Semiconductor Band DiagramAtomic organization

Silicon atom

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Covalent linkageEK LevelsL LevelsM Levels(Valence)Conduction levels

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EK BandL BandM Band (Valence)Conduction Band

Band Gap

Band diagram

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Crystalline structure (Crystal cell)Polycrystalline structureAmorphous Silicon

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Free Charge Carriers

EValence BandConduction BandSi

Band Gap (1.12 eV)Energy (E> Band Gap)

Charge carrier generation

electronhole Electron is a free charge carrier Electron has a negative charge

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EValence BandConduction BandSi

Band Gap (1.12 eV)

Holes as charge carriers

electronhole

Electron movementHole movement Hole is a free charge carrier Hole has a positive charge

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EValence BandConduction BandSi

Band Gap (1.12 eV)

Charge carrier recombination

electronhole

Energy

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SummaryBoth electrons (negative charge) and holes (positive charge) are free charge carriers

There is an equilibrium between electrons-hole pairs generated (G) and recombined (R).

This equilibrium between R and G depens on the energy applied to the system (mainly due to temperature)

So, electron concentration (n) = hole concentration (p) = ni (T)

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Free Charge Carriers Movement: Currents

Free charge carriers (Electrons or holes)

Diffusion currentElectrical charge movement => Electric currentCharge carriers movementIf charge carriers are electronsDiffusion currentIf charge carriers are holesDiffusion current

Drift current

ElectronsHoles Electric Field Drift currentDrift current

n = 1350 cm2/V sp = 480 cm2/V sq = 1.6 10-19 Cni = 1.5 1010 cm-3

s = 4.39 10-6 S/cm= 4.39 10-4 S/m

Materials (S/m)

Silver6,30 107Gold5,96 107Copper4,10 107Aluminium3,50 107PET10-21TEFLON10-25 - 10-23

Summary

Since both charge carriers have an electrical charge, they can move when applying an electric field, causing a Drift Current (Ia)

If the concentration of a charge carrier type is high in a determined semiconductor region, the repulsing forces between equal charges produces the diffusion of the charge carriers, causing a Diffusion Current (Id)

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Doping Semiconductors

Doping semiconductors

Si

Intrinsic semiconductorWe have as many holes as electronsn=pnp=ni 2

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Doping semiconductors

Si

P

N- Type semiconductorWe are generating free electrons without generating holes (n>p)+

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Doping semiconductors

Si

Si

BP- Type semiconductorWe are generating holes without generating electrons (n

Resistivity of n-type (red curve) and p-type (blue curve) silicon versus doping density

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