Q.Define mobility.

The magnitude of a charge carrier's drift velocity per unit applied electric field, $\mu=v_d/E=e\tau/m$, SI unit $\text{m}^2\,\text{V}^{-1}\,\text{s}^{-1}$. Depends only on the carrier's charge-to-mass ratio and the material's relaxation time $\tau$ -- not on the field strength itself, since $v_d\propto E$ makes the field cancel out of the ratio. Combined with the free-electron density $n$, …