Monte Carlo simulation of impact ionization in SiGeHBTs

Measurements and Monte Carlo simulations of impact ionization in the base-c ollector region of SiGe HBTs are presented. A device with low germanium con centration (graded from 0 to 12%) is considered and no differences are foun d between the experimental multiplication factor in that device and the cor responding silicon control. Because impact ionization (II) occurs inside th e 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 straine d SiGe layers. Full-Band Monte Carlo simulations are shown to reproduce the multiplication factors measured in SiGe devices featuring different collec tor profiles.