Editing Electron mobility (section)

===Derivation===
Starting with [[Newton's second law]]:
<math display="block">a = F/m_e^* </math>
where:
*''a'' is the acceleration between collisions.
*''F'' is the electric force exerted by the electric field, and
*<math>m_e^* </math> is the [[Effective mass (solid-state physics)|effective mass]] of an electron.

Since the force on the electron is −''eE'':
<math display="block">a = -\frac{eE}{m_e^*} </math>

This is the acceleration on the electron between collisions. The drift velocity is therefore:
<math display="block">v_d = a \tau_c = -\frac{e\tau_c}{m_e^*}E,</math> where <math>\tau_c</math> is the [[mean free time]]

Since we only care about how the drift velocity changes with the electric field, we lump the loose terms together to get 
<math display="block">v_d = -\mu_e E,</math> where <math>\mu_e = \frac{e\tau_c}{m_e^*}</math>

Similarly, for holes we have 
<math display="block">v_d = \mu_h E,</math> where <math>\mu_h = \frac{e\tau_c}{m_h^*}</math>
Note that both electron mobility and hole mobility are positive. A minus sign is added for electron drift velocity to account for the minus charge.