Is the standard equation for drift velocity of electrons also applicable to conduction in, say, a vacuum tube? I assume it is, and if so what is the relation between the drift velocities at the interface of vacuum-metal at the anode of a vacuum diode? The current, electronic charge and Area remain the same so I suppose it is only the ratio of the carrier density? (Let's assume we are operating the diode in its saturation current region) The conductor density in a metal is fixed (say, 8.5×1028 electrons per m³ for Copper) but what determines the density in the vacuum (again, in the saturation region)? Beyond saturation, even on a bias voltage increase the current remains the same. The explanations I've read say this is because all the electrons emitted by thermionic emission have been used up. But wouldn't it be possible to increase current via an increase in their drift velocity? Wouldn't increasing voltage increase the field thereby accelerating electrons ultimately increasing drift velocities? Why does current saturate then?