# Electronic Science (Test 4)

## Ugc Net Entrance Exams : Science

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Q.1
Hole is a vacancy or lack of an electron and can travel through the semiconductor material. It can
A. only serves as a vacant spot and cannot conduct current
B. move only if an electron stops
C. Responsible for current carrier in semicondustor
D. None of these
Explaination / Solution:

Holes and electrons are the two types of charge carriers responsible for current in semiconductor materials. Holes in a metal or semiconductor crystal lattice can move through the lattice as electrons can, and act similarly to positively-charged particles. They play an important role in the operation of semiconductor devices such as transistors, diodes and integrated circuits. However they are not actually particles, but rather quasiparticles; they are different from the positron, which is the antiparticle of the electron.

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Q.2
The electron and hole concentration in a semiconductor in thermal equilibrium is given by
A. nenh=2n2i
B. nenhn2i
C. 2nenh=n2i
D. nenh=n3i
Explaination / Solution:

The number of carriers in the conduction and valence band with no externally applied bias is called the equilibrium carrier concentration.

For majority carriers, the equilibrium carrier concentration is equal to the intrinsic carrier concentration plus the number of free carriers added by doping the semiconductor. Under most conditions, the doping of the semiconductor is several orders of magnitude greater than the intrinsic carrier concentration, such that the number of majority carriers is approximately equal to the doping.

At equilibrium, the product of the majority and minority carrier concentration is a constant, and this is mathematically expressed by the Law of Mass Action.

Where ni is the intrinsic carrier concentration and neand ph are the electron and hole equilibrium carrier concentrations.

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Q.3
To obtain a p-type germanium semiconductor, it must be doped with
A. Antimony
B. Arsenic
C. Indium
D. Phosphorus
Explaination / Solution:

p-type germanium semiconductor is formed when it is doped with a trivalent impurity atom. So it should be doped with indium.

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Q.4
Majority current carriers in N – types are
A. negative ions
B. holes
C. positive ions
D. electrons
Explaination / Solution:

The more abundant charge carriers are called majority carriers, which are primarily responsible for current transport in a piece of semiconductor. In n-type semiconductors they are electrons, while in p-type semiconductors they are holes.

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Q.5
To produce N – type crystal Ge or Si may be doped with a substance that is
A. tetravalent
B. pentavalent
C. divalent
D. trivalent
Explaination / Solution:

If a pentavalent element from the Vth Group such as Antimony (Sb) is added to pure geranium (Ge) or silicone (Si), then four electrons of (Sb) will form covalent bonds with four (Ge) or (Si) atoms. The fifth electron of 'Sb' is free to move which makes (Ge) or (Si) a good conductor. This type of material is called n-type substance.

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Q.6
Which of the following statements about magnetic field lines true?
A. The smaller the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B.
B. The perpendicular to the field line at a given point gives direction of magnetic field
C. The larger the number of field lines crossing per unit area, the weaker is the magnitude of the magnetic field B.
D. The magnetic field lines of a magnet (or a solenoid) form continuous closed loops
Explaination / Solution:
No Explaination.

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Q.7
if   =magnetic intensity,  =susceptibility, magnetic moment per unit volume  equals

A. 2χH⃗
B.  χH⃗
C. χ2H⃗
D. πχH⃗
Explaination / Solution:
No Explaination.

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Q.8
If you made a map of magnetic field lines at Melbourne in Australia, would the or of the ground?
A. lines seem to come out at 45 degrees to the ground
B. lines seem to be horizontal to the ground
C. lines seem to come out of the ground
D. lines seem to go into the ground
Explaination / Solution:
No Explaination.

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Q.9
permeability of a paramagnetic material is expected to decrease with increasing temperature because
A. random thermal motion increases magnetic moment alignment
B. random thermal motion reduces magnetic moment alignment
C. electrons go into forbidden gap
D. electrons stop moving
Explaination / Solution:
No Explaination.

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Q.10
A bar magnet of magnetic moment 1.5 J/T lies aligned with the direction of a uniform magnetic field of 0.22 T. What is the amount of work required by an external torque to turn the magnet so as to align its magnetic moment normal to the field direction?
A. 0.33J
B. 0.23J
C. 0.38J
D. 0.43J