Monte Carlo Simulation of Impact Ionization in SiGe HBTs

Measurements and Monte Carlo simulations of impact ionization in the base-collector region of SiGe HBTs are presented. A device with low germanium concentration (graded from 0 to 12%) is considered and no differences are found between the experimental multiplication factor in that device and the corresponding silicon control. Because impact ionization (II) occurs inside the bulk-Si collector, phonon and II scattering rates for bulk silicon can be used in the Monte Carlo simulation, avoiding the need to model the strained SiGe layers. Full-Band Monte Carlo simulations are shown to reproduce the multiplication factors measured in SiGe devices featuring different collector profiles.