A factor to note is that the slope of each curve is almost identical. This implies that the individual activation energies
of the diffusion coefficients and hence the mechanism by which each impurity diffuses through the silicon are similar. In
fact, there are two principal diffusion mechanisms; (a) vacancy diffusion and (b) interstitial diffusion (see Figure 4).
Vacancy diffusion involves an impurity atom "hopping" from a lattice site to an adjacent, empty lattice site (vacancy).
In contrast, an interstitial impurity is not bonded into the lattice and in consequence is able to move with relative ease
between the host (Si) atoms.
Figure 4(a): Diagram to show vacancy diffusion in a semiconductor.
Figure 4(b): Diagram to show interstitial diffusion in a semiconductor.
The ease with which an impurity can diffuse through the lattice is reflected in the value of EA; EA
for vacancy diffusion is in the range 3 - 5 eV, while EA for interstitial diffusion is between 0.5 - 1.5 eV. Impurities
which diffuse through a semiconductor via vacancies diffuse more slowly (slow diffusant) than impurities which diffuse interstitially (fast diffusant).
