### D.30 Number of conduction band electrons

This note finds the number of electrons in the conduction band of a semiconductor, and the number of holes in the valence band.

By definition, the density of states is the number of single-particle states per unit energy range and unit volume. The fraction of electrons in those states is given by . Therefore the number of electrons in the conduction band per unit volume is given by

where is the energy at the bottom of the conduction band and that at the top of the band.

To compute this integral, for the Maxwell-Boltzmann expression (6.33) can be used, since the number of electrons per state is invariably small. And for the density of states the expression (6.6) for the free-electron gas can be used if you substitute in a suitable effective mass of the electrons and replace by .

Also, because decreases extremely rapidly with energy, only a very thin layer at the bottom of the conduction band makes a contribution to the number of electrons. The integrand of the integral for is essentially zero above this layer. Therefore you can replace the upper limit of integration with infinity without changing the value of . Now use a change of integration variable to and an integration by parts to reduce the integral to the one found under ! in the notations section. The result is as stated in the text.

For holes, the derivation goes the same way if you use from (6.34) and integrate over the valence band energies.