Field Emission

Author: J.B. Hoag

Suppose the voltage on the plate of a two-electrode vacuum tube or diode be increased to a very, very large value. Then it will be found that the current passing through the tube will be much greater than the saturation or total emission value discussed in previous sections of this chapter. The electric field between the plate and filament, due to the very-high-voltage battery, causes an increase in the number of electrons escaping from the filament at a given temperature. This so-called field emission is negligible at the usual voltages used on ordinary vacuum tubes. Intense electrical fields, however, are able to assist the electrons inside the metal of the filament to escape into the vacuum outside. Thus, due to the combined pull of the electric field and their own thermal movements, more electrons escape than would do so by thermal emission alone.

With field emission, it is the strength of the electric field rather than the voltage of the battery which counts. In other words, if the filament and the plate are close together, a given voltage of comparatively modest amount can set up an electric "field gradient" of value equal to that produced by a much larger voltage applied to a more widely separated filament and plate. Yet the field emission in the two cases will be the same.