The process by which atoms move in the crystal lattice is called diffusion. The movement of an impurity in a lattice takes place in a series of random jumps in all three dimensions and a flux of diffusing species results if there is a concentration gradient.

The different methods by which these jumps take place are discussed below:

1. Interstitial Diffusion:

An interstitial atom moves through the crystal lattice jumping from one interstitial site to the next adjacent site. This may also occur by dissociative mechanism.

In such, diffusion type, the impurity atom does not replace the silicon atom, but instead moves into the interstitial voids in the lattice.

The main types of impurities diffusing by such mechanism are Gold, copper, and nickel.

Because of the large size of such metal atoms, they do not usually substitute in the silicon lattice. Interstitial diffusion is generally faster and there are many more interstitial sites than vacancy sites to jump to.

2. Substitutional Diffusion:

At high temperature many atoms in the semiconductor move out of their lattice site, leaving vacancies into which impurity atoms can move.

The impurities, thus, diffuse by this type of vacancy motion and occupy lattice position in the crystal after it is cooled.

Thus, substitutional diffusion takes place by replacing the silicon atoms of parent crystal by impurity atom.

In other words, impurity atoms diffuse by moving from a lattice site to a neighboring one by substituting for a silicon atom which has vacated a usually occupied site as shown in the figure below.

Substitutional diffusion mechanism is applicable to the most common diffusants, such as boron, phosphorus, and arsenic.

These dopants atoms are too big to fit into the interstices or voids, so the only way they can enter the silicon crystal is to substitute for a Si atom.

3. Interstitial – Substitutional Diffusion:

In this impurity atom occupies substitutional as well as interstitial site. They only move at significant rate in interstitial site.

A substitutional impurity atom can become interstitial leaving behind vacancy, by dissociative mechanism.

Alternative for interstitial diffusion is the kick out mechanism. Here a rapidly moving interstitial diffuser can move into a substitutional site by displacing an atom which is already in place, resulting in interstitial substitutional diffusion.

4. Interstitialcy Diffusion:

It is a modified version of substitutional diffusion.

Interstitial host atoms can be wiped out by pushing substitutionally located impurity atom into interstitial site.

These impurities can diffuse to adjacent substitutional sites and create new self-interstitial.

Interstitialcy diffusion is somewhat faster than substitutional diffusion since there is an increase in concentration of vacant substitutional site over its equilibrium value.

For example diffusion of boron and phosphorus in silicon.

5. Interchange Diffusion:

Two or more atoms diffuse by an interchange process to cause interchange diffusion.

This is a direct process of interchange when two atoms, impurity and host are involved and is a cooperative interchange when larger numbers of host atoms are involved.

6. Gain boundary Diffusion:

Diffusion occurs by moment along dislocation and grain boundaries.

7. Combination effects:

Combination of these mechanisms can occur. For example, a certain fraction of impurity atoms may diffuse substitutionally and the rest interstitially.