For an estimation of the mean electron velocity *v* the time
derivative in the equation of motion was approximated by the
relaxation time τ:

The current density *j* is easily expressed in terms of the
carrier velocity *v*, their density *n* and their charge *e*:

Earlier we defined the specific conductivity σ as product of carrier density, carrier charge, and mobility.

Thus, we are able to relate the carrier mobility μ and the relaxation time τ:

In the next paragraphs we will look closer at the different
types of collisions, or scattering events, and try to relate
them to the relaxation time τ:

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