Base profile of self-aligned bipolar transistors for power amplifier applications

1. A bipolar transistor structure having a transistor top and a transistor bottom, said structure comprising:

a silicon substrate located at said transistor bottom and having a collector region of a first conductivity type;

an epitaxial base layer of a second conductivity type overlying, relative to said transistor top and said transistor bottom, a portion of said collector region, said epitaxial base layer having a bottom surface contacting said silicon substrate and a top surface opposite said bottom surface,

a top region, relative to said transistor top and said transistor bottom, of said epitaxial base layer comprising a concentration of germanium having atomic compositions between 1% and 15% at said top surface, said concentration of germanium being sufficient to avoid impacting transistor parameters and sufficient to be resistant to selective chemical etching; and

a silicon emitter layer of said first conductivity type overlying, relative to said transistor top and said transistor bottom, a portion of said epitaxial base layer adjacent said top surface of said epitaxial base layer.

2. The structure according to claim 1 , further comprising:

a trench surrounding said collector region; and

an insulator within said trench.

3. The structure according to claim 1 , said silicon emitter layer further comprising an emitter having sidewalls and sidewall spacers attached to said sidewalls of said emitter.

4. The structure according to claim 1 , said base layer further comprising:

a silicon layer overlying, relative to said transistor top and said transistor bottom, said top region of said base layer.

5. The structure according to claim 1 , said epitaxial base layer comprising SiGe, and, said emitter layer and said silicon substrate comprising Si.

6. The structure according to claim 1 , said concentration of germanium in said epitaxial base layer varying according to a predetermined profile between said top surface and said bottom surface, said predetermined profile being, in cross-section between said transistor top and said transistor bottom, one of:

triangular shaped; and

trapezoidal shaped.

7. A device having a device top and a device bottom, said device comprising:

an n-type doped collector of a bipolar transistor disposed in an upper portion, relative to said device top and said device bottom, of a silicon (Si) substrate layer;

a base comprising p-type doped epitaxial silicon germanium (SiGe) disposed on said Si substrate layer above, relative to said device top and said device bottom, said n-type doped collector, said base contacting said Si substrate layer,

a top region of said base, relative to said device top and said device bottom, comprising a concentration of germanium (Ge) having atomic compositions between 1% and 15% at a top surface of said base, said concentration of germanium being sufficient to avoid impacting transistor parameters, and sufficient to be resistant to selective chemical etching; and

an n-type doped silicon (Si) emitter contacting said base.

8. The device according to claim 7 , further comprising:

a trench surrounding said collector; and

an insulator within said trench.

9. The device according to claim 7 , said n-type doped Si emitter further comprising:

sidewalls and sidewall spacers attached to said sidewalls of said emitter.

10. The device according to claim 7 , said base comprising an intrinsic base portion and an extrinsic base portion, said device further comprising:

silicide pads and electrical contacts connected to said emitter, said collector, and said extrinsic base portion.

11. The device according to claim 7 , said base further comprising:

a silicon layer overlying, relative to said device top and said device bottom, said top region of said base.

12. The device according to claim 7 , said concentration of germanium in said base varying according to a predetermined profile between said device top and said device bottom, said predetermined profile being, in cross-section between said device top and said device bottom, one of:

triangular shaped; and

trapezoidal shaped.

13. A bipolar transistor comprising:

a collector region of a first conductivity type within a semiconductor substrate;

a base of a second conductivity type contacting a portion of said collector region;

an etch-stop region in a surface layer of said base, said surface layer being located opposite said semiconductor substrate, said etch-stop region having atomic compositions of germanium between 1% and 15% at said surface layer, said atomic compositions of germanium being sufficient to avoid impacting transistor parameters and sufficient to be resistant to selective chemical etching; and

an emitter of said first conductivity type contacting said etch-stop region of said base.

14. The bipolar transistor according to claim 13 , said base comprising:

a single crystal, epitaxially grown directly on said collector region.

15. The bipolar transistor according to claim 13 , said emitter comprising:

a single crystal, epitaxially grown directly on said base.

16. The bipolar transistor according to claim 13 , further comprising:

a trench surrounding said collector region; and

an insulator within said trench.

17. The bipolar transistor according to claim 13 , further comprising:

sidewall spacers attached to sidewalls of said emitter.

18. The bipolar transistor according to claim 13 , said base comprising an intrinsic base portion and an extrinsic base portion, said bipolar transistor further comprising:

silicide pads attached to each of said emitter, said collector, and said extrinsic base portion; and

electrical contacts connected to each of said silicide pads.

19. The bipolar transistor according to claim 13 , said atomic compositions of germanium in said base varying according to a predetermined profile, said predetermined profile being, in cross-section, one of:

triangular shaped; and

trapezoidal shaped.

20. The bipolar transistor according to claim 13 , said surface layer further comprising:

a silicon layer overlying said etch-stop region.