# Boltzmann Distribution: Feynman's treatment of p-n junction

• I
In Vol III, 14-4 and 14-5 of the Feynman Lectures (http://www.feynmanlectures.caltech.edu/III_14.html), Feynman gives a discussion of the p-n junction, in which he derives the diode characteristic equation via a nice, simple and convincing application of the Boltzmann distribution to the relative numbers of charge carriers on either side of a junction with a potential difference V.

I am however completely confused: do electrons/holes in a semi-conductor not obey Fermi-Dirac statistics? If so, why does he not mention or rely on this, and if so, how is his derivation valid?

I'll point out that I know not a great deal about solid state physics, or stat. mech., so the explanation may be utterly trivial.

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phyzguy
In a semiconductor, the conduction and valence bands are far from the Fermi level in units of kT. In this limit, Fermi-Dirac statistics are very closely equal to Maxwell-Boltzmann statistics:

Fermi-Dirac: $N \propto \frac{1}{\exp(\frac{E-E_F}{kT})+1}$

But in this case $\exp(\frac{E-E_F}{kT}) >> 1$, so we can ignore the 1 in the denominator. It then reduces to Maxwell-Boltzmann statistics to a sufficient approximation.

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• strauser
In a semiconductor, the conduction and valence bands are far from the Fermi level in units of kT. In this limit, Fermi-Dirac statistics are very closely equal to Maxwell-Boltzmann statistics:

Fermi-Dirac: $N \propto \frac{1}{\exp(\frac{E-E_F}{kT})-1}$
OK, thanks, things are slightly clearer.

However, I'm not sure either what your ##N## or the expression on the RHS are. Is ##N## related to the density of states (i.e. what I'd call ##N(E)##), or is it the concentration of charge carriers? And the expression on the RHS looks suspiciously like the Fermi function, but that has a +1.

Could you clarify this a bit more, please?

But in this case $\exp(\frac{E-E_F}{kT}) >> 1$, so we can ignore the 1 in the denominator. It then reduces to Maxwell-Boltzmann statistics to a sufficient approximation.
Right. So is it correct to say that, to a very good approximation, electrons in the conduction band of a semiconductor obey Maxwell-Boltzmann statistics, which is Feynman's tacit starting point?

phyzguy