CH274 - Electrons in Solids and Materials Exercise 3.1
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
CH274 - Electrons in Solids and Materials
Exercise 3.1 – electron density in the conduction band
a) Estimate the electron density in the CB at room temperature (300 K) for pure (intrinsic) Si and diamond, knowing that their band gaps are 1.12 and 5.50 eV, respectively (assume the same effective density of states for Si and for diamond).
b) Compare the results of a) with the fraction of valence electrons that participate in conduction at 300 K for Na (body-centred cubic structure with unit cell parameter
4.29 Å).
Model answer
a)
The concentration of conduction electrons for an intrinsic semiconductor (pure material) can be estimated from the formula:
n = NC eEg / 2kT ,
where Eg is the semiconductor band gap and NC is the effective density of states and is assumed to be a constant which, for Si at room temperature, is equal to ~ 1025 m-3 (the
same value is assumed also for diamond).
At room temperature, for Si we can thus write:
nSi = 1025 e1. 12/(20.0256) = 3. 161015 m-3,
and for diamond:
ndiamond = 1025 e5.5 /(20.0256) = 2.221022 m-3 .
b)
The concentration of fraction of valence electrons that participate in conduction at temperature T in a metal can be evaluated by following the procedure described in exercise 2.2.
In the simplest approximation, the energetic region where the Fermi-Dirac distribution is significantly different form either 0 or 1 is equivalent to an energy interval of 2kT, on each side of EF. This results in a fraction of valence electron participating in conduction of ~1.6% (see exercise 2.2).
So we must now determine the total density of valence
electrons for Na. Na has one valence electron per each atom.
The atomic density of Na can be evaluated by considering its
crystal structure, body-centred cubic. There are 2 atoms for
each unit cell and therefore the number density for Na is:
pNa = = 2.53 1028 m-3 .
As a consequence, the concentration of valence electrons is
nNa = 2.531028 m-3 .
Therefore, the effective density of electrons that participate in conduction in Na at room temperature can be evaluated as:
nNa 4.1026 m-3 .
In conclusion, Na has around 11 orders of magnitude more conduction electrons than Si. While Na is an extremely good conductor, at room temperature Si has a much lower conductivity, though a non-vanishing one. On the contrary, the number of conduction electrons in diamond is effectively zero at room temperature, making this material an optimal insulator.
2022-09-17