Define mobility of a charge carrier. Write the relation expressing mobility in terms of relaxation time. Give its SI unit.

Explanation

Mobility of electrons (or other charge carriers) in a conductor means drift speed of conduction electrons per unit electric field.
Mobility `mu = (v_d)/(E )`
We know that drift speed of electrons `v_d = (eE)/(m) tau`, where `tau` = relaxation time
`therefore mu = ( (eE tau)/(m) )/(E ) = e/m tau`
SI unit of mobility is `m^2 V^(-1) s^(-1)`