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Extrinsic SemiconductorsECE 2204
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Definitions•Intrinsic
▫Pure▫There are an equal number of electrons
and holes•Extrinsic
▫Contains impurities (donors, acceptors, and traps)
▫The material properties are dominated by the concentration of the impurities and their effect on the crystal Usually, n ≠ p
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Back to the Periodic Table
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Column V Atoms
Have 5 outer shell electrons
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The extra electron on the phosphorous atom is easily removed and becomes a free electron without generating a hole.
The phosphorous atom becomes positively charged (ionized).
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Back to the Periodic Table (again)
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Column III Atoms
Have 3 outer shell electrons
Missing outer shell electron when compared to Si
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The gallium atom ‘steals’ an electron from a neighboring Si atom to complete the four bonds with the surrounding Si atoms, generating a hole at the neighboring Si atom. The boron atom becomes negatively charged (ionized).
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n-type Semiconductors
•Are doped with donor atoms, which have an extra electron that they donate to the crystal▫When the concentration of donor atoms is
much greater than the intrinsic carrier concentration, the electron concentration is composed of these donated electrons.
DNn
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p-type Semiconductors
•Are doped with acceptor atoms, which generate holes in the crystal▫When the concentration of acceptor atoms is
much greater than the intrinsic carrier concentration, the hole concentration is composed of the holes generated by the acceptors.
ANp
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Minority Carrier Concentrations
A
i
A
i
N
nn
Np
pnn
2
2
n-type semiconductor
p-type semiconductor
D
i
D
i
N
np
Nn
pnn
2
2
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Energy Diagrams
EF
EF
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Resistivity
Dn
D
ipDn
pn
Nq
Nn
Nq
pnq
1
1
1
2
Ap
ApA
in
pn
Nq
NN
nq
pnq
1
1
1
2
n-type semiconductor
p-type semiconductor
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Temperature Effects
•Unfortunately, the simplification of the equation for resistivity gets rid of its temperature dependence of the resistivity. Why? ▫(Always blame the intrinsic carrier
concentration for temperature effects – it is usually the cause.)