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Electronics

CHAPTER-17
Electronics
Text Book Question and Answers
I. Four alternatives are given to each of the following incomplete statement / question. Choose the right answer
1. The conductivity of a semiconductor is
(a)greater than the conductivity of metals
(b)less than the conductivity of metals
(c)equal to the conductivity of metals
(d)intermediate between the conductivity of metals and insulators
Answer: (d)intermediate between the conductivity of metals and insulators
2. The conductivity of a semi-conductor can be increased by
(a)heating the semi-conductor (b)doping the semi-conductor (c)both (a) and (b) (d)none
Answer: (c)both (a) and (b)
3. In a n-type semi-conductor which of the following statement is true.
(a)electrons are majority carrier and trivalent impurity are the dopants.
(b)electrons are minority carrier and pentavalent atoms are dopants.
(c)holes are minority carrier and pentavalent atoms are dopants.
(d)holes are majority carrier and trivalent atoms are the dopants.
Answer: (c)holes are minority carrier and pentavalent atoms are dopants.
4. In a p-type semi-conductor which of the following statement is true.
(a)electrons are majority carrier and trivalent impurity are the dopants.
(b)electrons are minority carrier and pentavalent atoms are dopants.
(c)holes are minority carrier and pentavalent atoms are dopants.
(d)holes are majority carrier and trivalent atoms are the dopants.
Answer: (d)holes are majority carrier and trivalent atoms are the dopants.
5. When a forward bias is applied to p-n junction it
(a)raises the potential barrier (b)reduces the majority carrier current to zero
(c)lowers the potential barrier (d)none of the above.
Answer: (c)lowers the potential barrier
6. For transistor action which of the following statements is correct?
(a)base, emitter and collector regions should have similar size and doping concentration
(b)the base region must be very thin and lightly doped.
(c)the emitter junction is forward biased and collector junction is reverse biased.
(d)both emitter junction and collection junction are forward biased.
Answer: (c)the emitter junction is forward biased and collector junction is reverse biased.

II. Fill in the blanks
1. In p-n-p transistor the majority carriers are holes
2. In n-p-n transistor the majority carriers are electrons
3. During reverse bias of the diode there is a small current of the order of micro ampere due to high resistance
4. In an npn transistor p type semi-conductor is sandwiched between two n type semi-conductor
5. Pentavalent impurity is called donor impurity
6. Triavalent impurity is called acceptor impurity
7. The traces of impurity added for doping are called dopants.

III. Answer the following
1. What is electronics?
Electronics is a field of both pure and applied science dealing with the design and application of electronics devices
2. What are conductors and insulators? Give examples.
Substances which allow electric current to pass through them are known as conductor
Eg: copper, silver, gold, aluminium, etc…
Substances which do not allow electric current to flow through them are known as insulator.
Eg: plastic, paper, rubber, wood, etc…
3. Mention any four electronic gadgets.
i. Cell phone
ii. Television
iii. Dvd player
iv. Music player
v. Computer etc….
4. Which are the areas influenced by electronics?
The areas influenced by electronics are industrial and military requirements.
5. What are most recent advancements in the field of electronics?
The most recent advancements in the field of electronics include digital circuits, micro processor digital computers, laser technology, Nano technology etc..
6. What are semi-conductors? Give examples
Substances whose conductivity lies in between that of conductors and insulators are called semiconductors.
Eg: Silicon and Germanium
7. How many electrons are there is the outermost shell of silicon atom?
The outermost shell of silicon atom has four electrons.
8. What is called a hole?
when an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom, can drop into this vacancy, leaving the neighbour with a vacancy. In this way, the vacancy usually called a ‘hole
9. Distinguish between extrinsic and intrinsic semi-conductor.
Intrinsic semiconductorExtrinsic semiconductor
They are pure semiconductorThey are formed when minute traces of other elements are added to intrinsic semiconductor
They have equal number of holes and electronsIn an n-type semiconductor, electrons are majority carriers and holes are majority carriers.
In a p-type semiconductor, electrons are minority carriers and holes are majority carriers.
Their conductivity is very smallThe conductivity of the semiconductor is more
Eg: Ge, Si,Eg: n-type and p-type semiconductor.
n-type semiconductorsp-type semiconductors
1. It is prepared by doping with a pentavalent impurity.1. It is prepared by doping with a trivalent impurity.
2. The impurity atoms added will donate electrons.2. The impurity atoms added will accept electrons.
3. The majority of charge carriers are electrons.3. The majority of charge carriers are holes.
diode? Indicate in figure.
Conductorssemiconductors
Conduction is very highConduction is very poor.
Resistance is lowResistance is higher than in conductor
On heating resistance increases.On heating resistance decreases

10. Explain the term ‘doping’
Doping is the process of adding minute traces of impurities to enhance the conductivity of semiconductors.
11. Distinguish between n type and p type semi-conductor.

12. Name two pentavalent impurities.
Antimony, Arsenic and Phosphorous
13. Name two trivalent impurities.
Boron, Indium, Gallium and Aluminium.
14. What is a semi-conductor diode or junction diode?
When a donor impurity is added to one side and an acceptor impurity is added to the other side of a pure semiconductor, the first side becomes n-type and the other side becomes p-type. Thus a p-njunction is formed this is called a junction diode or a semiconductor diode.
15. What is meant by forward bias and reverse bias of a

Forward bias:
When the positive terminal of a cell (source of e.m.f) is connected to p-side and the negative terminal to the n-side of a p-n junction, it is said to be forward biased
image

Reverse biased:
When the positive terminal of a cell is connected to the n side and the negative terminal to the p side of a p-n junction it is said to be reverse biased.
image

16. Give the circuit symbol of a diode.
image

17. Mention important characteristics and applications of a diode.
Characteristics:
i. Diode has ‘n’ type semiconductor nature on one side and ‘p’ type semiconductor nature on the other side.
ii. When an external source of e.m.f is connected to a p-n junction, it is said to be biased and the applied voltage is called bias voltage.
iii. A diode may be forward biased or reverse biased.
iv. A diode allows current to pass through in one direction only.
Applications:
i. Diodes are used to convert A C into D C
ii. They are used in voltage regulation systems
iii. The rectifier diodes finds application in power supplies.
18. What is transistor? Distinguish between p-n-p and n-p-n transistor.
Transistor is a specially constructed three terminal semiconductor device
19. What are functions of three regions of a transistor?
1) Base – Controls the flow of charge carriers into the collector.
2) Emitter – Emits electrons or holes into the base.
3) Collector – Collector the charge carriers from the base.
20. Mention the applications of a transistor.
Transistor is used in (i) amplifier (ii) oscillator and (iii) switching circuits.

Additional Questions and Answers
1. Distinguish between conductors and semiconductors .


2. Explain the conductivity in semiconductors
Atoms of Silicon and Germanium have four electrons in their outer most shell. These electrons form covalent bond with the neighbouring atoms and are not free at low temperature. Hence they behave like insulators.
However when a small amount of thermal energy is available from the surroundings a few covalent bonds are broken and few electrons are set free to move. Even at room temperature good numbers of electrons are dissociated from there and this number increases with rise in temperature. This leads to conductivity.
3. Name the types of semiconductor.
i. Intrinsic semiconductor
ii. Extrinsic semiconductor
4. Name the types of extrinsic semiconductor.
Depending upon the type of impurity atom doped in the intrinsic semiconductors there are two types of extrinsic semiconductors. They are classified.
i. N-type semiconductors
ii. P-type semiconductors
5. Write a note on n-type semiconductors
· When an impurity from the 5th group of elements having 5 valence electrons are added to germanium or silicon, 4 valence electrons of the impurity atom forms a covalent bond with a germanium or silicon. The fifth electron is a free electron and is responsible for increased conductivity of germanium or silicon.
· Here the number of free electrons is more than the number of holes. In other words the electrons are the majority carriers and holes are the minority carriers. This is why the doped semiconductor is called n-type semiconductor where ‘n’ stands for negative.
· The impurities are Donors.
· Some 5th group elements used for Doping: Arsenic, Antimony, Phosphorous
6. Write a note on p-type semiconductors
· When an impurity form the 3rd group of elements having 3 valence electrons are added to germanium or silicon, each impurity atom takes away on electron from the neighbouring germanium or silicon atoms to form covalent bonds. This result in the creation of holes
· Electrons from the neighbouring atoms can fill up the holes, resulting in the apparent movement of the holes.
· Here the number of holes is more than the number of free electrons. In other words, the holes are the majority carriers and electrons are the minority carriers. This is why the doped semiconductor is called p-type semiconductor where ‘p’ stands for positive.
· The impurities are Acceptors
· Some 3rd group elements used for Doping: Boron, Indium, Aluminium, Gallium,
7. Define biasing of a p-n junction
Connecting a cell across the p-n junction is called biasing of a p-n junction.
8. Give the circuit symbol of a n-p-n and p-n-p transistor.
image

9. How is the transistor biasing done?
For normal operation of the transistor
i. Emitter base junction is forward biased
ii. Collector base junction is reverse biased
10. Define Super conductivity
The property by virtue of which certain materials show almost zero resistance at a very low temperature is called super conductivity
11. Define critical temperature.
The temperature below which a material becomes a super conductor is called critical temperature (Tc)
12. Give the uses of super conductors.
i. Super conductors are used in powerful electromagnets.
ii. High temperature super conductors are used in microwave devices.
iii. Super conductor magnets are used in magnetic resonance imaging (MRI)