Define drift velocity and Mobility. Write its unit.

Drift velocity (\(v_d\)): When an electric field is applied to a conductor or semiconductor, the free charge carriers acquire a small average velocity opposite to (electrons) or along (holes) the field, superimposed on their random thermal motion. This average velocity is the drift velocity. \(v_d=\mu E\), and the current density is \(J=nev_d\). Unit: m/s.

Mobility (\(\mu\)): The drift velocity acquired per unit electric field: \(\mu=\dfrac{v_d}{E}\). It shows how easily carriers move through the material (\(\sigma=ne\mu_e+pe\mu_h\)). Unit: m²/V s (m² V⁻¹ s⁻¹).