It is the process of introduction of dopant atoms into the semiconductor. It alters the type of conductivity of the semiconductor.

Diffusion is the movement of impurity atoms in a semiconductor material at high temperatures.

Diffusion is applied to anneal the crystal defects after ion implantation or to introduce dopant atoms into silicon from a chemical vapor source. In the last case the diffusion time and temperature determine the depth of dopant penetration.

Diffusion is used to form the source, drain, and channel regions in a MOS transistor. But diffusion can also be an unwanted parasitic effect, because it takes place during all high temperature process steps.

Methods – Diffusion from a

1. Chemical source
2. Doped oxide source
3. Diffusion and annealing from an ion implanted layer

Advantages:

1. No crystal damage like its alternatives.
2. Batch processing (handling many wafers in single process) is possible.
3. High quality junction with minimum leakage current is possible.

Disadvantages:

1. High temperature method
2. It cannot independently control the dopant concentration and junction depth.