This set of Basic Electronics Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Junction Diode – Semiconductor Theory and Energy Band Structure”.
1. Semiconductors can conduct current when ______
a) temperature is reduced
b) temperature is increased
c) kept under high pressure
d) kept on a room temperature
View Answer
Explanation: When the temperature is increased, after a certain point, the electrons move
from valence band to conduction band. The excited electrons relax by emitting heat. The energy level of electrons is enough to migrate freely from bond and hence they can move freely and conduct electricity. If the temperature is reduced, the semiconductors acts more as an insulator than a conductor. At absolute temperature i.e. 0 Kelvin, it loses all its conductivity.
2. N-type of extrinsic semiconductor is formed by adding a _______ impurity.
a) trivalent
b) hexavalent
c) bivalent
d) pentavalent
View Answer
Explanation: N-type of extrinsic semiconductors is formed by pentavalent impurity. Pentavalent impurities have five electrons in the last shell, which allows them to form bonds with semiconductors, such as, Phosphorous, Antimony, Arsenic, etc. P-Type semiconductors have trivalent impurity such as Boron. Hexavalent and bivalent impurities are used in any case as it would increase the mass.
3. A special type of level is seen in the energy bands of solids, that level is called as ______
a) isolation level
b) resistant level
c) fermi level
d) threshold level
View Answer
Explanation: Fermi Level is the point indicating energy level of electrons at absolute zero temperature. It is only found in conductors and semiconductors energy bands. Fermi function of every material is different and it depends on physical conditions plus the valence electrons. It depends on the valence of the metal or semiconductors. Fermi level plays an important role in fermi velocity and conduction of electricity.
4. The Fermi Level is close to the Valence band in ___________
a) Intrinsic Semiconductor
b) P-Type Semiconductor
c) N-Type Semiconductor
d) All Metals
View Answer
Explanation: Due to addition of trivalent impurity, the energy level of holes is increased and so the fermi level is shifted down near to the valence band. The majority carriers in N-Type are the holes and are more valent in nature. N-Type Semiconductors have pentavalent impurity and have the fermi level near to the conduction band.
5. For a semiconductor with 1.1 eV at temperature 100K the conduction electron population will be _________ electrons/m3.
a) 1.12×10-3
b) 1.50×10-3
c) 2.12×10-3
d) 0.9232×10-3
View Answer
Explanation: Ncb = AT-3/2 e-Egap/2KT where,
A = 4.83×1021 which is the electron density.
Egap = 1.1 eV, 100K = -173.149°C, K is the Boltzmann Constant.
6. Impurities which generate holes and are trivalent are generally known as _______
a) acceptors
b) generators
c) receptors
d) moderators
View Answer
Explanation: The impurities which are trivalent, their addition causes the semiconductor to create the holes in it. Such impurities are Boron, Aluminium and Gallium. Receptors have majority carriers as electrons and hence they conduct easily. As it goes by the name Acceptors, they accept electrons from the trivalent impurities used.
7. In LED, the relaxation of excited electrons is done by _______ than generation of heat.
a) applying electric field
b) electric energy
c) emitting light
d) applying magnetic field
View Answer
Explanation: Light emitting diodes consists of impurity added semiconductors such that they emit light when the electrons relax while a current is applied. For example Gallium Nitride Silicon which emits blue light. Applying electric field would cause gating and hence excite electrons rather than relaxing the electrons. This would give enough energy to electrons to leave their orbit shell and move freely.
8. If two different types of extrinsic semiconductors are joined together then it will be form a ___________
a) homojunction
b) heterojunction
c) junction
d) fusion
View Answer
Explanation: When a heterojunction is formed, it results in exchange of holes from p-doped and exchange of electrons from n-doped parts. The N-Type have excess of electrons which are required by the acceptors i.e. P-Type part, whereas the P-Type have excess of holes. This transfer continues until equilibrium is reached. Homojunction is formed when same types of semiconductors are used.
9. _______ is an alternate way instead of doping and is effective for conduction.
a) Coiling
b) Bombarding
c) Gating
d) Relucting
View Answer
Explanation: Gating is the process in which we apply an electric field across the semiconductor and excited the electrons for conduction of electricity. Bombarding is process of emitting electrons which releases energy. Relucting is a chemical process in which hydrogen compounds are made of elements using various methods.
10. Materials which have wider band gaps than usual one’s they are often referred as _________
a) semiconductors
b) metals
c) conductors
d) semi insulators
View Answer
Explanation: Semi-insulators have a wider band gap between the valence band and conduction band. Wider the gap, the greater is the resistivity. In such type of materials there is covalent bond which makes the material a polynomial or a long chain complex compound. Their conductivity is same as that of electrical insulators. They are semiconductors which are not doped for example, Gallium Arsenide.
Sanfoundry Global Education & Learning Series – Basic Electronics Engineering.
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