a base structure for high performance silicon germanium : carbon based heterojunction bipolar transistors with phosophorus atomic layer doping is disclosed . the ald process subjects the base substrate to nitrogen gas and provides an additional sige : c spacer layer . during the ald process , the percent concentrations of germanium and carbon are substantially matched and phosphorus is a preferred dopant . the improved sige : c hbt is less sensitive to process temperature and exposure times , and exhibits lower dopant segregation and sharper base profiles .