Diffusion

Diffusion is a process by which controlled amount of impurity is introduced into semiconductor. Impurity atoms are introduced onto the surface of a crystal wafer and diffuse into the lattice because of their tendency to move from regions of high to low concentration. Diffusion of impurity atoms into crystal takes place only high temperatures. There are mainly two types of physical mechanisms by which the impurities can diffuse into the lattice.

1. 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. This mechanism is known as Substitutional diffusion.

2. Interstitial Diffusion

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

Flick's Law of Diffusion

The diffusion rate of impurities into semiconductor lattice depends on the following :-

•  Mechanism of diffusion
• Temperature
• Physical properties of impurity
• The properties of the lattice environment
• The concentration gradient of impurities
• The geometry of the parent semiconductor

The rate of transfer of solute atoms per unit area of the diffusion flux density (atoms/cm².sec)

N is the concentration of solute atoms

x is the direction of solute flow.

t is the diffusion time, and

D is the diffusion constant (cm²/sec)