= Solution
<Charge carrier mobility> measures weak-field drift speed per <electric field>. For <electrons> and <electron holes>,
$$
\mathbf v_e=-\mu_e\mathbf E,\qquad\mathbf v_h=+\mu_h\mathbf E,\qquad\boxed{\sigma=e(n\mu_e+p\mu_h).}
$$
Both mobilities are positive, with SI units $\mathrm{m^2\,V^{-1}s^{-1}}$; the opposite drift signs reflect the opposite carrier charges. In a simple isotropic relaxation-time model, <electron mobility> is $\mu_e=e\tau_e/m_e^*$ and <hole mobility> is $\mu_h=e\tau_h/m_h^*$. These are momentum relaxation times, not the <minority carrier lifetime> for <electron-hole recombination>.
Scattering by <phonons>, ionized impurities and defects limits mobility. Increasing <phonon> populations commonly lowers mobility at high <temperature>; ionized-impurity scattering can dominate at low <temperature> and high <semiconductor doping>. Approximate addition of independent inverse relaxation times is the <Matthiessen's rule>. At sufficiently strong field, velocity saturation invalidates the constant-mobility drift law.
The <Hall effect> provides a measurement: a current-carrying bar in a perpendicular <magnetic field> develops a transverse voltage. In a one-carrier model $|R_H|=1/(en)$ and $\mu=\sigma|R_H|$, combining the transverse voltage with longitudinal <electrical conductivity>. Multiple carrier types and scattering-dependent Hall factors require a more detailed interpretation.
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