US Patent 7566921 Silicon germanium emitter

Disclosed are an improved hetero-junction bipolar transistor (HBT) structure and a method of forming the structure that incorporates a silicon-germanium emitter layer with a graded germanium profile. The graded germanium concentration creates a quasi-drift field in the neutral region of the emitter layer. This quasi-drift field induces valence bandgap grading within the emitter layer so as to accelerate movement of holes from the base layer through the emitter layer. Accelerated movement of the holes from the base layer through the emitter layer reduces emitter delay time and thereby, increases the cut-off frequency (fT) and the maximum oscillation frequency (fMAX) of the resultant HBT.