Electronic Science (Test 3)

Ugc Net Entrance Exams : Science

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Electronic Science
| Electric Circuits | | Electrical and Electronic Measurements | | Power Electronics | | Electronic Devices | | Analog Circuits | | Digital Circuits | | Digital Logic | | Electrostatic Potential and Capacitance | | Current Electricity | | Moving Charges and Magnetism | | Magnetism and Matter | | Electronic Devices |
Q.1
In the case of semiconductors the valence and conduction bands have
A. overlap, energy gap = 0
B. no overlap, energy gap = 0
C. no overlap, energy gap is small
D. no overlap, energy gap is large
Answer : Option C
Explaination / Solution:

The materials can be classified by the energy gap between their valence band and the conduction band. The valence band is the band consisting of the valence electron, and the conduction band remains empty. Conduction takes place when an electron jumps from valence band to conduction band and the gap between these two bands is forbidden energy gap. Wider the gap between the valence and conduction bands, higher the energy it requires for shifting an electron from valence band to the conduction band.

• In the case of conductors, this energy gap is absent or in other words conduction band, and valence band overlaps each other. Thus, electron requires minimum energy to jump from valence band. The typical examples of conductors are Silver, Copper, and Aluminium.
• In insulators, this gap is vast. Therefore, it requires a significant amount of energy to shift an electron from valence to conduction band. Thus, insulators are poor conductors of electricity. Mica and Ceramic are the well-known examples of insulation material.
• Semiconductors, on the other hand, have an energy gap which is in between that of conductors and insulators. This gap is typically more or less 1 eV, and thus, one electron requires energy more than conductors but less than insulators for shifting valence band to conduction band.

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Q.2
in p-type semiconductor
A. nh=ne
B. nh>>ne
C. nh
D. nh>ne
Answer : Option B
Explaination / Solution:

P-type semiconductors have a larger hole concentration than electron concentration. The term P-type refers to the positive charge of the hole. In P-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities (or doping an n-type semiconductor). A common P-type dopant for silicon is boron. For P-type semiconductors the Fermi level is below the intrinsic Fermi level and lies closer to the valence band than the conduction band.

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Q.3
A small impurity is added to germanium to get p-type semiconductor. This impurity is a
A. Monovalent substance
B. Pentavalent substance
C. Trivalent substance
D. Bivalent substance
Answer : Option C
Explaination / Solution:

In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbors. Each ionic core, consisting of the nucleus and non-valent electrons, has a net charge of +4, and is surrounded by 4 valence electrons. Since there are no excess electrons or holes In this case, the number of electrons and holes present at any given time will always be equal. Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole balance will be changed. This impurity will only be able to contribute three valence electrons to the lattice, therefore leaving one excess hole. Since holes will "accept" free electrons, a Group 3 impurity is also called an acceptor.

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Q.4

A P– type crystal is obtained by doping

A) silicon with arsenic,

B) silicon with aluminum,

C) Ge with boron,

D) Ge with phosphorus.

A. B and C are correct
B. B and D are correct
C. A and D are correct
D. A and C are correct
Answer : Option A
Explaination / Solution:

Doping means the introduction of impurities into a semiconductor crystal to the defined modification of conductivity. In P-Type Semiconductor, The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes".

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Q.5
The number of terminals in a transistor is/are
A. Three
B. one
C. four
D. two
Answer : Option A
Explaination / Solution:

A transistor is an electronic component that can be used as an amplifier, or as a switch. It is made of semiconductor material. Transistors are found in most electronic devices.

A transistor has three connectors or terminals. In the older bipolar transistor they are the collector, the emitter, and the base. The flow of charge goes in the collector, and out of the emitter, depending on the charge flowing to the base

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Q.6
When iron filings are sprinkled on a sheet of glass placed over a short bar magnet then,
A. The iron filings form a pattern suggesting that the magnet has two poles similar to the positive and negative charge of an electric dipole
B. The iron filings form a random pattern of suggesting that the magnet has random pole
C. The iron filings form a pattern of suggesting that the magnet has north pole similar to the positive charge
D. The iron filings form a pattern of suggesting that the magnet has south pole similar to the negative charge
Answer : Option D
Explaination / Solution:

North pole act as positive end and South pole act as negative end. The iron fillings in the presence of magnetic field gets magnetized and form a tiny magnet which then gets attracted to the poles of bar magnet. North pole of bar magnet attracts south pole of tiny magnet and vice-versa. So all the iron fillings are arranged as the magnetic field lines of bar magnet.

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Q.7
magnetic intensity is defined by

A.
B⃗ μ30M⃗
B. B⃗ μ
0
M⃗
C. 2B⃗ μ0M⃗
D. B⃗ μ20M⃗
Answer : Option B
Explaination / Solution:
No Explaination.

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Q.8
The angle of dip at a location in southern India is about  . Dip angle in Britain would be

A. 180
B.
smaller

C. greater
D. 0.0
Answer : Option C
Explaination / Solution:
No Explaination.

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Q.9
A cube-shaped permanent magnet is made of a feromagnetic matrerial with a magnetization M of about The side length is 2 cm. Magnetic field due to the magnet at a point 10 cm from the magnet along its axis is
A. 0.002 T
B. 0.001 T
C. 0.003 T
D. 0.004 T
Answer : Option B
Explaination / Solution:
No Explaination.

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Q.10
A closely wound solenoid of 800 turns and area of cross section carries a current of 3.0 A.It is free to turn about the vertical direction and a uniform horizontal magnetic field of 0.25 T is applied.Magnitude of torque on the solenoid when its axis makes an angle of with the direction of applied field
A. 0.075J
B. 0.085J
C. 0.080J
D. 0.070J
Answer : Option A
Explaination / Solution:
No Explaination.

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