Drift velocity
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The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. Since particles can accelerate arbitrarily close to the speed of light in the absence of other forces, the term "drift velocity" can only really apply to carriers in materials, and not to particles in a vacuum. Particles in solids, for example, actually collide or scatter with the crystal lattice (or phonons), which slows them down.Drift Velocity is non-uniform as it involes electric field as an externally accelerating agent.
In a semiconductor, the two main carrier scattering mechanisms are ionized impurity scattering and lattice scattering.
Jdrift = ρ * ʋavg where ρ is charge density in units C/cm^3, and ʋavg is the average velocity of the carriers
ʋavg = μ * E where μ is the mobility of the carriers (cm^2 / V-s) and E is the electric field (V/cm)