Velocity modulation in III/V-HBTs

Velocity modulation is shown to have a strong impact on the base/collector capacitance and the collector transit-time delay which dominate the high-speed performance of state-of-the-art HBTs. The authors present a theoretical analysis of the velocity modulation effects, which is the basis of a method to assess their strength from measured S-parameters. Monte Carlo simulations are in good agreement with the measurements, providing strong support for the theory. As a consequence, the authors find that the carrier velocity is much lower than estimated from transittime measurements when neglecting velocity modulation and that base-pushout occurs at much lower current levels than commonly expected.