Physics - Electronic Devices

Question - 1

The correct relation for the cohesive energy is 

  • A cohersive energy = free energy of atoms - crystal energy
  • B cohersive energy = free energy of atoms + crystal energy
  • C cohersive energy = \(\frac { free\quad energy\quad of\quad atoms }{ crystal\quad energy } \)
  • D cohersive energy = free energy of atoms x crystal energy

Question - 2

In an unbaised p-n junction, holes diffuse from the p-region to n-region because

  • A free electrons in the n-region attract them
  • B they move across the junction by the potential difference
  • C hole concentration in p-region is more as compared to n-region
  • D all the above

Question - 3

A spectrum of silicon is to be made p-type semiconductor. For this one atom of indium, on an average, is doped in 5 x \({ 10 }^{ 7 }\)silicon atoms. If the number density of silicon is 5 x \({ 10 }^{ 28 }\)atoms/\({ m }^{ 2 }\), then the number of acceptor atoms per \({ cm }^{ 3 }\)will be 

  • A 2.5 x\({ 10 }^{ 30 }\)
  • B 1.0 x \({ 10 }^{ 13 }\)
  • C 1.0 x \({ 10 }^{ 15 }\)
  • D 2.5 x \({ 10 }^{ 36 }\)

Question - 4

In the ration of the concentration of electrons and of holes in a semiconductor is 7/5 and the ration of currents is 7/4, then what isthe ration oftheir drift velocities?

  • A 4/7
  • B 5/8
  • C 4/5
  • D 5/4

Question - 5

The conductivity of a semiconductor increases with increase in temperature because

  • A number density of free current carriers increase
  • B relaxation time increases
  • C both number density of carriers and relaxation time increase
  • D number density of current carriers increases, relaxation time decreases but effect of decerese in relaxation time is much less than increase in number density

Question - 6

The ratio of electron and hole currents in a semiconductor is \(\frac { 5 }{ 4 } \) and the ration of drift velocities of electrons and holes is \(\frac { 7 }{ 4 } \), then the ration of concentrations of electrons and holes will be 

  • A \(\frac { 25 }{ 49 } \)
  • B \(\frac { 49 }{ 25 } \)
  • C \(\frac { 7 }{ 5 } \)
  • D \(\frac { 5 }{ 7 } \)

Question - 7

The resistivity of an n-type extrinsic semiconductor is 0.25 \(\Omega \)-m. If the electron mobility is 8.25 \({ m }^{ 2 }\)/v-s, then the concentration of donor atoms will be (in \({ m }^{ -3 }\))

  • A 3.0 x \(10^{ 16 }\)
  • B 3.0 x \(10^{ 17 }\)
  • C 3.0 x \(10^{ 18 }\)
  • D 3.0 x \(10^{ 19 }\)

Question - 8

Assume that the number of hole- electron pairs in am intrinsic semiconductor is proportional to \(e^{ -\triangle E/2kT }\). Here , \(\triangle E\) = energy. gap and k = 8.62 x \({ 10 }^{ -5 }\)eV/K. The ration of electron-hole pairs at 300 k and 400 k, is 

  • A \({ e }^{ -5.31 }\)
  • B \({ e }^{ +5 }\)
  • C e
  • D \({ e }^{ +3 }\)

Question - 9

THe contribution in the total current flowing through a semiconductor due to electrons and holes are \(\frac { 3 }{ 4 } \) and \(\frac { 1 }{ 4 } \). If the drit velocity of the electron is \(\frac { 5 }{ 2 } \) times that of holes at this temperature, then the ratio of concentration of electrons and holes is 

  • A 6 : 5
  • B 5 : 6
  • C 3 : 2
  • D 2 : 3

Question - 10

The number of densities of electrons and holes in pure silicon at \({ 27 }^{ 0 }\)C are equal and its value is 1.5 x \({ 10 }^{ 16 }\)\({ m }^{ -3 }\). On doping with indium, the hole density increase to 4.5 x \({ 10 }^{ 27 }\)\({ m }^{ -3 }\). The electron density in doped silicon will be 

  • A 50 x \({ 10 }^{ 9 }\)\({ m }^{ -3 }\)
  • B 5 x \({ 10 }^{ 9 }\)\({ m }^{ -3 }\)
  • C \({ 10 }^{ 8 }\)\({ m }^{ -3 }\)
  • D \({ 10 }^{ 7 }\)\({ m }^{ -3 }\)
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