Profile scaling constraints for ion-implanted and epitaxial bipolar technology designed for 77 K operation

The scaling constraints on vertical profile design which are unique to liquid nitrogen temperature operation (LNT equivalent to 77 K) of ion-implanted and epitaxial Si and SiGe bipolar technologies are investigated experimentally. While conventional on-implantation techniques can yield transistors with cutoff frequencies as high as 36 GHz at LNT, these devices have limited extendibility for circuit applications due to excessive base freeze-out. A more advanced epitaxial SiGe technology can be used to simultaneously achieve an fT for 59 GHz and superior base freeze-out properties at low temperatures, yielding a very aggressive ECL (emitter coupled logic) gate delay of 28 ps at LNT. For SiGe devices, however, the optimum collector profile design is constrained by a barrier induced at the SiGe-Si heterojunction under high injection which limits the device transconductance and fT at LNT.