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Thus Ohm's law can be explained in terms of drift velocity. The law's most elementary expression is: =, where u is drift velocity, μ is the material's electron mobility, and E is the electric field. In the MKS system, drift velocity has units of m/s, electron mobility, m 2 /(V·s), and electric field, V/m.
At low fields, the drift velocity v d is proportional to the electric field E, so mobility μ is constant. This value of μ is called the low-field mobility. As the electric field is increased, however, the carrier velocity increases sublinearly and asymptotically towards a maximum possible value, called the saturation velocity v sat.
Typical values of saturation velocity may vary greatly for different materials, for example for Si it is in the order of 1×10 7 cm/s, for GaAs 1.2×10 7 cm/s, while for 6H-SiC, it is near 2×10 7 cm/s. Typical electric field strengths at which carrier velocity saturates is usually on the order of 10-100 kV/cm.
The drift velocity deals with the average velocity of a particle, such as an electron, due to an electric field. In general, an electron will propagate randomly in a conductor at the Fermi velocity. [5] Free electrons in a conductor follow a random path. Without the presence of an electric field, the electrons have no net velocity.
is the mobility (m 2 /(V·s)). In other words, the electrical mobility of the particle is defined as the ratio of the drift velocity to the magnitude of the electric field: =. For example, the mobility of the sodium ion (Na +) in water at 25 °C is 5.19 × 10 −8 m 2 /(V·s). [1]
The physical quantity ion mobility K is defined as the proportionality factor between an ion's drift velocity v d in a gas and an electric field of strength E. = After making the necessary adjustments to account for the n0 standard gas density, ion mobilities are often expressed as reduced mobilities.
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The drift velocity is the average velocity of the charge carriers in the drift current. The drift velocity, and resulting current, is characterized by the mobility; for details, see electron mobility (for solids) or electrical mobility (for a more general discussion). See drift–diffusion equation for the way that the drift current, diffusion ...