Fick's  law    In a crystal, if there is a concentration variation of impurity atoms  (or vacancies), the impurity atoms will diffuse from high concentration  region  to low concentration region. According to Fick's  law  the net flux of impurity atoms of one species is proportional to the concentration gradient of this species,

Jn,x = - D(dn /dx)

Here   Jn,x  is  the  number  of  atoms  crossing  unit  area  held perpendicular to x axis per unit time. D is a constant known as diffusion constant  or diffusivity (m ² /s). The concentration gradient  exists  in the  x direction. The minus sign means that diffusion occurs opposite  the direction of  concentration gradient.