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 conduction and valence bands in Si-based graded bandgap HBT's using de and ac simulation. For a fixed total bandgap offset, a conduction band pushed up by the total offset, together with a valence band pushed up by 2x the total offset gives the best ac performance, and allows the highest operational current for high frequency applications in an n-p-n HBT. A retrograded mole fraction profile, when properly optimized, can produce nearly identical ac performance for different bandgap offset distributions. These suggest that contrary to popular belief, applying careful optimization can yield excellent transistor 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 reserved.