extrinsic semiconductors

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.)