Drift velocity
The drift velocity is the average velocity that a particle, such as an electron, attains in a material due to an electric field. It can also be referred to as axial drift velocity. In general, an electron will propagate randomly in a conductor at the Fermi velocity. An applied electric field will give this random motion a small net flow velocity in one direction.
In a semiconductor, the two main carrier scattering mechanisms are ionized impurity scattering and lattice scattering.
Because current is proportional to drift velocity, which in a resistive material is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.
The most elementary expression of Ohm's law is:
where u is the drift velocity, μ is the electron mobility (with units m2/(V⋅s)) of the material and E is the electric field (with units V/m).
The formula for evaluating the drift velocity of charge carriers in a material of constant cross-sectional area is given by:
where u is the drift velocity of electrons, j is the current density flowing through the material, n is the charge-carrier number density, and q is the charge on the charge-carrier.
In terms of the basic properties of the right-cylindrical current-carrying metallic ohmic conductor, where the charge-carriers are electrons, this expression can be rewritten as:
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