer are made of the same semiconductor material.

2. The heterojunction bipolar transistor according to claim 1 , wherein a thickness of the third collector layer is smaller than a thickness of the first collector layer and the second collector layer.

3. The heterojunction bipolar transistor according to claim 2 , wherein a thickness of the second collector layer is 150 nm or larger.

4. The heterojunction bipolar transistor according to claim 3 , wherein an impurity concentration of the second collector layer is 5×10 16 cm −3 or lower.

5. The heterojunction bipolar transistor according to claim 4 , wherein an impurity concentration of the third collector layer is 1×10 17 cm −3 or lower.

6. The heterojunction bipolar transistor according to claim 4 , wherein an impurity sheet concentration of the third collector layer is 2×10 12 cm −2 or lower.

7. The heterojunction bipolar transistor according to claim 1 , further comprising:

a first doping slope layer formed between the second collector layer and the third collector layer, the first doping slope layer having an impurity concentration distribution with an impurity concentration gradually increasing from an impurity concentration of the second collector layer to an impurity concentration of the third collector layer in a thickness direction.

8. The heterojunction bipolar transistor according to claim 1 , further comprising:

a second doping slope layer formed between the first collector layer and the third collector layer, the second doping slope layer having an impurity concentration distribution with an impurity concentration gradually increasing from an impurity concentration of the first collector layer to an impurity concentration of the third collector layer in a thickness direction.

9. The heterojunction bipolar transistor according to claim 1 , wherein the third collector layer has an impurity concentration distribution with a maximum concentration point of the impurity of the first conductivity type existing in the third collector layer and with an impurity concentration gradually increasing from an impurity concentration of the first collector layer to the maximum concentration point and gradually decreasing from the maximum concentration point to an impurity concentration of the second collector layer.

10. The heterojunction bipolar transistor according to claim 9 , wherein an impurity concentration at the maximum concentration point is 1×10 15 cm −3 or higher and lower than 2×10 18 cm −3 .

11. The heterojunction bipolar transistor according to claim 4 , further comprising:

a high bandgap layer formed between the subcollector layer and the first collector layer, the high bandgap layer containing a semiconductor with a wider bandgap than the first collector layer.

12. The heterojunction bipolar transistor according to claim 11 , wherein the high bandgap layer is made of InGaP with a composition ratio of In of 0.45 or higher and lower than 0.50.

13. The heterojunction bipolar transistor according to claim 12 , wherein InGaP as a material of the high bandgap layer is InGaP grown under a condition favoring formation of natural superlattice or InGaP grown to create a bandgap of 1.82 to 1.87 Ev.

14. The heterojunction bipolar transistor according to claim 11 , further comprising:

a high doped layer formed between the high bandgap layer and the first collector layer, the high doped layer containing an impurity of the first conductivity type at 5×10 17 cm −3 or higher and lower than 3×10 18 cm −3 .

15. The heterojunction bipolar transistor according to claim 14 , wherein the high doped layer is made of GaAs or the same semiconductor material as the high bandgap layer.