2_1_2_1_class 9 slidestemp - EE 2 Fall 2007 Class 9 slides...

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1 EE 2 Fall 2007 Class 9 slides
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2 Outline 1. Review of last class 2. Extrinsic semiconductors 3. Donor and acceptor impurities 4. Majority and minority carries in extrinsic semiconductors 5. Thermal equilibrium carrier densities 6. Law of mass action 7. Compensated semiconductor
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3 Extrinsic semiconductors Extrinsic semiconductors are those in which controlled (and trace) amount of specific impurities are incorporated in the semiconductor lattice to increase the electron density or the hole density. Depending on the type of impurity the electron or the hole density will be increased by the number of impurities added. Whereas in an intrinsic semiconductor the electrons and holes arise due to thermal excitation of electrons from the valence band to the conduction band, in an extrinsic semiconductor electrons (or holes) will be excited to the conduction band (or to the valence band) from the impurity atoms with energy levels in the band gap close to E c for increasing the electron density (or E v for increasing the hole density). The material is called extrinsic since its properties depend on the externally added impurities.
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4 Extrinsic semiconductors The impurities are said to be substitutionally added since the impurity atoms substitute the original atoms in the material. For example in silicon an impurity atom will sit in the site previously occupied by a silicon atom. If you recall the discussion in an earlier class the semiconductors crystallize in diamond structure because in such a structure each atom say silicon is surrounded by four nearest neighboring silicon atoms The s 2 p 2 electron configuration in silicon gives rise to the covalent bond whereby each atom shares an electron with the four neighbors forming the diamond structure.
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6 Extrinsic semiconductors
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7 Extrinsic semiconductors
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8 Extrinsic semiconductors Elements of group V of the periodic table have five valence electrons. When one of these impurity atoms is substituted for silicon in the crystal lattice, four of the five electrons complete the four covalent bonds and the fifth electron is not participating in the covalent bond and is weakly bound to the site of the impurity atom.
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