Inverse base-width modulation and collector space-charge-region widening: degrading effects at high current densities in highly scaled BJT´s (and HBT´s)

Degrading effects on BJT speed performance due to current-induced perturbation of the collector-base junction space-charge region (SCR) prior to base pushout are analyzed and assessed. Inverse base-width modulation (IBWM)-a widening of the quasi-neutral base width-and collector SCR-width widening (SCRW) in highly scaled BJT´s and HBT´s are identified as important mechanisms governing device speed degradation at high currents. IBWM, which increases the base transit time, is described analytically to distinguish it from base pushout, or quasi-saturation. MMSPICE simulations of an aggressive SiGe-base HBT technology, supported by measured data, show that the speed (f_T) degradation associated with IBWM is significant at currents well below the onset of base pushout, which underlies the speed degradation of lesser scaled BJT´s. Simulations of further scaled devices show that SCRW can be the predominant mechanism of speed degradation at high current densities