Tag: energy bands

Questions Related to energy bands

The level formed due to impurity atom, in the forbidden energy gap, very near to the valence band in a P-type semiconductor is called

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

  1. an acceptor level

  2. a donor level

  3. a conduction level

  4. none of these

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Correct Option: A
Explanation:

The level formed due to impurity atom, in the forbidden energy gap, very near to the valency band in a p-type semiconductor is called an acceptor level.

The level formed due to impurity atom, in the forbidden energy gap, very near to the conduction band in a p-type semiconductor is called a donor level.
Therefore option $A$ is correct.

At absolute zero temperature a semiconductor behaves like :

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

  1. an insulator

  2. a super conductor

  3. a good conductor

  4. a variable resistor

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Correct Option: A
Explanation:

At absolute zero, all the electrons by definition are found in valence band. Hence, at absolute zero temperature, a semiconductor behaves like an insulator.

The band gaps of a conductor, semiconductor and insulator are respectively ${Eg} _{1}, {Eg} _{2}$ and ${Eg} _{3}$. The relationship between them can be given as.

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

  1. ${ Eg } _{ 1 }={ Eg } _{ 2 }={ Eg } _{ 3 }$

  2. ${ Eg } _{ 1 }<{ Eg } _{ 2 }<{ Eg } _{ 3 }$

  3. ${ Eg } _{ 1 }>{ Eg } _{ 2 }>{ Eg } _{ 3 }$

  4. ${ Eg } _{ 1 }<{ Eg } _{ 2 }>{ Eg } _{ 3 }$

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Correct Option: B
Explanation:

Band gap of the insulator is largest as it restricts the flow of electrons through it. So, $E _{g _{3}} > E _{g _{2}} > E _{g _{1}}$.

Energy gap in case of Germanium is about

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

  1. 0.82 eV

  2. 0.12 eV

  3. 0.72 eV

  4. 0.02 eV

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Correct Option: C

The energy gap is highest in the case of

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

  1. Metal

  2. Insulator

  3. Semiconductor

  4. Diode

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Correct Option: B

At absolute zero, a semiconductor is an insulator because _________.

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

  1. No electron is present in the conduction band

  2. All electrons occupy the valence band

  3. The value of $E _G$ is large

  4. All of the above

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Correct Option: D
Explanation:

At absolute zero, the band gap in a semiconductor is of order $\sim 3eV$. The electrons in the valence band do not have sufficient energy to jump to the conduction band and hence semiconductor, behaves as an insulator at 0K.

With rise in temperature the resistance of germanium,

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

  1. increases

  2. decreases

  3. remains the same

  4. first increases then decreases

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Correct Option: B
Explanation:

germanium is a semiconductor and as we know  when a semiconductor is heated, electron-hole pair is generated in it which increases its conductivity as the number of charge carriers are increasing.
Now, as conductivity increases resistance decreases because both are opposite of each other.

The energy bands are present only when the substance is present in the solid state.

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
Explanation:

This is because in a solid, there are several atoms placed closely together. The energy levels of inner orbit electrons of an atom are not influenced by the neighboring atoms as these electrons are tightly bound to their parent nucleus.
However, energy levels of outer orbit electrons of an atom are altered as these are influenced by the neighboring atoms. Therefore, the energy levels corresponding to outer shell electrons are spread up to form a band of energy. Therefore, the closely spaced energy levels of atoms in the solid combine to form an energy band.

The energy gap $E _G$ of a semi-conductor decreases with rise in temperature.

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
Explanation:

As the temperature is increased, the thermal energy to the electron within the semiconductor material also increases. Therefore, now lower energy is required to break the bond. This reduction in Bond energy also reduces the band gap. So, the band gap of a semiconductor decreases with increasing temperature.

With rise in temperature, resistance of a semiconductor material (germanium or silicon)

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

  1. increases

  2. decreases

  3. remains the same

  4. first increases then decreases

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Correct Option: B
Explanation:

With rise in temperature, the number of charge carriers (holes and electrons) increases in the case of a semiconductor dominating the effect of decreasing relaxation time, increasing collision frequency.
So, as a whole the resistance of the material decreases with rise in temperature.