Tag: electron energies in solids

Questions Related to electron energies in solids

Bands in solids are formed due to a group of closely spaced ________ .

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

  1. conductor bands

  2. valance bands

  3. energy levels

  4. solid bands

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Correct Option: C
Explanation:

Closely space energy levels combine to form an energy band in solids. This is because, the outer orbit of an atom in solids, are common to several neighboring atoms. Therefore, energy levels corresponding to outer orbit electrons spread up to form a band of energy called energy band.

The energy gap in glass at room temperature is :

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

  1. greater than that in a semiconductor

  2. less than that in a good conductor

  3. greater than that in a good conductor

  4. both (A) and (C) are true

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Correct Option: D
Explanation:

Glass is an insulator. The energy gap of an insulator is $\sim 6 eV$. Whereas for conductors, the energy gap is $\sim 0 eV$.
for semiconductors, energy gap $\sim 3 eV$.
So energy gap for glass is greater in conductors or a semiconductor.

A pure germanium crystal at absolute zero is :

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

  1. an insulator

  2. a good conductor

  3. a semiconductor

  4. none of the above

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Correct Option: A
Explanation:

A pure Ge crystal at absolute zero has electrons only in the valence band. No electron in the conduction band is there. So, it acts as an insulator at 0 Kelvin.

A pure semiconductor at absolute zero has :

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

  1. absence of electrons in the conduction band

  2. all the electrons occupying the valence band

  3. large ${E} _{g}$ value

  4. all of the above

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Correct Option: D

The band structure determines the _________ behaviour of a solid.

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

  1. chemical

  2. electrical

  3. mechanical

  4. molecular

Show answer
Correct Option: B
Explanation:

The band structure, i.e. valence band, conduction band and forbidden energy band (Eg) tells on the basis of the energy difference between valence and conduction band, that whether the given solid is a metal, insulator or a semiconductor. If the two bands overlap, then the solid is a conductor, i.e, it has high electrical conductivity. If Eg $\sim 6$ eV; then it is an insulator and has minimum electrical conductivity otherwise if Eg $\sim 3$ eV, it is a semiconductor whose electrical conductivity lies between conductor and insulator.

In insulators (CB is conduction band and VB is valence band)

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

  1. VB is partially filled with electrons

  2. CB is partially filled with electrons

  3. CB is empty and VB is filled with electrons

  4. CB is filled with electrons and VB is empty

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Correct Option: C
Explanation:

In insulators conduction band is empty and valence band is filled with electrons.

The energy gap in a semiconductor 

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

  1. Increases with temperature

  2. Does not change with temperature

  3. Decreases with temperature

  4. Is zero

Show answer
Correct Option: C
Explanation:

The energy bandgap of semiconductors tends to decrease as the temperature is increased.  the interatomic spacing increases when the amplitude of the atomic vibrations increases due to the increased thermal energy. This effect is quantified by the linear expansion coefficient of a material. An increased interatomic spacing decreases the potential seen by the electrons in the material, which in turn reduces the size of the energy bandgap. A direct modulation of the interatomic distance, such as by applying high compressive (tensile) stress, also causes an increase (decrease) of the bandgap.

The variance of energy gap with temperature is given by
$E _g(T)=E _g(0)-\dfrac{\alpha T^2}{T+\beta}$

When the band gap for a semiconductor is low 

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

  1. conductivity of that material is low

  2. conductivity of that material is high

  3. the resistance of that material is high

  4. none of the above

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Correct Option: B
Explanation:

When the band gap for a semiconductor is low, it means it is easy for the valance electrons to jump into conduction band i.e. less energy is required for the electrons to enter into conduction band. Hence, the resistance of the material is low and conductivity is high.

Which of the following has least band gap energy at $273K$.

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

  1. InSb

  2. InAs

  3. InP

  4. GaSb

Show answer
Correct Option: A
Explanation:

The energy gaps of the given semiconductors at $273K$ are given:

   $InSb=0.16eV$ (Indium antimonide),
   $InAs=0.33eV$ (Indium arsenide),
   $InP=1.29eV$ (Indium phosphide),
  $GaSb=0.67eV$ (Galiumium antimonide),
It is clear that $InSb$ has the least energy gap.

What is the optimum band gap energy for a material to be considered as a semiconductor?

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

  1. greater than $6eV$

  2. less than $6eV$

  3. $0eV$

  4. $0.5-3eV$

Show answer
Correct Option: D
Explanation:

Semiconductor materials have low but finite band gap energy, due to which there is an easy jump of electron from valence band to conduction band upon provision of external thermal energy.

The typical range of band gap energy for a semiconductor material is $0.5eV-3eV$.