The impact of bandgap offset distribution between conduction and valence bands in Si-based graded bandgap HBT's

This work examines the impact of bandgap offset distribution between conduc tion and valence bands in Si-based graded bandgap HBT's using de and ac sim ulation. For a fixed total bandgap offset, a conduction band pushed up by t he total offset, together with a valence band pushed up by 2x the total off set gives the best ac performance, and allows the highest operational curre nt for high frequency applications in an n-p-n HBT. A retrograded mole frac tion profile, when properly optimized, can produce nearly identical ac perf ormance for different bandgap offset distributions. These suggest that cont rary to popular belief, applying careful optimization can yield excellent t ransistor performance for any arbitrary band alignment for both n-p-n and p -n-p graded bandgap HBT's. (C) 1999 Elsevier Science Ltd. All rights reserv ed.