1. SEMICONDUCTORS

2. 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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3. INTRINIC SEMICONDUCTORS
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4. The Silicon, Si, Atom
Silicon 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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5. Silicon – the crystal lattice
If 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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6. Electron Movement in Silicon
At 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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7. Hole Movement in Silicon
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8. Hole Movement in Silicon
This hole can also move…
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9. Heating Silicon
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10. Intrinsic Conduction Take a piece of silicon…
And apply a potential difference across it…
This sets up an electric field throughout the silicon – seen here as dashed lines
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11. Intrinsic Conduction
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12. Intrinsic Conduction
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13. 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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14. 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

15. Intrinsic Semiconductors at T >0 K
Electrons and holes contribute to the current when a voltage is applied
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16. 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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18. Electron and hole conductivity
• In a semiconductor, there can be electrons and holes:
• Total Electrical Conductivity thus given by:
# electrons/m 3
electron mobility
# holes/m
hole mobility
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19. 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.
μh is ~20% of μe
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21. EXTRINIC SEMICONDUCTORS
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22. 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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23. Doping Pentavalent Group VA elements Trivalent Group III A elements
Phosphorous
Arsenic
Antimony
Trivalent
Group III A elements
Boron
Gallium
Indium
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24. The Phosphorus Atom Phosphorus 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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25. Doping – Making n-type Silicon
We now have an electron that is not bonded – it is thus free for conduction
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26. Doping – Making n-type Silicon
As more electrons are available for conduction we have increased the conductivity of the material
Phosphorus is called the dopant
If we now apply a potential difference across the silicon…
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27. Extrinsic Conduction – n-type Silicon
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31. This crystal has been doped with a pentavalent impurity.
The free electrons in n type silicon support the flow of current.
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34. 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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35. The Boron Atom Boron 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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36. Doping – Making p-type Silicon
Notice we have a hole in a bond – this hole is thus free for conduction
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37. Doping – Making p-type Silicon
Boron is the dopant in this case
If we now apply a potential difference across the silicon…
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38. Extrinsic Conduction – p-type silicon
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42. 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 direction
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45. 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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46. Ef=Edonor= Ec-0.05eV
Ef=Eacceptor= Ev+0.05eV
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47. • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n)
• 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)
• N-type Extrinsic: (n >> p)
• P-type Extrinsic: (p >> n)
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48. Variation of carrier concentration with temperature in intrinsic semiconductors

49. Variation of carrier concentration with temperature in extrinsic semiconductors