Methods of p-n junction fabrication

• Diffusion

Dopant diffuses in under heating so that the surface acceptor concentration exceeds the donor concentration.
A junction appears when N _d =N _a

• Ion implantation

Starting material (n-type) is bombarded with the required species of ions (acceptors).
This produces sharper junctions, but causes damage to the crystal lattice structure increasing the number of dislocations and interstitial atoms.

• Epitaxial deposition

Starting material is a single crystal - it is possible to grow further crystal layers which are in register with the starting crystal.

Ions of the semiconductor together with dopants are fired at the crystal surface.

Under the right conditions (ultrahigh vacuum, correct ion fluxes, correct substrate temperature) the crystal grows epitaxially with the required dopant included.

This technique can produce very sharp junctions.

Related Reading

Gregorian, R., Temes G. C., Analog MOS integrated circuits for signal processing, Wiley, 1986.

Johns, D. A., Martin, K., Analog integrated circuit design, Wiley, 1997.

Razavi, B., RF Microelectronics, Prentice Hall, 1998.