While there is considerable scatter in the experimental data, for noncompensated material (no counter doping) for heavily doped substrates (i.e. 10 18 c m − − 3 and up), the mobility in silicon is often characterized by the empirical relationship : μ μ = μ μ o + μ μ 1 1 + ( N N ref ) α α where N is the doping concentration (either N D or N A), and N ref and α are fitting parameters. At room temperature, the above equation becomes: