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The noise behavior resulting from impact ionization (II) was investigated at room temperature for silicon-germanium (SiGe) heterojunction bipolar transistors with box Ge profile ("true" HBT5), featuring a maximum transit frequency of fT = 80 GHz. Noise parameters (NPs) were measured over a wide range of collector-emitter voltages. Modeling was performed using a generalized hydrodynamic (HD) device simulation with a local temperature approach for avalanche generation, drift-diffusion (DD) simulation with a local field model, and the compact model (CM) HICUM/L2 with a conventional local field Chynoweth's law for avalanche generation. Local temperature model parameters were calibrated by matching the avalanche multiplication factor (M) to results obtained from full-band Monte Carlo (MC) simulations. The spectral density of!! current noise, obtained from the CM, is in fair agreement with the HD model. Verification of NPs (NFmin, Rn, and ΓOPT), obtained with compact and HD model, against experimental values proved that the weak avalanche model is accurate enough to capture II noise in investigated SiGe HBTs. [ABSTRACT FROM AUTHOR]