抄録
The well known formula of isotropic diffusion coefficient Dv of electrons does not give exact values of lateral diffusion coefficient DT under an electric field. Dv implies the ensemble average of the instantaneous probability of diffusion, while DT is obtained as the time average of diffusion flux. The cause of the discrepancy between Dv and DT is the change of the collision frequcncy within respective flights of electrons due to the electric field. Problems concerning Dv and DT are made clear from relevant quantities calculated by the analytical method and by the Monte-Carlo simulation for various model cross sections.