Inverse base-width modulation and collector space-charge-regionwidening: degrading effects at high current densities in highly scaledBJT'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