Charge-control analysis of collector transit time for (AlGa)As/GaAs HBTs under a high injection condition

Charge-control analysis of collector transit time (τ_C ) for (AlGa)As/GaAs n-p⁺-n heterojunction bipolar transistors has been carried out under the restriction of low-frequency approximation to explain the remarkable τ_C reduction at the onset of the Kirk effect. The fundamental idea of the model is that the increment of the current density must depend not only on the increment of the carrier density but also on the increment of the carrier drift velocity in the collector space-charge region. The obtained model well explains the previous Monte Carlo results, thus indicating the validity of the above idea as well as the possibility that the actual τ_C is by far smaller than that estimated from the conventional model. It also leads to a concept for the reduction of transit times, i.e. that an increasing rate of the electron overshoot velocity is as essential for the reduction of τ_C as the magnitude of the overshoot velocity itself