Patent ID: 8357953

Claim:
A bipolar transistor, comprising: an emitter; an intrinsic base underneath said emitter, said intrinsic base extending laterally to an extrinsic base, said extrinsic base comprising at least: a first semiconductor material with a first bandgap (energy gap) and a second semiconductor material with a second bandgap formed on said first semiconductor material in a recessed surface portion thereof, said second bandgap being smaller than said first bandgap; and a collector underneath said intrinsic base, wherein: said recessed portion of said first semiconductor material forms a stepped portion of said first semiconductor material, said recessed portion of said first semiconductor material comprises a first portion with a first thickness and a second portion with a second thickness, different from said first thickness, said extrinsic base is defined by a set of spacers formed on top of said first semiconductor material and adjacent to sidewalls of said emitter; and said set of spacers are formed such that edges of said spacers are in substantial alignment with edges of a set of isolation structures which underlies said extrinsic base, and an edge of said recessed portion.