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: