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Pure sodium (Na) is a good conductor of ...

Pure sodium `(Na)` is a good conductor of electricity because the `3s` and `3p` atomic bands overlap to from a partially filled conduction band. By contrast the ionic sodium chloride `(NaCl)` crystal is

A

insulator

B

conductor

C

semiconductor

D

None of these

Text Solution

Verified by Experts

The correct Answer is:
A
In NaCl, the `Na^(+) and Cl^(-)` ions both have noble gas electronic configuration corresponding to completely filled bands. Since, the bands do not overlap, there must be a gap between the filled bands and the empty bands above them. Thus, NaCl is an insulator.
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In a ideal crystal there nust be regular repeating arrangement of the constuting particles and its entropy must be zero at absolute zero at absolute zero temperature. However, it is impossible to obtain an ideal crystal and it suffers from certain defects called imperfections. In pure crystal these defects arise either due to disorder or dislocation of the movement of the particles even at absolute zero temperature. Such defect increases with rise in temperature. In addition ti this, certain defects arise due to the pressure of some impurities. Such defects not only modify the existing properties of the crystalline solid but also impart certain new characteritics to them. In pure crystal, e.g, silicon or germanium at 0K, the electrons are prsent in fully occupied lowest energy states and are not xpected to conduct any electricity. However at temperature above 0K, some electron leave their bonds and become free to move in the crystal lattice, giving rise to and become free to move in the crystal lattice, giving rise to electrical conductivity. The electron deficient bonds, called holes (+vely charged) and thermally mobile electrons move in opposite direction under the electric field. Stoichiometric ppoint defects include (a) Schottky defects, which arise due to missing of both cations and anions from their lattice sites without disturbing the stoichiometry and (b) Frenked defects, which arise due to misplacement of certian ions in the crystal lattice. The former defect gives rise to no change of density. Another type of defects are non-stoichometry defects where the cetions and anion are not present in the stoichiometry ratio. In metal excess defect, metal ions or positive ions are in excess as compared to anions of non-metals stoichiometrycally. On the other hand in metal deficiency defect, the cations are in lesser proportion than stoichiometric value. Since the crystal is neutral electrically, the balance of charge is maintained by free electrons or extra positive charges. The metal excess defects gives rise to conduction of electricity due to the presence of free electrons. Also crystals having metal excess defects are paramagnetic and coloured due to the presence of electrons in the anion vacancies. Impurity defects arise when some foreign atoms are present at the lattice sites in place of the host atoms or at the vacant interstitial sites. When 15 group elements like P or are doped into Si or Ge, the added impurity atoms occupy the lattice sites forming four covalent bonds with 4 Si/Ge atoms leaving an extra electron free to move. Such a crystal is said to be n-type semi conductor because the conduction of electricity is due to movement of extra unbounded electrons. If doping of a covalent crystal of 14 group elements are caused by addition of small amounts of elements are caused by addition of small amounts of elements of group 13, e.g, Al or Ga with three valence electrons, one covalent bond formed will be electron deficient and acts as a positive hole. The presence of such holes in the crystal leads to electrical conductivity and the the crystal is said to be p-type semiconductor. The type of semiconduction shown by crystal capable of showing Schottky defect, will be :

MHTCET PREVIOUS YEAR PAPERS AND PRACTICE PAPERS-SEMICONDUCTORS-Exercise 1 ( MISCELLANEOUS PROBLEMS )
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  4. The combination of gates shown below yields

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  5. The input resistance of a silicon transistor is 100Omega base current ...

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  6. Pure sodium (Na) is a good conductor of electricity because the 3s and...

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  7. In a common base amplifier circuit, calculate the change in base curre...

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  8. In the figure, potential difference between A and B is

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  9. The circuit shown in the figure contains two diodes each with a forwar...

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  10. The diode used in the circuit shown in the figure has a constant volta...

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  11. In a common emitter amplifier, using output reisistance of 5000 ohm an...

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  12. The length of a germanium rod is 0.58 cm and its area of cross-section...

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  13. If a Zener diode (V(Z)=5 and I(Z)=10 mA) is connected in series with a...

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  14. A red LED emits light of 0.1 watt uniformaly around it. The amplitude ...

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  15. The circuit has two oppositively connected ideal diodes in parallel wh...

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  16. A 2V battery is connected across AB as shown in the figure. The value ...

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  17. The value of the resistor, R(S), needed in the dc voltage regulator ci...

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  18. An LED is constructed from a p-n junction based on a certain semi-cond...

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  19. In a triode, gm=2xx10^(-3)ohm^(-1) , mu=42, resistance load, R = 50 ki...

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  20. The input characteristics of a transistor in CE mode is the graph obta...

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