Slide 41 of 64
This is a scanned copy of the cross section of the Hitachi SiGe HBT for which these low power delay products were obtained. Notice that the extrinsic base is extremely vestigial. Contacts to the base region which traditionally have been made from the top of the base region are instead made to the side of the emitter stack where a sheet of polysilicon provides the connection to the SiGe base sideways. This not only compacts the device, but greatly reduces the extrinsic base and associated lateral base pushout effect. In addition, the opening into which the base is eventually deposited is used for selective doping.