12th Class Physics Semiconducting Devices Question Bank

done C. B. (MCQs) - Semiconductor Electronics: Materials, Devices and Simple Circuits Total Questions - 10

• question_answer1)

  Directions (Q. No. 1 to 5)

  Read the following text and answer the following questions on the basis of the same:

  Band theory of solid:

  Consider that the Si or Ge crystal contains N atoms.

  Electrons of each atom will have discrete energies in different orbits. The electron energy will be same if all the atoms are isolated, i.e., separated from each other by a large distance. However, in a crystal, the atoms are close to each other (\[2\overset{o}{\mathop{A}}\,\,\,to\,\,3\overset{o}{\mathop{A}}\,\]) and therefore the electrons interact with each other and also with the neighbouring atomic cores. The overlap (or interaction) will be more felt by the electrons in the outermost orbit while the inner orbit or core electron energies may remain unaffected. Therefore, for understanding electron energies in Si or Ge crystal, we need to consider the changes in the energies of the electrons in the outermost orbit only. For Si, the outermost orbit is the third orbit (n = 3), while for Ge it is the fourth orbit (n = 4).

  The number of electrons in the outermost orbit is 4 (2s and 2p electrons). Hence, the total number of outer electrons in the crystal is 4N. The maximum possible number of outer electrons in the orbit is 8 (2s + 6p electrons). So, out of the 4N electrons, 2N electrons are in the 2N s-states (orbital quantum number \[l\]= 0) and 2N electrons are in the available 6N p-states. Obviously, some p-electron states are empty. This is the case of well separated or isolated atoms.

  The energy of electrons of atoms of a substance will be same if:
  A)
  atoms are isolated. done clear
  B)
  atoms are closely spaced. done clear
  C)
  atoms are excited. done clear
  D)
  atoms are charged. done clear
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• question_answer2)
  In a crystal, the distance between two atoms is:
  A)
  \[200\overset{o}{\mathop{A}}\,\,\,to\,\,300\overset{o}{\mathop{A}}\,\] done clear
  B)
  \[2\overset{o}{\mathop{A}}\,\,\,to\,\,3\] micron done clear
  C)
  \[2\overset{o}{\mathop{A}}\,\,\,to\,\,3\overset{o}{\mathop{A}}\,\] done clear
  D)
  2 mm to 3 mm done clear
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• question_answer3)
  The overlap (or interaction) will be more felt by the electrons when they are:
  A)
  in the outermost orbit. done clear
  B)
  in the innermost orbit. done clear
  C)
  free. done clear
  D)
  in any orbit. done clear
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• question_answer4)
  For Silicon and Germanium the outermost orbits are respectively:
  A)
  n=3 and n=5 done clear
  B)
  n = 4 and n =3 done clear
  C)
  n=5 and n=4 done clear
  D)
  n=3 and n=4 done clear
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• question_answer5)
  The maximum possible electrons in an orbit is:
  A)
  8 (2s + 6p electrons) done clear
  B)
  8 (6s + 2p electrons) done clear
  C)
  8 (4s + 4p electrons) done clear
  D)
  8 (1s + 7p electrons) done clear
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• question_answer6)

  Directions (Q. No. 6 to 10)

  Read the following text and answer the following questions on the basis of the same:

  Light Emitting Diode:

  It is a heavily doped p-n junction which under forward bias emits spontaneous radiation. The diode is encapsulated with a transparent cover so that emitted light can come out. When the diode is forward biased, electrons are sent from n \[\to \] p (where they are minority carriers) and holes are sent from p\[\to \]n (where they are minority carriers).

  At the junction boundary, the concentration of minority carriers increases as compared to the equilibrium concentration (i.e., when there is no bias).

  Thus at the junction boundary on either side of the junction, excess minority carriers are there which recombine with majority carriers near the junction. On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted.

  When the forward current of the diode is small, the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum. Further increase in the forward current results in decrease of light intensity. LED's are biased such that the light emitting efficiency is maximum.

  The V-I characteristics of a LED is similar to that of a Si junction diode. But, the threshold voltages are much higher and slightly different for each colour.

  The reverse breakdown voltages of LED's are very low, typically around 5 V. So care should be taken that high reverse voltages do not appear across them. LED's that can emit red, yellow, orange, green and blue light are commercially available.

  LED is a:
  A)
  lightly doped p-n junction diode. done clear
  B)
  heavily doped p-n junction diode. done clear
  C)
  moderately doped p-n junction diode. done clear
  D)
  two back to back p-n junction diode. done clear
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• question_answer7)
  LED emits light:
  A)
  when reversed biased. done clear
  B)
  when forward biased. done clear
  C)
  when forward or reverse biased. done clear
  D)
  when heated. done clear
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• question_answer8)
  During recombination at the junction, emitted photons have:
  A)
  energy greater than the band gap. done clear
  B)
  energy equal to or slightly less than the band gap. done clear
  C)
  energy which has no relation with the band gap. done clear
  D)
  very low energy compared to band gap.   done clear
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• question_answer9)
  Threshold voltage of LED is:
  A)
  lower compared to other p-n junction diodes and slightly different for each colour. done clear
  B)
  higher compared to other p-n junction diodes and slightly different for each colour. done clear
  C)
  higher compared to other p-n junction diodes and same for all colours. done clear
  D)
  lower compared to other p-n junction diodes and same for all colours. done clear
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• question_answer10)
  The reverse breakdown voltages of LED's are:
  A)
  very low and typically around 0.5 V. done clear
  B)
  very low and typically around 5 V. done clear
  C)
  very high and typically around 50 V. done clear
  D)
  very low and typically around 0.05 V. done clear
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