# Homework Help: Mobility of charged particles in classical gas

1. Nov 7, 2011

### Sigurdsson

This is problem 9.2 in solid state physics by Ibach and Luth.

1. The problem statement, all variables and given/known data
Using the stationary Boltzmann equation, show that the mobility of the charged particles in a classical gas is given by

$\mu = \frac{e \langle v^2 \tau(\mathbf{k}) \rangle}{m \langle v^2 \rangle}$

2. Relevant equations

The stationary Boltzmann equation is
$\mathbf{v} \cdot \nabla_{\mathbf{r}} f - \frac{e}{\hbar} \vec{\mathscr{E}} \cdot \nabla_{\mathbf{k}} f = \left( \frac{\partial f}{\partial t} \right)_s = -\frac{f(\mathbf{k}) - f_0(\mathbf{k}) }{\tau(\mathbf{k} ) }$

The mobility is defined as the proportionality between the drift velocity of the electrons and the electric field
$-\frac{v_D}{\mathscr{E} } = \mu$

and the function $f$ is the fermi distribution.

3. The attempt at a solution

Now the first thing I notice is the in the final answer we have the electron mass $m$ but not the effective mass $m^*$. Now I'm going to assume that the distribution does not depend on position so we have
$- \frac{e}{\hbar} \vec{\mathscr{E}} \cdot \nabla_{\mathbf{k}} f = -\frac{f(\mathbf{k}) - f_0(\mathbf{k}) }{\tau(\mathbf{k} ) }$

The term on the left is called drift term. From there we should get the drift velocity. The electrical conductivity is
$\sigma = j/\mathscr{E} = n e \mu$

Where $j, e, n$ are in correct order the current density, electron charge, electron density.

Now I have that
$\sigma = j_x/\mathscr{E}_x = \simeq \frac{e^2}{8 \pi^3 \hbar} \int_{E = E_F} \frac{v_x^2(\mathbf{k}) }{v(\mathbf{k})} \tau(\mathbf{k}) \ df_E$

What is really bugging me is the final answer has the mean value of the velocity and the classical mass of the electron but not the effective mass. Any tips on what happens when I'm working with the Boltzmann equation in classical electron gas?

Cheers