Heterojunction bipolar transistor

1. A heterojunction bipolar transistor having a base layer made of SiGe mixed crystal, the base layer comprising:

a spacer layer formed in contact with a collector layer with no base impurities diffused therein; and

an intrinsic base layer formed in contact with an emitter layer with base impurities diffused therein,

wherein the spacer layer contains C at a first concentration,

the intrinsic base layer comprises:

a first region containing C at a second concentration on the collector side; and

a second region containing C at a third concentration on the emitter side, and

the third concentration is higher than the first concentration and the second concentration.

2. The heterojunction bipolar transistor of claim 1 , wherein the second region is in contact with the emitter layer.

3. The heterojunction bipolar transistor of claim 1 , wherein the second region has a region in which the C concentration decreases from the emitter side toward the collector side in a portion closer to the first region.

4. The heterojunction bipolar transistor of claim 1 , wherein the intrinsic base layer further comprises a third region between the second region and the emitter layer, and

the third region contains C at a fourth concentration, and the fourth concentration is lower than the third concentration.

5. The heterojunction bipolar transistor of claim 4 , wherein the second region has a region in which the C concentration increases from the emitter side toward the collector side in a portion closer to the third region.

6. The heterojunction bipolar transistor of claim 1 , wherein the spacer layer has a high-concentration region containing C at a concentration higher than the first concentration in a portion closer to the collector layer.

7. The heterojunction bipolar transistor of claim 6 , wherein the high-concentration region has a graded region in which the C concentration increases from the emitter side toward the collector side in a portion closer to the intrinsic base layer.

8. The heterojunction bipolar transistor of claim 1 , wherein the first and second concentrations are in a range of 0.05 to 0.3 at %.

9. The heterojunction bipolar transistor of claim 1 , wherein the third concentration is in a range of 0.4 to 2.0 at %.

10. The heterojunction bipolar transistor of claim 4 , wherein the fourth concentration is in a range of 0.4 to 2.0 at %.

11. The heterojunction bipolar transistor of claim 1 , wherein a cap layer made of Si is formed on the base layer,

an emitter electrode made of polycrystalline Si with emitter impurities diffused therein is formed on the cap layer, and

an emitter layer containing emitter impurities diffused from the emitter electrode is formed in the cap layer.

12. The heterojunction bipolar transistor of claim 11 , wherein an extrinsic base layer made of SiGe mixed crystal is formed on the side of the base layer, and

the extrinsic base layer is formed in a self-aligned manner by ion implantation using the emitter electrode as a mask.