Bipolar effects on the signal delay time in HBTs at high currents

The signal delay time for GaAs-based n-p-n HBTs operating at high current densities has been investigated using a phenomenological transport model. It is shown that once base pushout commences, the electronic contribution to the signal delay decreases because of the reduced intervalley scattering associated with the weakened electric field in the collector. However, more importantly for practical considerations, it is also shown that the overall signal delay time increases because of the dominant role played by the slow-moving holes in the base pushout process