Tag: semiconductors

Questions Related to semiconductors

The highest energy band which is filled at zero Kelvin is called ________.

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

  1. conduction band

  2. valence band

  3. insulation band

  4. filled band

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Correct Option: B
Explanation:

At zero Kelvin, all the electrons are present in valence band while conduction band is empty. So valence band is the highest energy band which is filled at zero kelvin.

If the energy gap of a semiconductor is 1.1 e V it would be:

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

  1. Transparent to the ultraviolet radiation

  2. Opaque to the visible light

  3. Transparent to the visible light

  4. None of these

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Correct Option: B
Explanation:

Visible light lies in the range of about 2.0 eV to 3.2 eV on the electromagnetic spectrum. 

This energy is sufficient to excite the valence electrons in the semiconductor and is hence absorbed by it. 
As a result, photons of lower energy are emitted which do not fall in the visible range of light. 
This causes the opacity of the semiconductor to visible light.

Which of the following statement(s) is/are correct: 

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

  1. The gap between the top of the valance band and the bottom of the conduction band is called energy band gap

  2. More band gap results in less electron transfer

  3. A semiconductor is a material with a small but non-zero band gap that behaves as an insulator at absolute zero

  4. All of the above

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Correct Option: D
Explanation:
In a material the gap between top of the valance band and bottom of the conduction band is called energy gap as the energy equal to this gap is required for the valance electrons to jump into conduction band.

When the band gap is high, it means it is difficult for the valance electrons to jump into conduction band i.e. higher energy is required for the electrons to enter into conduction band. This results in less electrons transfer.

A semiconductor is a material with small band gap which can fulfilled by some special treatment. At absolute zero temperature, the valance electrons don't have energy to cross this energy gap therefore semiconductors behave as insulators.

Fill in the blank.
Energy band gap size for insulators is in the range ________ eV. 

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

  1. $1-2$

  2. $2-3$

  3. $3-4$

  4. $>4$

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Correct Option: C

In the bandgap between valence band and conduction band in a material is $5.0eV$, then the material is

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

  1. semiconductor

  2. good conductor

  3. superconductor

  4. insulator

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Correct Option: D
Explanation:

For an insulator, the band gap i.e. the energy difference between valence and conduction band is high i.e. $5-6$ eV.

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

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

  1. $0.5$

  2. $0.9$

  3. $0.7$

  4. $1.1$

Show answer
Correct Option: A
Explanation:

We know that $E=\dfrac{hc}{\lambda}$

$\Rightarrow E=\dfrac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{2480 \times 10^{-9}}=0.5 ev$
Therefore the band gap for the semiconductor is $0.5ev$
So the correct option is $A$

State whether given statement is True or False
The probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature decreases exponentially with increasing bandgap.

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 probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature decreases exponentially with increasing bandgap because it is more difficult for the electrons to jump to the conduction band from valence band if the band gap between them is large.

Therefore the given statement is TRUE.


A transformer has 500 turns in its primary and 1000 turns in its secondary winding.The primary voltage is 200 V and the load in the secondary is 100 ohm.Calculate the current in the primary,assuming it to be a ideal transformer.
physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

  1. 25 A

  2. 45 A

  3. 8A

  4. 22 A

Show answer
Correct Option: C
Explanation:

In usual rotation we have -

$\Rightarrow \dfrac{V _s}{V _p}=\dfrac{N _s}{N _p}$
$\therefore V _S=V _p\times \dfrac{N _s}{N _p}$
          $=200v\times \dfrac{1000}{500}$
          $=400V$
There is load resistance of $100\Omega$ in the secondary circuit. Therefore current in the secondary 
$\Rightarrow i _s=\dfrac{V _s}{R _s}=\dfrac{400V}{100\Omega}=4A$
In an ideal transformer the output power and input power are equal that is 
$\Rightarrow V _s\times i _s=V _p\times i _p$
$\therefore i _p=i _s \times \dfrac{V _s}{V _p}=4A\times \dfrac{400V}{200V}$
                            $=4A\times 2$
                            $=8A$
Hence, the answer is $8A.$