A simple model gives the approximate relation between scattering time (average time between scattering events) and mobility. It is assumed that after each scattering event, the carrier's motion is randomized, so it has zero average velocity. After that, it accelerates uniformly in the electric field, until it scatters again. The resulting average drift mobility is: μ μ = q m ∗ ∗ τ τ ¯ ¯ where q is the elementary charge, m * is the carrier effective mass, and τ is the average scattering time.