M.A.M.Abed said:
why does work function measured from fermi level
while work function is defined by lowest energy required to remove an electron from surface of materials
and fermi level the level have 50% chance to occupation that's mean there is level above it have electron inside so why we don't measure from those levels
Since in
metals electrons are filled up to the Fermi level, the minimum energy required to extract
an electron from a metal is assigned as its work function.
One cannot extract electrons from the bulk of a material without extracting it from its surface.
So, strictly speaking, work function is a surface property - it usually varies with the surface conditions of the same material.
For all practical purposes, work function can be taken as a bulk property
.
However if one looks up the same Work Function
for semiconductors, there are energy band gaps, and so electrons may not be coming out of the Fermi level.
There are some electrons available at the bottom of the conduction band,
so the photoelectric effect experiment gives a different quantity assigned as the electron affinity of the semiconductor.
Now if one calculates the work function, the energy difference between the bottom of the conduction band and the Fermi level must be added .
That is why the work functions of p- and n-type of the same semiconductor (say, Si) are different.