SEMICONDUCTORS

Semiconductors

Semiconductors have a resistivity/resistance between that of conductors and insulators

Their electrons are not free to move but a little energy will free them for conduction

Their resistance decreases with increase in temperature

The two most common semiconductors are silicon and germanium

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INTRINIC SEMICONDUCTO

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The Silicon, Si, AtomSilicon has a valency of 4 i.e. 4 electrons in its outer shell

Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms

These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms

This picture shows the shared electrons

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Silicon – the crystal latticeIf we extend this arrangement throughout a piece of silicon…

We have the crystal lattice of silicon

This is how silicon looks when it is 0K

It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator

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Electron Movement in SiliconAt room temperature

An electron may gain enough energy to break free of its bond…

It is then available for conduction and is free to travel throughout the material

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Hole Movement in Silicon

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Hole Movement in SiliconThis hole can also move…

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Heating Silicon

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Intrinsic ConductionTake a piece of silicon…

This sets up an electric field throughout the silicon – seen here as dashed lines

And apply a potential difference across it…

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Intrinsic Conduction

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Intrinsic Conduction

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Intrinsic Semiconductors• Consider nominally pure

semiconductor at T = 0 K• There is no electrons in

the conduction band

• At T > 0 K a small fraction of electrons is thermally excited into the conduction band, “leaving” the same number of holes in the valence band

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• This hole is positive, and so can attract nearby electrons which then move out of their bond etc.

• Thus, as electrons move in one direction, holes effectively move in the other direction

Electron moves to fill hole

As electron moves in one direction hole effectively moves in other

Intrinsic Semiconductors at T >0 K

• Electrons and holes contribute to the current when a voltage is applied

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Carrier Concentrations at T >0 K

• The number of electrons equals the number of holes, ne = nh

• The Fermi level lies in the middle of the band gap• ne = nh increase rapidly with temperature

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• Total Electrical Conductivity thus given by:

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# electrons/m3 electron mobility

# holes/m3

hole mobility

• In a semiconductor, there can be electrons and holes:

+ -

electron hole pair creation

+ -

no applied electric field

applied electric field

valence electron Si atom

applied electric field

electron hole pair migration

Electron and hole conductivity

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Intrinsic carriers• With intrinsic systems (only), for every free

electron, there is also a free hole.

# electrons = n = # holes = p = ni

--true for pure Si, or Ge, etc.

• Holes don’t move as easily (mobility of holes is always less than for electrons), but still there are so many that they will contribute at least an extra 10-20% to the intrinsic conductivity.

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μh is ~20% of μe

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EXTRINIC SEMICONDUCTO

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• Prepared by adding (doping) impurities to intrinic semiconductors

• Doping is the incorporation of [substitutional] impurities (trivalent or pentavalent) into a semiconductor according to our requirements• In other words, impurities are introduced in a controlled manner

• Electrical Properties of Semiconductors can be altered drastically by adding minute amounts of suitable impurities to the pure crystals

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Doping

• Pentavalent • Group VA elements

– Phosphorous– Arsenic– Antimony

• Trivalent • Group III A elements

– Boron – Gallium – Indium

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The Phosphorus AtomPhosphorus is number 15 in the periodic table

It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell

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Doping – Making n-type Silicon

We now have an electron that is not bonded – it is thus free for conduction

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Doping – Making n-type Silicon

As more electrons are available for conduction we have increased the conductivity of the material

If we now apply a potential difference across the silicon…

Phosphorus is called the dopant

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Extrinsic Conduction – n-type Silicon

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The free electrons in n type silicon support the flow of current.

This crystal has been doped with a pentavalent impurity.

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Donor electrons• Unlike for intrinsic semiconductors, free electron doesn’t

leave a mobile free hole behind. Instead, any holes are trapped in donor state and thus will not contribute substantially to conductivity as for intrinsic semiconductors (thus p~0).

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The Boron AtomBoron is number 5 in the periodic table

It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell

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Doping – Making p-type Silicon

Notice we have a hole in a bond – this hole is thus free for conduction

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Doping – Making p-type Silicon

If we now apply a potential difference across the silicon…

Boron is the dopant in this case

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Extrinsic Conduction – p-type silicon

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This crystal has been doped with a trivalent impurity.

The holes in p type silicon contribute to the current.

Note that the hole current direction is opposite to electron current so the electrical current is in the same directionM V V K Srinivas Prasad

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Extrinsic conductivity—p type• Every acceptor generates excess mobile holes

(p=Na).• Now holes totally outnumber electrons, so

conductivity equation switches to p domination.

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Ef=Edonor= Ec-0.05eV

Ef=Eacceptor= Ev+0.05eV

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• Intrinsic: # electrons = # holes (n = p) --case for pure Si

• Extrinsic: --n ≠ p --occurs when DOPANTS are added with a different # valence electrons than the host (e.g., Si atoms)

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• N-type Extrinsic: (n >> p)• P-type Extrinsic: (p >> n)

no applied electric field

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom

no applied electric field

Boron atom

valence electron

Si atom

conduction electron

hole

3+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

hei en

310*, inpn

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Variation of carrier concentration with temperature in intrinsic semiconductors

Variation of carrier concentration with temperature in extrinsic semiconductors