Movement impurity is caused by combination of substitutional and interstitials diffusion. Their motion is influenced by concentration and charge state of lattice defects.

Impurities which move interstitially in silicon are lithium, potassium, sodium and gases like argon, helium and hydrogen. They occupy interstitial sites and are electronically inactive.

When diffusivity is plotted against temperature value, apparent activation energy (Ed) can be obtained. Its value for Arsenic and Antimony is about 4 eV which is expected for substitutional diffusers in silicon.

On the other hand corresponding value of phosphorus and all p-type impurities are significantly lower (3 – 5 eV) indicating presence of, Interstitialcy diffusion component.

Details of individual impurities are listed below:

1. Arsenic in Silicon –

   Arsenic have low misfit factor and can be incorporated t a high concentration without causing strain in lattice. It has low diffusivity and abrupt doping profile which makes it suitable for shallow diffused structures.

2. Boron in Silicon –

   Boron diffusions are often made with low surface concentration so that constant diffusivity can be used to characterize them. For low surface concentration diffusion, field enhancement factor is usually neglected. Boron profile is abrupt and degree of abruptness increases with higher surface concentration.

3. Phosphorus in Silicon –

   Phosphorus has high solubility in silicon and used for $n^+$ region in MOS and bipolar structure. Its misfit factor is low and capable of achieving active carrier concentration of $3 \times 10^{20} cm{-3}$