The impact of non-equilibrium transport on breakdown and transit time in bipolar transistors

Impact ionization and velocity overshoot in the base-collector junction of bipolar transistors are studied using Monte Carlo simulation and the hydrodynamic energy-balance equation. For advanced bipolar transistors, the carrier energy lags the electric field; therefore, the maximum impact ionization rate occurs deep into the junction. A simplified solution of the energy-balance equation can accurately model this nonlocal behavior. Excellent agreement with measurements of the multiplication factor for a variety of base-collector profiles is obtained. As a consequence of this nonequilibrium effect, velocity overshoot is expected and its trade-off with breakdown is analyzed in detail.