Slide 33 of 64
This slide shows the relative emitter sizes for currently available SiGe HBTs and GaAs HBTs. Note that for the moment SiGe seems to be more aggressive in the shrinkage of the emitter. Note also that it may be possible to make smaller emitters in these processes, but these are the recommended sizes attainable for 10K HBT yields and higher.
The design rules have been purposely blacked out to protect proprietary data for both IBM and Rockwell.