There are also emission regimes where the internal electrons are not in thermodynamic equilibrium and the emission current is, partly or completely, determined by the supply of electrons to the emitting region.
12.
The emission current varies strongly with the local work function in accordance with the Fowler-Nordheim equation; hence, the FEM image displays the projected work function map of the emitter surface.
13.
The surface property of primary interest is the work function, which is the barrier that limits electron emission current from the surface and essentially is the heat of vaporization of electrons from the surface.
14.
Experiments measure the emission current " i " from some defined part of the emission surface, as a function of the voltage " V " applied to some counter-electrode.
15.
Where voltage-drop effects occur, then the theory of the emission current may, to a greater or lesser extent, become theory that involves internal transport effects, and may become very complex.
16.
Simple solvable models of the tunneling barrier lead to equations ( including the original 1928 Fowler Nordheim-type equation ) that get predictions of emission current density too low by a factor of 100 or more.
17.
In 1911 and then 1914, then-graduate student Franz Rother, employing Earhart's method for controlling and measuring the electrode separation but with a sensitive platform galvanometer, directly measured steady field emission currents.
18.
For CFE, basic theoretical treatments provide a relationship between the local emission current density " J " and the local barrier field " F ", at a local position on the emitting surface.
19.
At the time of writing, the most promising forms of large-area field emission source ( certainly in terms of achieved average emission current density ) seem to be Spindt arrays and the various forms of source based on CNTs.
20.
Photoemission can occur from any material, but it is most easily observable from metals or other conductors because the process produces a charge imbalance, and if this charge imbalance is not neutralized by current flow ( enabled by conductivity ), the potential barrier to emission increases until the emission current ceases.