V dp(r)=V th??(r)(U E r???U?? E r)U?? E r) wherein Q is a charge of carrier, k is Boltzmann's constant, T is temperature, ??p is carrier momentum relaxation time, Vth is carrier thermal velocity, Er is electric field vector at the position vector r, ??Er is electric field vector at the position vector r in which one of said linearly changing quasi-Fermi potential profiles is considered, UEr is uni