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Mean Free Time

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

Overview[edit]

Charge carriers in a material are in random thermal motion with average velocity Equation mathematical left-angle v mathematical right-angle . When an electric field Equation upper E is applied, the carriers acquire drift velocity Equation v Subscript d 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 Equation tau is the average time before a randomly picked electron makes it next collision. Note that collisions are random, so Equation tau does not depend on the elapsed time since prior collision.

Relation to mean free path[edit]

The mean free path is the average distance the electron travels between collisions. In other words, it is the average distance the electron travels in Equation tau time.

Equation lamda equals mathematical left-angle v mathematical right-angle tau


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