This paper presents a unified physical formulation for the avalanche effect in silicon-germanium heterojunction bipolar transistors (SiGe HBTs) at different injection levels. Based on an analytical description of the resulting electric-field distribution, a closed-form analytical expression for the multiplication factor is derived and has been implemented in the HICUM compact model. The model accuracy close to and beyond the common-emitter breakdown voltage BV CEO has been assessed over a wide temperature range in comparison to measurements of SiGe HBTs with different collector doping profiles and emitter geometries. Index Terms-Avalanche, compact model, high injection, impact ionization, kirk effect, safe operating area (SOA), silicon-germanium heterojunction bipolar transistors (SiGe HBTs).