Energy-gap and electron-affinity contractions and their importance in bipolar device simulators

Simulations using bipolar device simulators show that moderate injection can exist in heavily doped portions of a transistor base and that very high injection can exist throughout a current-extended or pushed out part of a quasineutral base layer of a high-speed digital device. Physical models now in bipolar device simulators neglect the dependence of the effective intrinsic density and the corresponding effective energy-gap contraction on high concentrations of mobile electrons and holes. Further they assume that the conduction-band and valence-band shifts are equal in magnitude and thus equal to half of the magnitude of the generalized electron-affinity contraction. New results from experiment and theory demonstrate the incompleteness of these physical models, and the engineering significance of this incompleteness is assessed.<>