Tag: band theory of solids, a brief introduction

Questions Related to band theory of solids, a brief introduction

Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy band gap respectively equal to $(E _g) _C,\, (E _g) _{Si}$ and $(E _g) _{Ge}$.Which of the following statements is true?

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. $(E _g) _{Si}\, <\, (E _g) _{Ge}\, <\, (E _g) _C$

  2. $(E _g) _{C}\, <\, (E _g) _{Ge}\, >\, (E _g) _{Si}$

  3. $(E _g) _{C}\, >\, (E _g) _{Si}\, >\, (E _g) _{Ge}$

  4. $(E _g) _{C}\, =\, (E _g) _{Si}\, =\, (E _g) _{Ge}$

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Correct Option: C
Explanation:
For given elements, the energy band gap of carbon is the maximum and of germanium is the least.
Hence option C is correct.

The band gap is larger for which of the following 

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. Conductor

  2. Insulator

  3. Semiconductor

  4. None of the above

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Correct Option: B
Explanation:

In conductors, the conduction band is partially filled or overlapped with valance band.

In semiconductors, the band gap is small.
In insulators, the band gap is largest.

The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than $2480\ nm$ is incident on it. The band gap in $(eV)$ for the semiconductor is.

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. $0.5\ eV$

  2. $0.7\ eV$

  3. $1.1\ eV$

  4. $2.5\ eV$

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Correct Option: A

The difference between the highest energy in a band and lowest energy in the next higher band is called the band gap between the two energy bands.

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. True

  2. False

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Correct Option: A

In extrinsic semiconductors :

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. the conduction band and valence band overlap

  2. the gap between conduction band and valence band is near about $16\ eV$

  3. the gap between conduction band and valence band is near about $ 1\ eV$

  4. the gap between conduction band and valence band will be $100\ eV$ and more

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Correct Option: C
Explanation:

In extrinsic semiconductors, formation of donor or acceptor level takes place in forbidden energy gap. Thus, some of the electrons in valance band acquire enough energy to jump to donor level or acceptor level present in forbidden energy gap. Thus, to obtain large number of electrons in conduction band some energy to be supplied to the electrons in valance band. For extrinsic semiconductors, it is nearly equal to $1\ eV$.

Among the following, the wrong statement in the case of semiconductor is:

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. Resistivity is in between that of a conductor and insulator.

  2. Temperature coefficient of resistance is negative.

  3. Doping increases conductivity.

  4. At absolute zero temperature, it behaves like a conductor.

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Correct Option: D
Explanation:

For semiconductor, as the temperature coefficient of resistance is negative, at absolute zero temperature, it behaves like an insulator.

At absolute zero, Si acts as

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. non-metal

  2. metal

  3. insulator

  4. semiconductor

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Correct Option: C
Explanation:

The semiconductor (Si) has negative temperature coefficient of resistivity. At absolute zero temperature, its resistance becomes infinite and it act like an insulator.

What is the resistivity of a pure semiconductor at absolute zero ?

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. Zero

  2. Infinity

  3. Same as that of conductors at room temperature

  4. Same as that of insulators at room temperature

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Correct Option: B
Explanation:

The electricity conductivity of a semiconductor at $0 K$ is zero. hence resistivity (= 1/electrical conductivity) is infinity.

Which one is the weakest type of bonding in solids ?

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. Ionic

  2. Covalent

  3. Metallic

  4. Vander Wall's

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Correct Option: D
Explanation:

Vander Wall's force or bonding is the weakest exists between the two molecules of the solids.

The intrinsic semi conductor becomes an insulator at

physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. $0^{\circ}C$ 

  2. $0 K$

  3. $300 K$

  4. $-100^{\circ}C$

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Correct Option: B
Explanation:

At $0K (-273^{\circ}C)$ motion of free electron stop i.e., there is no electron in conduction band therefore at $0K$ intrinsic semiconductor becomes insulator.