Tag: electron energies in solids

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

Choose the correct statement(s):

  1. Band gap of a semiconductor increases as temperature increases.

  2. Band gap of a semiconductor decreases as the temperature increases.

  3. Band gap of a semiconductor is independent of temperature.

  4. Bandgap of a semiconductor decreases till critical temperature and increases after that. 
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The energy bandgap of semiconductors tends to decrease as the temperature is increased. This behaviour can be understood if one considers that 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. 

This is the reason behind the decrease in resistance of semiconductor materials by increasing temperature.

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

Which among  the following having highest band gap ?

  1. Diamond

  2. Silicon

  3. Germanium

  4. Gallium nitride
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The energy gaps of the given materials are given:

   Diamond $=6eV$  (insulator)
   Silicon $=1.1eVeV$  (semiconductor)
   Germanium $=0.6eV$  (semiconductor)
   Gallium nitride $=3.4eV$  (semiconductor)

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

What is the energy band gap of silicon and germanium respectively in $eV$  ?

  1. $1.1, 0.7$

  2. $0.7, 1.1$

  3. $-0.7, -1.1$

  4. $-1.1, -0.7$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A band gap, also called an energy band, is an energy range in a solid where no electron states can exist. It generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. The energy band gaps of silicon and germanium are $1.1eV $ and $0.7eV$ respectively.

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

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

  1. If the band gap becomes much more higher for a semiconductor then electrons from lower energy state can't move to higher energy state

  2. Lesser the band gap lesser is the conduction

  3. Lesser the band gap higher is the conduction

  4. none of the above
Reveal answer Fill a bubble to check yourself
A,C Correct answer
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 i.e. lesser the band gap higher is the conduction.

When the band gap becomes much higher for a semiconductor then electrons can't move to higher energy state from lower energy state.

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

At what temperature semiconductors behaves as an ideal insulator?

  1. 273.15 $^\circ C$

  2. -459.67 $^\circ\ F$

  3. -273 $^\circ\ F$

  4. Room temperature
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Semi-conductors behaves as an ideal insulator at absolute zero temperature($0K$) which is 0 Kelvin. Because at the absolute zero temperature the electrons in the valence band of semi-conductors do not posses enough thermal energy to overcome forbidden energy gap. so semi-conductors stop conducting and behaves as an insulator.

But  $0K=-459.67F$   
Hence correct answer is   $-459.67F$.

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

What is an energy gap?

  1. the space between two orbital shells

  2. the energy equal to the energy acquired by an electron passing a $1 V$ electric field

  3. the energy band in which electrons can move freely

  4. an energy level at which an electron can exist
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
  1. Orbiting electrons contains energy and are confirmed to definite energy levels.
    2. The various shells in an atom represent these levels.
    3. Therefore, to move an electron from the lower shell to a higher shell a certain amount of energy is required.
    3. Below the conduction band is the forbidden band or energy gap, electrons are never found in this band, but may travel back and forth through it, provided they do not come to rest in the band.
    4. As the electrons can also lose energy as well as receive it when an electron loses energy it moves to a lower shell.
    5. And supplying more energy than is needed will only cause the electron to move to the next higher shell.
    6. It means that an energy gap is the spacing between two orbital shells.
Multiple choice physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

 Fermi energy level for $p-type$ extrinsic semiconductors lies 

  1. At middle of the band gap

  2. Close to conduction band

  3. Close to valence band

  4. None of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In case of a p-type semiconductor, the number of holes in valence band is grater then number of electrons in conduction band. hence, the probability of occupation of energy levels by the holes in valence band is greater than probability of occupation of energy levels by electrons in conduction band. This probability of occupation of energy levels is represented in terms of Fermi level.

$\therefore $ female level in p-type semiconductor lies does to valence band.

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

Energy gap of conductor is

  1. $0 \ eV$

  2. $1 \ eV$

  3. $2 \ eV$

  4. $3 \ eV$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The range of energy of the valence electrons of an atom is known as valence band. The range of energy in which an electron must exist in order to participate in the conduction of electricity is known as conduction band. The difference between the valence band and conduction band is known as band gap or energy gap. In conductors, the valence band overlaps with the conduction band. Which means, electrons are already ready for conduction and energy gap in a conductor is zero.

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

Fermi energy level for intrinsic semiconductors lies 

  1. At middle of the band gap

  2. Close to valence band

  3. Close to conduction band

  4. None of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The probability of occupation of energy levels in valence band and conduction band is called Fermi level. As the temperature increases free electrons and holes gets generated. In intrinsic semiconductor, the number of holes in valence band is equal to the number of electrons in the conduction band. Hence, the probability of occupation of energy levels in conduction band and valence band are equal. Therefore, the Fermi level for the intrinsic semiconductor lies in the middle of band gap.

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

Energy gap of semiconductor is approx

  1. $1 eV$

  2. $0 \ eV$

  3. $6-7 eV$

  4. $> 8\ eV$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The energy gap of a semiconductor lies in between insulators and conductors. In case of conductors, band gap Eg $\sim 0$ eV. Whereas in case of insulators, Eg $\sim 3-4$ eV. So, Band gap in case of a semiconductor is of order $\sim 1$ eV.