If a piece of metal or semiconductor carrying a current I is placed in a transverse magnetic field B then an electric field E is induced in the direction perpendicular to both I and B. This phenomenon is known as Hall effect.

Hall effect is normally used to determine whether a semi-conductor is n-type or p-type.

To find whether the semiconductor is n-type or p-type

i) In the figure. above, If I is in the +ve X direction and B is in the +ve Z direction, then a force will be exerted on the charge carriers (holes and electrons) in the –ve Y direction.

ii) This force is independent of whether the charge carriers are electrons or holes. Due to this force the charge carriers ( holes and electrons) will be forced downward towards surface –1 as shown.

iii) If the semiconductor is N-type, then electrons will be the charge carriers and these electrons will accumulate on surface –1 making that surface –vely charged with respect to surface –2. Hence a potential called Hall voltage appears between the surfaces 1 and 2.

iv) Similarly when surface –1 is positively charged with respect to surface –2, then the semiconductor is of P-type. In this way, by seeing the polarity of Hall voltage we can determine whether the semiconductor is of P-type or N-type.

Applications of Hall effect

Hall effect is used to determine,

• carrier concentration, conductivity and mobility.
• The sign of the current carrying charge.
• Charge density.
• It is used as magnetic field meter.

Carrier lifetime (τ)

In a pure semiconductor, we know that number of holes are equal to the number of electrons. Thermal agitation however, continues to produce new hole electron pairs while other hole-electron pair disappear as a result of recombination.

On an average, a hole will exist for τp second and an electron will exist for τn second before recombination. This time is called the carrier lifetime or Mean lifetime.

The average time an electron or hole can exist in the free state is called carrier lifetime.