Negative base current and impact ionization phenomena in AlGaAs/GaAs HBTs

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/sub CBO/. At high current densities, three effects contribute to lower the measured M-1 value: voltage drops due to collector (R/sub C/) and base (R/sub 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/sub CB/ in AlGaAs/GaAs HBTs is accurately characterized.< >