Slide 43 of 64
This is the really clever part of the processing. The sidewall base contacts go on sideways before the base is deposited. This virtually eliminates the part of the extrinsic base that would extend out over the collector for an area bigger than the size of a contact. Even before that the Hitachi process ion implants a Selective Ion Collector doping (SIC) process through the opening that receives the eventual base epitaxy.