Extrinsic SemiconductorsECE 2204
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
Back to the Periodic Table
Column V AtomsHave 5 outer shell electrons
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).
Back to the Periodic Table (again)
Column III AtomsHave 3 outer shell electrons
Missing outer shell electron when compared to Si
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).
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
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
Minority Carrier Concentrations
A
i
A
i
Nnn
Nppnn
2
2
n-type semiconductor
p-type semiconductor
D
i
D
i
Nnp
Nnpnn
2
2
Energy Diagrams
EF
EF
Resistivity
Dn
D
ipDn
pn
Nq
NnNq
pnq
1
1
1
2
Ap
ApA
in
pn
Nq
NNnq
pnq
1
1
1
2
n-type semiconductor
p-type semiconductor
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.)
Recommended
