someone made us realize earlier today
while it's a common misconception that the speed electricity propagates down a wire is the same as the speed of individual electrons (it's not, by several orders of magnitude)
it is actually true that the speed of electrons is the determining factor in BJTs, diodes, and other devices based on forward-biased junctions (thyristors for example, but not jfets) turning on and off
It's commonly said, and it's a useful abstraction (it's not really wrong!) that these are current-controlled devices. Actually, what determines the state of a BJT is not the current through its base but the charge stored in its base. It's just that this charge quickly dissipates due to low-impedance paths to the collector and emitter. It's this charge that alters the potential energy to allow charge carriers through the base region, not current
the reason it's so useful to think in terms of current controlled devices is simply that as this charge dissipates quickly, it must be constantly replenished, so what the equilibrium charge in the base region is depends on how quickly that charge is being restored--the magnitude of the current
(similar arguments apply to diodes, but here it's the depletion region equilibrium width)
