DIFFUSION Impurities are introduced into selected regions of

DIFFUSION Impurities are introduced into selected regions of a semiconductor for the purpose of altering its electronic properties

MECHANISMS l l SUBSTITUTIONAL DIFFUSION INTERSTITIALCY DIFFUSION SUBSTITUTIONAL-INTERSTITIAL

SUBSTITUTIONAL • Impurity jumps from one lattice site to another thus substituting for the original host atom • Vacancies must be present • Occurs at a much slower rate than interstitial diffusion • V=Vo e-Es/k. T e –En /k. T And D= Do e –Ed /k. T where Ed=Es+En. • Es- Energy of formation of schottky defects. • En – Potential barrier associated with impurity migration • Vo- frequency of lattice vibration

INTERSTITIAL DIFFUSION Na & Lithium l l l Jumps via interstitial voids. Process is relatively fast. V= Vo e –Em/k. T and D=Do e –Em/k. T Em –Activation energy of impurity migration V- the jump frequency is the frequency with which thermal energy fluctuations occur.

INTERSTITIALCY DIFFUSION Boron & Phosphorus l l Self interstitials push the substitutionally located impurity into interstitial sites These impurities diffuse to adjacent substitutional sites. V=Vo e-Es`/k. T e –En /k. T And D= Do e –Ed /k. T where Ed=Es`+En. Es` is 1 e. V lower than the energy of formation of schottky defect.

SUBSTITUTIONAL-INTERSTITIAL (Cu & Ni) l l A rapidly moving interstitial diffuser can move into a substitutional site by displacing an lattice atom –Kick out Substitutional impurity atom can become an interstitial leaving behind a vacancy – Dissociative

Advantages & Disadvantages l l l Ideally adapted to Batch processes where many slices are handled in a single operation. Does not produce crystal damage High quality junctions with minimum leakage current can be made Control of impurity concentration is at best 510% at high concentrations and becomes worse at low concentrations High temperatures are required.