# Microelectronics Questions and Answers – Properties of Semiconductor Materials

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Semiconductor Materials”.

1. What is the strength of relationship between the Fermi-Dirac distribution and Temperature?
a) Strong
b) Weak
c) Moderate
d) Linear

Explanation: The Fermi-Dirac distribution is exponentially related to the temperature. Hence, it is a strong function of temperature.

2. If the effective mass of electrons and holes are similar and the equilibrium electron concentration is equal to the equilibrium hole concentration, where would the Femi-level lie?
a) Mid-way between conduction and valence band
b) Shifted towards conduction band
c) Shifted towards valence band
d) Becomes 0

Explanation: If the effective mass of electron and hole are same, we find that the thermal equilibrium electron and hole concentrations become equal when the fermi level is between conduction and valence band.

3. If the effective density of states function in the conduction band is equal to the effective density of states function in the valence band, choose the correct conclusion.
a) Fermi level is shifted towards conduction band
b) The effective masses of electrons and holes are equal
c) The electron concentration is equal to hole concentration
d) The built in barrier potential is reduced

Explanation: The effective density of states function in the conduction band and in the valence band is linearly related to the effective mass of electrons and holes respectively. If they are equal, it implies that the effective mass of the electrons and the holes are equal.

4. If a semiconductor is doped non-uniformly with donor impurity atoms, assume that the dopant atoms are reducing from left to right, what would be the alignment of the bottom of the conduction band?
a) move upwards from left to right
b) move upwards from right to left
c) move downwards from right to left
d) move downwards from left to right

Explanation: The electric field would increase from left to right to the deficiency of impurity atoms. This is because the diffusion due to non-linear concentrations would separate the electrons and the holes and this would lead to development of electric field to oppose the diffusion process.

5. How many independent current mechanisms are possible in a semiconductor?
a) 4
b) 3
c) 5
d) 2

Explanation: There are 4 independent mechanisms which generate current in a semiconductor. Two are due to diffusion of electrons and holes and the other two are drift of electrons and holes.

6. When will impact ionization occur?
a) Very high electric field
b) Low electric field
c) Very high magnetic field
d) Low inducted magnetic field

Explanation: Impact ionization if the velocity of electrons trespasses the saturated velocity due to very high electric field. A lot of collisions lead to very high current which can destroy the piece of semiconductor.

7. In radiative recombination, which energy gets radiated as an electron falls from the conduction band?
a) Heat
b) Light
c) Sound
d) Wind

Explanation: In radiative recombination, electrons in the higher level of conduction band fall to the bottom by dissipating energy as heat. Thereafter, it loses one photon of energy and falls to the top of valence band.

8. In semiconductor devices, which of the following leads to propagation delay?
a) Finite switching time of transistor
b) Capacitive effects
c) Finite switching time of transistor and Capacitive effects
d) Inductive effects

Explanation: The transistor in inverters exhibit a finite switching time to switch from one state to another. This is one of the causes of propagation delay in integrated circuits. Another cause is the charging and discharging effect of the capacitors present in the circuit. The total propagation delay is due to both of these bottlenecks.

9. In thermal equilibrium state, the concentration of electrons and holes is independent of which of the following parameter?
a) Temperature
b) Electric field
c) Potential
d) Time

Explanation: In thermal equilibrium state, the concentration of electrons and holes doesn’t get affected with time. However, a change in temperature or electric field or potential would disrupt the equilibrium state of the semiconductor and the concentration changes.

10. What does the Auger recombination process involve?
a) Heat generation in the Conduction Band
b) Heat generation in the Valence Band
c) Photon generation
d) Phonon generation

Explanation: Instead of generating photon, the energy emitted by recombination is given to another electron in the bottom of the conduction band which goes up the band. This will release energy as heat and again fall at the bottom of the conduction band.

11. If the top of the valence band and the bottom of the conduction band occur for the same momentum, which of the following event occurs?
a) Indirect recombination takes place
b) Direct recombination takes place
c) Heat is generated during recombination
d) Heat is absorbed during recombination

Explanation: Since the top of the valence band and the bottom of the conduction band occur for the same momentum, electrons won’t have to change their momentum to fall into the valence band and direct recombination takes place.

12. For an n-type semiconductor, what is the relation between intrinsic Fermi-level and the new Fermi-level?
a) Intrinsic Fermi-level > new Fermi-level
b) Intrinsic Fermi-level < new Fermi-level
c) Intrinsic Fermi-level = new Fermi-level
d) Intrinsic Fermi-level =< new Fermi-level

Explanation: If the Fermi-level shits towards conduction band, it means that the donor concentration has increased since the difference between the bottom of the conduction and the fermi level is inversely proportional to the logarithm of the donor concentration.

Sanfoundry Global Education & Learning Series – Microelectronics.

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