Tag: semiconductors

Questions Related to semiconductors

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

Show answer
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.

The forbidden gap for a pure silicon at the room temperature is _______eV.

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

  1. Less than one

  2. 1.1

  3. 3

  4. 9

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Correct Option: B
Explanation:

The forbidden gap for a pure Si at room temperature is 1.1 eV.

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:

At absolute zero, a semiconductor is an insulator.

State whether True or False :

A semiconductor at absolute zero become a insulator.

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:

The conductivity and non-conductivity of material are dependent on the motion of electrons. If the motion is happening it is conductor.

Now, at absolute zero, the temperature is so low that it stops the motion of the electrons. Thus making it a complete insulator.
$\Rightarrow$  Statement (A) is the correct answer.

What is forbidden energy gap(Eg) or band gap?

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

  1. Forbidden energy gap ${ E } _{ g }$ is the difference of energy levels of conduction band and within conduction band

  2. Forbidden energy gap ${ E } _{ g }$ is the difference of energy levels of conduction band and valence band.

  3. when there is no energy

  4. conduction band

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Correct Option: B
Explanation:

Forbidden energy gap ${ E } _{ g }$ is the difference of energy levels of conduction band and valence band.

Give band gap Eg value for conductor, insulator and semiconductor?

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

  1. Conductor$<1MeV$, insulation $\ge 6eV$, semiconductor $\approx 8eV$.

  2. Conductor$<1MeV$, insulation $\ge 6eV$, semiconductor $\approx 1eV$.

  3. Conductor$<2MeV$, insulation $\ge 6eV$, semiconductor $\approx 1eV$.

  4. Conductor$<1MeV$, insulation $\ge 7eV$, semiconductor $\approx 1eV$.

Show answer
Correct Option: B
Explanation:

Conductor$<1MeV$, insulation $\ge 6eV$, semiconductor $\approx 1eV$.