CLASS 1ST YEAR PHYSICS SHORT QUESTIONS NOTES WRITTEN BY MR SAMIULLAH MOB # 0344-9167826
1. Explain why in a transistor (a) the base is thin lightly doped and (b) the collector is large in size?
Ans: (a) Statement:- In a transistor the base is thin and lightly doped.
Reason:- It is because to increase the current gain.
Explanation:As we know that
I
Current gain = β = C ……….(1)
IB
From equation (1) is cleared that
 Greater the collector current greater will be the current gain and vice versa.
 Smaller the base current greater will be the current gain and vice versa.
Conclusion:- As conclusion we find that in a transistor the base is thin and lightly doped.
(b) Statement:- In a transistor the collector is large in size.
Reason:- It is because to collect large number of electrons.
Explanation:- As we know that
 The collector has less concentration of impurity as compared to the emitter.
 The collector is comparatively large in size than the emitter.
 The collector’s function is to collect majority charge carries through the base, so for this it need to
be large in size.
Conclusion:- As conclusion we find that in a transistor the collector is large in size.
2. Explain why the base current is weak as compared to collector current?
Ans: Statement:- The base current is weak as compared to collector current.
Reason:- It is because that
 Smaller recombination of charges occur at the base.
 Size and doping ratio of base of a transistor is very small as compared to collector.
Explanation:- As we know that in the manufacture of the transistor, base is very thin and also it
is doped slightly than collector. Very small number of electrons and holes recombine to constitute the
base current. Majority of charge carriers to flow emitter towards the collector and thus large current
is constituted in the collector region.
Conclusion:- As conclusion we find that the base current is weak as compared to collector
current.
3. Why the emitter base junction is forward biased and collector base junction is reversed biased
Ans: Statement:- The emitter base junction is forward biased while the collector base junction is
reversed biased.
Reason:- It is because to increase the current gain.
Explanation:- In the normal operation of transistor, emitter-base junction is forward biased, so
that it has low resistance and thus minimize the depletion region. As a result the charges can flow
easily from emitter towards the base which is quite thin and lightly doped. So, in order to increase the
current gain, most of these charges must be received at the collector. This is only possible when the
collector-base junction is reversed biased. Hence most of the charges that are injected in the base
through forward biased configuration are received by collector through reversed biased
configuration.
Conclusion:- As conclusion we find that the emitter base junction is forward biased while
collector base junction is reversed biased.
Q5: How NPN and PNP transistor work?
CLASS 1ST YEAR PHYSICS SHORT QUESTIONS NOTES WRITTEN BY MR SAMIULLAH MOB # 0344-9167826
Ans: (a) NPN Transistor:Definition:- When P-type substance is sandwiched between two n-type substances then device
formed is known as NPN transistor.
Symbol in circuit:Working:- Emitter-base junction is forward biased by
(b) PNP transistor:Definition:- When N-type substance ( Si or Ge ) is sandwiched between two n-type substances then
device formed is known as NPN transistor.
Symbol in circuit:Working:6: Distinguish between N-type semiconductor and P-type semiconductor?
BASIS OF
DIFFERENCE
p TYPE SEMICONDUCTOR
n TYPE SEMICONDUCTOR
Group of Doping In P type semiconductor III group element is In n type semiconductor V group element
Element
added as doping element.
is added as doping element.
Nature of Doping Impurity added creates vacancy of electrons Impurity added provides extra electrons
Element
(holes) called as Acceptor Atom.
and is known as Donor Atom.
Type of impurity Trivalent impurity like Al, Ga, In etc. are
added
added.
Pentavalent impurity like P, As, Sb, Bi etc.
are added.
Majority Carriers Holes are majority carriers
Electrons are majority carriers
Minority Carriers Electrons are minority carriers
Holes are minority carriers
Density of
Electrons and
Holes
The hole density is much greater than the The electron density is much greater than
electron density.
the hole density.
nh >> ne
ne >> nh
Energy level
The acceptor energy level is close to the
The donor energy level is close to the
valence band and away from the conduction conduction band and away from the
band.
valence band.
CLASS 1ST YEAR PHYSICS SHORT QUESTIONS NOTES WRITTEN BY MR SAMIULLAH MOB # 0344-9167826
BASIS OF
DIFFERENCE
Fermi level
p TYPE SEMICONDUCTOR
n TYPE SEMICONDUCTOR
Fermi level lies between acceptor energy
level and the valence band.
Fermi level lies between donor energy level
and the conduction band.
Movement of
Majority carriers move from higher to lower Majority carriers move from lower to
Majority carriers potential.
higher potential.
7. A P-type semiconductor has a large number of holes but still it is electrically natural. Why?
Ans: Statement:- A P-type semiconductor has a large number of holes but still it is electrically
natural.
Reason:- It is because of number of electron = number of holes
Explanation:- As we know that Ge or Si crystal formed after adding a trivalent impurity from 3 rd
group such as Indium is called P-type semiconductor. It has a large number of holes as positive charge
carriers but still it is electrically neutral. It is because the large number of holes in excess of their
intrinsic value exactly equal to accepter negative ions present in the semiconductor materials.
Conclusion:- As conclusion we find that a P-type semiconductor has a large number of holes but
still it is electrically natural.
8. Explain why CE configuration is widely used in amplifier circuits?
Ans: Statement:- CE configuration is widely used in amplifier circuits.
Reason:- It is because of
 It has current gain.
 It has high voltage gain.
 It has high power gain.
 It has high input resistance and low output resistance.
 Its signal received at the output is more stable.
Conclusion:- As conclusion we find that CE configuration is widely used in amplifier circuits.
9. Why transistor is called current amplification device?
Ans: Statement:- Transistor is called current amplification device.
Reason:- It is because it rises the strength of a weak signal.
Explanation:- As we know that amplification is the process in which a small signal of voltage or
current is converted into large signals. The device which is used for amplification is known as
amplifier. A transistor commonly used as amplifier.
 It has low output resistance.
 It has higher current gain.
 It has high power gain.
 It increase the strength of weak signals.
 It has moderate ratio between the input and out impedance.
Conclusion:- As conclusion we find that the transistor is called current amplification device.
Q.10:- A doped semiconductor has 𝟏𝟎𝟏𝟎 silicon atoms and 10 trivalent atoms. If the temperature is
25𝑪𝒐 , How many free electrons and holes are their inside the semiconductor?
CLASS 1ST YEAR PHYSICS SHORT QUESTIONS NOTES WRITTEN BY MR SAMIULLAH MOB # 0344-9167826
Ans: Statement:- A doped semiconductor has 1010 silicon atoms and 10 trivalent atoms. If the
temperature is 25𝐶 𝑜 there will be 10 hole and zero free electrons.
Explanation:- As we know that each trivalent atom contributes one hole to the
semiconductor, therefore the semiconductor will have 10 holes produced by doping process.
At 25𝐶 𝑜 the heat energy is negligible small and thus almost no free electrons will be available to
move from valence band to the conduction band.
Conclusion:- As conclusion we find that a doped semiconductor has 1010 silicon atoms and 10
trivalent atoms. If the temperature is 25𝐶 𝑜 there will be 10 hole and zero free electrons.