In the field of
solid state physics,
the electron affinity is defined differently than in chemistry and
atomic physics. For a semiconductor-vacuum interface (that is, the
surface of a semiconductor), electron affinity, typically denoted by
EEA or
χ, is defined as the energy obtained by moving an electron from the vacuum just outside the semiconductor to the bottom of the
conduction band just inside the semiconductor:
[6]
In an intrinsic semiconductor at
absolute zero,
this concept is functionally analogous to the chemistry definition of
electron affinity, since an added electron will spontaneously go to the
bottom of the conduction band. At nonzero temperature, and for other
materials (metals, semimetals, heavily doped semiconductors), the
analogy does not hold since an added electron will instead go to the
Fermi level
on average. In any case, the value of the electron affinity of a solid
substance is very different from the chemistry and atomic physics
electron affinity value for an atom of the same substance in gas phase.
For example, a silicon crystal surface has electron affinity 4.05 eV,
whereas an isolated silicon atom has electron affinity 1.39 eV.
The electron affinity of a surface is closely related to, but distinct from, its
work function. The work function is the
thermodynamic work
that can be obtained by reversibly, isothermally moving an electron
from the vacuum to the material; this thermodynamic electron goes to the
Fermi level on average, not the conduction band edge:
. While the
work function of a semiconductor can be changed by
doping,
the electron affinity ideally does not change with doping and so it is
closer to being a material constant. However, like work function the
electron affinity does depend on the surface termination (crystal face,
surface chemistry, etc.) and is strictly a surface property.
In semiconductor physics, the primary use of the electron affinity is
not actually in the analysis of semiconductor–vacuum surfaces, but
rather in heuristic
electron affinity rules for estimating the
band bending that occurs at the interface of two materials, in particular
metal–semiconductor junctions and semiconductor
heterojunctions.
In certain circumstances, the electron affinity may become negative.
[7] Often negative electron affinity is desired to obtain efficient
cathodes
that can supply electrons to the vacuum with little energy loss. The
observed electron yield as a function of various parameters such as bias
voltage or illumination conditions can be used to describe these
structures with
band diagrams
in which the electron affinity is one parameter. For one illustration
of the apparent effect of surface termination on electron emission, see
Figure 3 in
Marchywka Effect.
