Mean free time or collision time or conductivity relaxation time () is the average time between scattering events. For example, the average time between collisions in a semiconductor.

Overview

Charge carriers in a material are in random thermal motion with average velocity . When an electric field is applied, the carriers acquire drift velocity in the same direction, opposite to the field. Constant collisions with lattice ions means that the the gained kinetic energy is lost pretty frequently. The mean free time is the average time before a randomly picked electron makes it next collision. Note that collisions are random, so does not depend on the elapsed time since prior collision.

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