Impact Ionization Noise in SiGe HBTs: Comparison of Device and Compact Modeling With Experimental Results

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" HBTs), featuring a maximum transit frequency of f_T = 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 II current noise, obtained from the CM, is in fair agreement with the HD model. Verification of NPs (NF_min, R_n, and Gamma_OPT), 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.