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  Usually, n ≠ p

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Column V Atoms Have 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 Atoms Have 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.

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.

Minority Carrier Concentrations n-type semiconductor p-type semiconductor

Energy Diagrams EFEF EFEF

Resistivity n-type semiconductorp-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.)