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Extrinsic Semiconductors

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

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Extrinsic Semiconductors

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  1. Extrinsic Semiconductors ECE 2204

  2. 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

  3. Back to the Periodic Table

  4. Column V Atoms Have 5 outer shell electrons

  5. 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).

  6. Back to the Periodic Table (again)

  7. Column III Atoms Have 3 outer shell electrons Missing outer shell electron when compared to Si

  8. 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).

  9. 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.

  10. 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.

  11. p-type semiconductor Minority Carrier Concentrations n-type semiconductor

  12. Energy Diagrams EF EF

  13. Resistivity n-type semiconductor p-type semiconductor

  14. 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.)

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