Editing Fermi level (section)

===Fermi level referencing and the location of zero Fermi level===

Much like the choice of origin in a coordinate system, the zero point of energy can be defined arbitrarily. Observable phenomena only depend on energy differences.
When comparing distinct bodies, however, it is important that they all be consistent in their choice of the location of zero energy, or else nonsensical results will be obtained.
It can therefore be helpful to explicitly name a common point to ensure that different components are in agreement.
On the other hand, if a reference point is inherently ambiguous (such as "the vacuum", see below) it will instead cause more problems.

A practical and well-justified choice of common point is a bulky, physical conductor, such as the [[electrical ground]] or earth.
Such a conductor can be considered to be in a good thermodynamic equilibrium and so its ''μ'' is well defined.
It provides a reservoir of charge, so that large numbers of electrons may be added or removed without incurring charging effects.
It also has the advantage of being accessible, so that the Fermi level of any other object can be measured simply with a voltmeter.

====Why it is not advisable to use "the energy in vacuum" as a reference zero====

[[File:Work function mismatch gold aluminum.svg|thumb|300 px|When the two metals depicted here are in thermodynamic equilibrium as shown (equal Fermi levels E<sub>F</sub>), the vacuum [[electrostatic potential]] ''ϕ'' is not flat due to a difference in [[work function]].]]

In principle, one might consider using the state of a stationary electron in the vacuum as a reference point for energies.
This approach is not advisable unless one is careful to define exactly where ''the vacuum'' is.<ref group=Note>Technically, it is possible to consider the vacuum to be an insulator and in fact its Fermi level is defined if its surroundings are in equilibrium. Typically however the Fermi level is two to five electron volts ''below'' the vacuum electrostatic potential energy, depending on the [[work function]] of the nearby vacuum wall material. Only at high temperatures will the equilibrium vacuum be populated with a significant number of electrons (this is the basis of [[thermionic emission]]).</ref> The problem is that not all points in the vacuum are equivalent.

At thermodynamic equilibrium, it is typical for electrical potential differences of order 1 V to exist in the vacuum ([[Volta potential]]s).
The source of this vacuum potential variation is the variation in [[work function]] between the different conducting materials exposed to vacuum.
Just outside a conductor, the electrostatic potential depends sensitively on the material, as well as which surface is selected (its crystal orientation, contamination, and other details).

The parameter that gives the best approximation to universality is the Earth-referenced Fermi level suggested above. This also has the advantage that it can be measured with a voltmeter.