Mobility of Charged Particle

Mobility of Charged Particle

• Consider a material having large number of free electrons, subjected to an external battery.

• There exists a drift current due to drifting of free electrons with a velocity called drift velocity denoted as v, measured in m/s.

• If 'l' is the length of the material and V is the applied voltage then the material gets subjected to an electric field E given by,

E = V/l  in V/m       …….. Electric field

• The drift velocity is proportional to the magnitude of an electric field

V ∝ E

i.e. v=μE

where μ = Constant of proportionality called the mobility of the electrons.

 μ = Mobility = V/E

• The units of μ are m²/ V‒sec.

• In semiconductor, the current is due to free electrons as well as holes. Both these particles have different mobilities denoted as,

μ_n = Mobility of free electrons

μ_p = Mobility of holes

Ex. 1.4.1 A bar of intrinsic germanium 6 cm long is subjected to a potential difference of 12 V. If the velocity of electrons in bar is 73 m/s, determine the mobility of electrons.

Solution: The given values are,

 1 = 6 cm = 6×10^‒2 m,

 V = 12 V,

v = 73 m/sec

E=Electric field intensity

= V / l =  12 / [6×10^‒2] = 200 V/m

Now

 v = μ E

73 = μ. × 200

Μ = Mobility = 73/200

= 0.3650 m²/V-sec

= 3650 cm²/V-sec

Review Question

1. Define mobility of a charged particle and state its units.