Charge carriers in semiconductors are negatively charged electrons and positively charged holes. Holes are
essentially vacancies in an otherwise filled band so that when an electron moves to such a vacancy, the effect is equivalent to movement of a hole in reverse direction.
In the absence of an electric field, charge carriers move randomly so that their average velocity is zero. When an
electric field is applied, the positve charge carriers move in the direction of the field and the negative charge carriers move against the field direction. This directed motion is superimposed over the random direction and is called drift.
Drift velocity is proportional to the direction of the field, the constant of proportionality is the mobility of the carrier.
If a magnetic field is applied on a flat strip of semiconductor in a direction perpendicular to the direction of current
flow, a voltage develops in a direction which is perpendicular to both the direction of the field and the magnetic field. This is known as Hall voltage.
Hall voltage arises due to Lorentz force that acts on charge carriers and provides a direct means of verifying
existence of holes.
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