Impact ionization phenomena in the collector region of AlGaAs/GaAs heterojunction bipolar transistors give rise to base current reduction and reversal. These phenomena can be characterized by extracting the M - 1 coefficient, which can be evaluated by measuring base current changes. Measurements of M - 1 are affected at low current densities by the presence of the collector-base junction reverse current I(CBO). At high current densities, three effects contribute to lower the measured M - 1 value: voltage drops due to R(C) and R(B) parasitic resistances, device self-heating, and lowering of the base-collector junction electric field due to mobile carriers. By appropriately choosing the emitter current value, parasitic phenomena are avoided and the behavior of M - 1 as a function of the collector-base voltage V(CB) in AlGaAs/GaAs HBT's is accurately characterized.