class 12 physics chapter 14 important questions
Dear students class 12 physics chapter 14 important questions with answers pdf have been collected at one place to get you all prepared for your 12th Bihar board exam, as well as for UP board exams ,Jharkhand board exam ,MP board exams ,Rajsthan board exams ,cbse board exams and other board exams. Visit my nawendu classes youtube channel for more help.
Q.(1)- What do you mean by semiconductor?
Bihar Board-2020
Answer- The material which has an small forbidden gap between conduction
band and valency band, is called semiconductor.
• Its conductivity is between conductor and non conductor.
• It has a few number of free electrons.
Q.(2)- What is doping?
Bihar Board-2012
Answer- The process of adding impurities to an intrinsic semiconductor (pure
semiconductor) is called doping.
• The impurity is called dopant.
• After doping, an intrinsic semiconductor becomes extrinsic semiconductor
e.g. - adding arsenic to pure silicon it becomes n type semiconductor.
Q.(3)- Explain n type of semiconductor.
Bihar Board-2021
Answer- n type semiconductor- When tetravalent atoms (e.g.- germanium-Ge ,silicon- Si) are doped with pentavalent atoms (e.g.- Phosphorus- P ,Arsenic- As, Antimony- Sb ,Bismuth- Bi) then it is called n type semiconductor.
OR
When a pentavalent impurity is added to a pure (an intrinsic) semiconductor then it is called n type semiconductor.
Four valence electron of arsenic form covalent bond with four surrounding germanium atoms and fifth electron becomes free. In such semiconductors electron are in majority. Electrons are negatively charged so it is called n type or donor type semiconductor.
Q.(4)- What do you mean by p type and n type semiconductors? Explain the
working of a p-n junction diode.
Bihar Board-2020
Answer- n type semiconductor- When tetravalent atoms (e.g.- germanium-Ge ,silicon- Si) are doped with pentavalent atoms (e.g.- Phosphorus- P ,Arsenic- As, Antimony- Sb ,Bismuth- Bi) then it is called n type semiconductor.
OR
When a pentavalent impurity is added to a pure (an intrinsic) semiconductor then it is called n type semiconductor.
Four valence electron of arsenic form covalent bond with four surrounding germanium atoms and fifth electron becomes free. In such semiconductors electron
are in majority. Electrons are negatively charged so it is called n type or donor
type semiconductor.
p type semiconductor- When tetravalent atoms (e.g.- germanium-Ge ,silicon- Si) are doped with trivalent atoms (e.g.- Indium-In ,Boron -B, Gallium- Ga ,Aluminum- Al) then it is called p type semiconductor.
OR
When a trivalent impurity is added to a pure (an intrinsic) semiconductor then it is called p type semiconductor.
Three valence electron of indium form covalent bond with three surrounding germanium
atoms.
But only one electron takes part in covalent bond with forth germanium atom
therefore a hole is produced in the bond in place of electron. Fools are in
majority carrier in such semiconductors.
Since holes are a positively charged and therefore such
semiconductor is called p type semiconductor or acceptor type semiconductor .
P-n junction- If p type and n type semiconductors are joined to each other then the boundary or interface between p type and n type semiconductor is called PN junction.
• It is denoted by
In pn junction, holes are in majority in p type semiconductor and electrons are in
majority carrier in n type semiconductor of a PN junction. Due to which
diffusion of holes and electrons begin.
After sometime, electrons arrange near junction in p type semiconductor and
holes arrange near junction in n type semiconductor. Due to which movement
of electrons and holes stop in thin layer which is called depletion layer. Width of
depletion layer is of the order of \[{{10}^{-6}}m\].
An small Voltage is developed across the junction due to arrangement of
holes and electrons near junction, which is called barrier voltage. Barrier
voltage is from 0.1 v to 0.5v. 
Q.(5)- Distinguish between n type and p type semiconductor.
Bihar Board-2013, 2015
Answer- p type semiconductor-
(i) In p type semiconductor holes are majority carriers and electrons
are minority carriers.
(ii) Impurities are added to p type semiconductor to increase the
number of holes.
(iii) p type semiconductor is obtained by adding tri valent impurity
(atoms) to pure germanium or silicon semiconductor.
n type semiconductor-
(i) In n type semiconductor electrons are majority carrier and holes
are minority carrier.
(ii) Impurities are added to n type semiconductor to increase the
number of electrons.
(iii) n type semiconductor is obtained by adding pentavalent impurity
(atoms) to pure germanium or silicon semiconductor.
Q.(6)- What happens when a forward bais is applied to a p-n junction?
Bihar Board-2014
Answer- 
when forward bias is applied to to a pn junction then the holes of p type semiconductor is repelled by positive terminal of the battery and the electrons of n type semiconductor is repelled by negative terminal of the battery then holes of p type moves towards n type and electrons of n type moves toward p-type. Thus forward bias opposes the potential barrier. Due to which potential barrier reduced and hence depletion layer becomes thin .The resistance of pn junction becomes low when forward biased. The value of current increases with increase in voltage.
when forward bias is applied to to a pn junction then the holes of p type semiconductor is repelled by positive terminal of the battery and the electrons of n type semiconductor is repelled by negative terminal of the battery then holes of p type moves towards n type and electrons of n type moves toward p-type. Thus forward bias opposes the potential barrier. Due to which potential barrier reduced and hence depletion layer becomes thin .The resistance of pn junction becomes low when forward biased. The value of current increases with increase in voltage.
Q.(7)- Draw the voltage current characteristic curve of a diode and mark its
important parameter.
Bihar Board-2014
Answer- 
Q.(8)- Draw the circuit diagram to explain the working of full wave PN junction
rectifier.
Bihar Board-2018
Answer- 
Q.(9)- Diode is called valve. Why?
OR
p-n junction is a valve .Explain .
Bihar Board-2013
Answer – Current flows through p-n junction diode only in forward bias but not in reverse bias. Hence diode is unidirectional since (valves of cycle tubes or heart)valves are also unidirectional this is why diode is called valve.
Q.(10)- Explain solar cell.
Bihar Board-2021
Answer- solar cell is a p-n junction diode which converts solar energy into
electrical energy.
• If energy of each Photon of incident light is greater than forbidden
gap then electron hole pair is produced in depletion layer. Electrons
moves toward n type and holes move toward p type semiconductor due to junction field. Due
to which p type becomes positive and n type becomes negative
which gives rise to photo Voltage between p and n type. Therefore
photo current flows if load is connected or circuit is completed .
•Silicon, gallium, arsenide are used in solar cell.
• It works on the principle of photodiode but only difference is that
and external voltage is not applied to solar cell.
• Uses-
(i) in artificial satellite
(ii) in the day to charge batteries
(iii) In calculators and wrist watches
(iv) In remote areas to produce electricity
Q.(11)- Give the symbol and truth table for AND gate.
Bihar Board-2012
Answer-
|
INPUT |
OUTPUT |
|
|
A |
B |
C=A.B |
|
0 |
0 |
0 |
|
0 |
1 |
0 |
|
1 |
0 |
0 |
|
1 |
1 |
1 |
Bihar Board-2014
Answer- 
Q.(13)- Explain NAND gate.
Bihar Board-2021
Answer- If output of AND gate is connected to the input of NOT gate then it is called NAND gate.
NAND gate is inverse of an AND gate .For a NAND gate , the output is high if one or both the inputs are low and the output is low if both the inputs are high. NAND gate is a universal logic gate.
INPUT | OUTPUT | |
A | B | $C=\overline{A\cdot B}$ |
0 | 0 | 1 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
Q.(14)- Identify the given gate and write its truth table.
Bihar Board-2011
Answer - given logic gate is a NOT gate.
|
INPUT |
OUTPUT |
|
A |
$C=\bar{A}$ |
|
0 |
1 |
|
1 |
0 |
Q.(15)- Write the truth table for a NAND gate connected in the given figure and
identify the exact logic operation carried out by this circuit.
Bihar Board-2014
Answer- For a NAND gate , the output is high if one or both the inputs are low and the output is low if both the inputs are high.
|
INPUT |
Output of AND gate OR Input of NOT
gate |
OUTPUT |
|
|
A |
B |
A.B |
$C=\overline{A\cdot B}$ |
|
0 |
0 |
0 |
1 |
|
0 |
1 |
0 |
1 |
|
1 |
0 |
0 |
1 |
|
1 |
1 |
1 |
0 |
The two inputs of the given logic gate are shorted here. So the given NAND gate has only one input and only one output. One input of a NAND gate may be 0 or 1. When input is 1 output will be 0 and when input is 0 output will be 1. So the given circuit carries out the operation of a NOT gate having following truth table.
INPUT | OUTPUT |
A | $C=\bar{A}$ |
0 | 1 |
1 | 0 |
Q.(16)- n-p-n transistors are used more than p-n-p transistors. Why?
Ans. – In n-p-n transistors
majority current carriers are free electrons and in p-n-p transistor majority
current carriers are holes. Mobility of electrons is greater than holes. This is why n-p-n transistors are used more than p-n-p transistors.
Q.(17)- Draw a circuit diagram of common emitter amplifier.
Bihar Board-2010
Answer- 
Q.(18)- What is ASCII?
Ans. – ASCII is a short form of
“American standard code for Information interchange” in is a system of encoding
a letter or a number.
Q.(19)- Write the truth table and Boolean expression for NOT and OR gate.
Bihar Board-2022
Answer- for NOT gate-
for OR gate-
Boolean expression
Boolean expression
$C=\bar{A}$
truth table-
INPUT | OUTPUT |
A | $C=\bar{A}$ |
0 | 1 |
1 | 0 |
Boolean expression
C=A +B
truth table-
INPUT | OUTPUT | |
A | B | C=A +B |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
Q.(20)- Explain binary number system.
Bihar Board-2022
Answer- A number system where a number is represented by using only two digits (two symbols) 0 and 1 with a base 2 is called binary number system .
* In computer science 1 is used for ON (or TRUE) and 0 is used for OFF (or FALSE)
* Each digit of a binary number is called bit . 4 bit is called nibble and 8 bit is called byte .
* The leftmost digit of a binary number is called most significant digit (MSD) and the rightmost digit of a binary number is called least significant digit (LSD).
Q.(21)- Explain (i) zener diode (ii) LED (Light Emitting Diode)
Bihar Board-2017
Answer- Go through nawendu physics classes notes
chapter 1 ELECTRIC CHARGES AND FIELDS short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 3 CURRENT ELECTRICITY short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 3 CURRENT ELECTRICITY short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
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