The diffusion coefficient (D) determines rate with which element moves in a given solid by diffusion. D depends strongly on temperature and varies between elements by orders of magnitude. For example in the case of diffusion in silicon diffusion coefficient for gold (Au), is in the range of 10-3 cm2 /sec (fast diffusant) while for Antimony (Sb) is in the range of 10-17 cm2 /sec. Since D is a function of temperature, the flux (F) is also a function of temperature. Negative sign indicates that the flow is down the concentration gradient.
Fick's second law is used in non-steady or continually changing state diffusion, i.e., when the concentration within the diffusion volume changes with respect to time. Because diffusion in solids is slow, diffusion is almost always transient.
This can be derived from the First Fick's law and the mass balance assuming the diffusion coefficient D to be a constant. This equation state that the dopant will redistribute until it is spread uniformly throughout the material. The time scale of this redistribution is given by the coefficient of diffusion.
