Collector signal delay in the presence of velocity overshoot

It is pointed out that the well-known expression equating the collector signal delay to one-half of the transit time through the base-collector depletion region is incorrect in the presence of a nonconstant carrier velocity, as occurs in the case of velocity overshoot. The correct expression yields a smaller signal delay than the conventional estimate for typical situations, further emphasizing the benefit of velocity overshoot in bipolar devices.>