Bipolar transistor with reduced topography

Claim 1

A bipolar transistor formed in a first silicon epitaxial layer above a silicon substrate and in a second silicon epitaxial layer above said first silicon epitaxial layer, comprising a base in said second epitaxial layer disposed above a base-collector junction and an emitter, having an emitter top surface, disposed above said base and connected thereto by an emitter-base junction, said base-collector junction being disposed above a collector portion of said first epitaxial layer extending downwardly to a buried conductor extending horizontally to a vertical collector extension that extends upwardly to a collector contact, said base and said collector extension being separated horizontally by an insulating member, in which said insulating member has an insulating member top surface substantially coplanar with a first epitaxial surface of said first silicon epitaxial layer and substantially below said emitter top surface; a base contact layer of polysilicon is disposed above said insulating member with a polysilicon top surface; said second epitaxial layer of silicon containing said emitter is disposed above said first epitaxial layer, having a top surface substantially coplanar with said polysilicon top surface; a doped layer of a first dielectric, doped with a base dopant of a first polarity, is disposed above said second epitaxial layer's top surface and said polysilicon top surface, and has a base aperture therein above said emitter, said base aperture being separated horizontally from said layer of polysilicon by a horizontal link distance; a doped polysilicon emitter contact member, doped with an emitter dopant of a second polarity opposite said first polarity, is disposed in contact with said emitter top surface in said base aperture; a sidewall spacer of a second dielectric different from said first dielectric and impermeable to said dopant of said first polarity is disposed within said base aperture between said polysilicon emitter contact and said first dielectric; said layer of polysilicon above said insulating member is doped with said base dopant having a concentration gradient with a maximum at said polysilicon top surface abutting said first dielectric; a link region of said second epitaxial layer below said link distance is doped with said base dopant having a concentration gradient with a maximum at said second epitaxial layer's top surface abutting said first dielectric, said layer of polysilicon and said link region of said second epitaxial layer each having a predetermined concentration of said base dopant sufficient to form a conductive path extending from a base contact area of said layer of polysilicon through said link region of said second epitaxial layer to said base; and said emitter has a concentration gradient of said emitter dopant having a maximum at that portion of said second epitaxial layer's top surface abutting said polysilicon emitter contact member.