Question:
The relaxation time τ is nearly independent of applied E field whereas it changes significantly with temperature T. First fact is responsible for Ohm’s law whereas the second fact leads to a variation of ρ with temperature. Elaborate why?
Solution:
Relaxation time is the time interval between two successive collisions of the electrons.
It is defined as
τ = mean free path/rms velocity of electrons
Usually, the drift velocity of the electrons is small because there is a frequent collision between the electrons.
ρ = 1/σ = m/ne2τ