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SEMICONDUCTORS
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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
M V V K Srinivas Prasad
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INTRINIC SEMICONDUCTO
RS
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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Hole Movement in Silicon
M V V K Srinivas Prasad
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Hole Movement in SiliconThis hole can also move…
M V V K Srinivas Prasad
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Heating Silicon
M V V K Srinivas Prasad
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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…
M V V K Srinivas Prasad
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Intrinsic Conduction
M V V K Srinivas Prasad
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Intrinsic Conduction
M V V K Srinivas Prasad
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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
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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:
hei epen
# 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
M V V K Srinivas Prasad
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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.
heihe enepen
μh is ~20% of μe
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EXTRINIC SEMICONDUCTO
RS
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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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
M V V K Srinivas Prasad
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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).
edehe eNenepen
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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
M V V K Srinivas Prasad
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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.
hahhe eNepepen
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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)
nee peh
• 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
he epen
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Variation of carrier concentration with temperature in intrinsic semiconductors
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Variation of carrier concentration with temperature in extrinsic semiconductors
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