extrinsic semiconductors
DESCRIPTION
Extrinsic Semiconductors. ECE 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 - PowerPoint PPT PresentationTRANSCRIPT
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.)