On the ambipolar diffusion of impurities into silicon

It is frequently proposed that at large impurity atom densities, diffusion into silicon is an ambipolar diffusion process involving mobile holes, electrons, and ionized impurity atoms. According to this proposal the density gradient attained during diffusion produces an electric field, and this field influences impurity atom transport within the semiconductor lattice. A qualitative discussion is presented here on the likelihood of an impurity atom gradient giving rise to an electric field, particularly at the temperatures used for device fabrication. It is suggested that an exceedingly small electrostatic shielding distance (Debye length) existent at diffusion temperatures places important limitations upon the electric field produced during the diffusion process.