Fermi Level, Fermi Function and
Electron Occupancy of Localized Energy States
- To move the Fermi level : mouse drag the magenta-colored
- To change temeprature: use the scollbar near the bottom.
Your browser doesn't understand the <APPLET> tag.
A snapshot of the applet is:
All magenta-colored horizontal bars (i.e., Ef and scroller) may be mouse-dragged.
Fermi distribution function, and its meaning on
the electron occupancy of energy states: This
applet shows the Fermi function at a given temperature T. Answer
the following questions.
- Set the temperature to the lowest possible value. (1) Sketch
the shape of f(E). (2) Move the Fermi level by mouse-drag so that
it coincides with one of the energy states. Verify that the probability
of occupancy is exactly 1/2. (there are 16 states per energy. how many
are occupied ?)
- The energy states, 16 of them per energy, are separated by 60 meV.
Increase the temperature, observe that the electron population
is more smeared out in the states around Ef.
After answering the above two questions, make a comment about the following
aspects (This part will be also graded as per your Technical
Communication Skills, which should convey your technical opinions/ideas
clearly in both quantitative and qualitative aspects.)
- Slect a Temperature, and find the electron occupancy (for states above
Ef) and the electron vacancy (for states below Ef)
from the Figure on the right hand side. Are these probabilities consistent
with the Fermi function f(E) ? (That is, count the number of
states occupied, divide it by the number of states at that energy. Calculate
the numerical value of Fermi function for that energy. Compare.)
- How many kT's away from Ef are the states completely
full or completely empty1 ??
- Does this applet help you understand better why they use 3kT or 4kT
as an energy limit in making approximation to f(E) ??
- Compared to your (previous) understanding of the Fermi function based
on the mathematical formula (eg, Eq. 1-10 of Yang) and the static illustration
(eg, Fig.1-17, p.18 of Yang), did this applet improve the clarity of the
physical meaning of the distribution function ? How did this
applet affect your understanding and perception. Make specific comments.
- Before using this applet (ie, from formula and static figure alone),
did you have an immediate 'feeling' that the energy states below Ef
must be occupied by electron ? Did this applet reinforce this notion ?
Provide some specific comments.
- Make comments on how helpful this applet is in learning the concepts
of Fermi level and Fermi funtion. Make any other comments here.
- Make comments on how to improve this applet for a more effective learning
of the concepts/principles ? Are you willing to participate
in developing these applets ?
full or completely empty: Because there are
16 energy states at each level, if the probability of occupancy is less
than 1/16 or so, none of the states may be occupied at that level (completely
empty). Likewise, if the probability of occupancy is greater
than 1.0 - 1/16, then all the states may be occupied (completely
full). This discussion is only valid within an accuracy
of 6x10-2 of the probability. It must not
lead you to conclude that EVERY states will be empty, no matter
how many states there are, whenever the E - Ef > 4kT. A
very clear example of a very small f(E) value, but a large number of state-occupying
electrons comes in the semiconductor band. For example, at 300K (kT
= 0.0259 eV), let us consider Ec - Ef = 0.5 eV. You note that Ec - Ef =
19.3kT >> 4kT. Does it mean that ALL states at or above E = Ec (the
conduction band edge) are empty and thus there are no conduction electons
? No ! There are in fact 1.16x1011 electrons per cm3
at energy states at or above Ec ! That is, one hundred
and sixteen billion electrons per cm3 ! This
is due to the simple arithmatic that (a very small number) times (a very
big number) is not necessarily very small ! Here, f(E) = f(Ec - Ef)
= f(0.5 eV) = 4.13 x 10-9, meaning that just about 4 states
will be occupied for every one billion states ! But in the conduction
band of Si, you have about 2.8x1019 states per cm3 (Nc
= 2.8x1019 cm-3), a number so large that I can not
even spell it out in English. Out of 2.8x1019 states,
a mere one hundred and sixteen billion states are occupied by electrons
( n = Nc * f(E) or 1.16E11 = 4.13E-9 * 2.8E19) !
Copyright (c) C.R.Wie, SUNY-Buffalo, 1996-1997