Electronics
EXERCISE
MULTIPLE CHOICE QUESTIONS
Choose the best possible answer:
1. In an N-type silicon, which of the following statement is true.
A. Electrons are majority carriers and trivalent atoms are the dopants.
B. Electrons are minority carriers and pentavalent atoms are the dopants.
C. Holes are minority carriers and pentavalent atoms are the dopants.
D. Holes are majority carriers and trivalent atoms are the dopants.
2. The barrier potential of silicon diode at room temperature is.
A. 0.3 V
B. 0.7 V
C. 0.9 V
D. 1.0 V
3.The width of depletion region of a junction A. Increases under forward bias
B. Is independent of applied voltage
C. Increases under reverse bias
D. Remains same
4.For common emitter configuration of NPN transistor, current gain is given by.
A. Ic/lb
B. Ic/l
C./lo
D. lo/Ir
5.The reverse saturation current in a PN diode is only due to
A. majority carriers
B. minority carriers
D. donor ions
C. acceptor ions
6.PN junction when reversed biased act as:
A. Capacitor
B. ON switch
C. Inductor
D. OFF switch
7. Improper biasing of a transistor circuit produces
A. Heavy loading of emitter current
B. Distortion in the output signal
C. Excessive heat at collector terminal
D. Faulty location of load line
8.When transistors are used in digital circuits they usually operate in the:
A. Active region
B. Breakdown region
C. Saturation and cutoff regions
D. Linear region
9. Most of the electrons in the base of an NPN transistor flow:
A. out of the base lead
B. into the collector
C. into the emitter
D. into the base supply
10.In a transistor, collector current is controlled by:
A. Collector voltage
B. base current
C. Collector resistance
D. all of the above
11. The minimum number of diodes required for full wave rectification are
A. 3 B. 4 C. 2 D. 1
12. In a transistor both collector-base and base-emitter junctions are reversed biased then it is
A. Active region
B. Cutoff region
C. Saturation region
C. Q-point
CONCEPTUAL QUESTIONS
1. Why in a transistor (a) the base is thin lightly doped and (b) the collector is large in size.
Ans. (a) Why Base is thin and lightly doped: The base region is meant for "only the control" of the strong flow of charge carriers from the Emitter towards the Collector. It is not supposed to absorb/emit any carriers on its own ideally. That can be done by making the base extremely thin to see that the carriers won't get enough time to recombine and also won't get a chance to recombine, thus weak ening the base current.
(b) Why Collector is large in size: The reason behind the collector is having the largest size in transistor is because it is the region which receives the majority of the current carriers, which for a npn transistor are electrons and for pnp transis tors are holes. The collector receives the current carriers from the base and the emitter regions of the transistor. When a transistor receives sufficient current at the base, current then travels from the emitter to the collector.
As it receives the most current carriers, therefore it generates more heat than all other regions. So, the collector region is the largest of all regions because it must dissipate more heat than the emitter or base regions. It is thus designed to be large, so that the extra surface allows dissipating all the extra heat. If the collector were smaller, it would overheat quicker, and; thus, the transistor would overheat easily. The larger area ensures that it has more surface area to dissipate heat.
2. Why the base current is weak as compared to collector current?
Ans. Base current is weak as compared to collector current because base is ex tremely thin and very lightly doped as compared to collector. Explanation: In the manufacture of the transistor, base is very thin and also doped lightly than collector. Therefore, smaller recombination of charges occurs at the base, which results in a weak base current. If base is heavily doped, then a very significant number of charges will recombine in the base to form base current and return to the battery. This will reduce the current gain of the transistor as it is the ratio of collector current to base current. Since collector is doped higher and its size is also larger than base so that majority of the charges can be received at the collector to make the collector current larger than base current.
3. Why the emitter base junction is forward biased and collector base junction is reverse biased?
Ans. For the normal operation of transistor (i.e. in active region), its emitter-base junction is forward biased because in forward biased the input resistance i.e. the resistance of the emitter base junction becomes very small. As a result, maximum charges are injected through the emitter into the base. These charge carriers reach the base, which is quite thin and lightly doped. So, in order to make current gain larger, most of these charges must be received at the collector. This is only possible when collector-base junction is reversed biased, because in reverse bi ased the output résistance, i.e. resistance of the collector base junction becomes very high. Hence, most of the charges that are injected in the base through for ward biased configuration are received by collector through reversed biased con figuration. This is the reason that in a transistor, emitter-base (EB) junction is for ward biased and collector-base (CB) junction is reverse biased. Note: The circuit with low input resistance and high output resistance acts as the best amplifier.
4. How NPN and PNP transistors works.
Ans. NPN Transistor: A transistor formed by sandwiching P-type material be tween two N-type materials is known as NPN transistor. Circuit Diagram: Circuit diagram and symbol for the NPN transistor is given in the figure.
Working of NPN Transistor: In the normal operation of an NPN transistor (active region), the base-emitter junction should be forward biased, and the collector base junction should be reversed biased.
When forward biased is applied to the emitter-base junction, the electrons in N region will repel from the negative terminal of the battery and will move towards the base region. The base re gion is very small as com pared to emitter and collec tor region. Besides, the dop ing intensity of base is lowest." Thus, it consists of fewer holes.
Due to few holes in the base region, only a few electrons will recombine with holes.
The other electrons which have not recombined yet will move towards collector region. This will constitute current in the circuit. The size of the collector is large so that it can collect more charge carriers and can dissipate heat. The current in NPN transistor is due to electrons because electrons are the major ity charge carriers in NPN transistor..
The emitter current in NPN transistor is equal to the sum of base and collector
current. Mathematically it can be written as:
PNP Transistor: A transistor formed Ly sandwiching N-type material between
two P-type materials is known as PNP transistor. Circuit Diagram: Circuit diagram and symbol for the PNP transistor is given in the figure.
Working of PNP Transistor: PNP transistor works when the emitter-base junction
is forward biased while collector-base junction is reverse biased.
The holes are the majority charge carrier in P-type semiconductor and electrons are the majo, ty charge carri ers in N-type semiconductor. The hole in emitter region will be repelled by the posi tive terminal of the battery and will be attracted by elec trons present in the N-re gion. Thus, the effective base width between both the junctions will be reduced.
Since the emitter is heavily doped than base and collector, and area of the emitter is also more than the base but less than the collector. The base is lightly doped so it has fewer electrons. As a consequence of which these few electrons will com bine with holes emitted from emitter region due to repulsion from the positive terminal of the battery. But only a few holes will combine with electrons present in the base region due to the small size of the base and light doping. The majority of the holes are remaining that has not combined with electrons of base terminal. These holes will flow towards collector. They will further move to wards the end of collector region because they are attracted by negative terminal of the battery through which collector is connected. The equation of current in PNP transistor is given below.
5. Distinguish between N-type semiconductors and P-type semiconductors.
Ans. The differences between N-type semiconductor and P-type semiconductor
are shown in the table given below.
1)N-type Semiconductor
1.N-type semiconductors formed by doping group-V ele- ments (Arsenic, Antimony, Phos- phorus or Bismuth) with group- IV elements (Ge, Si).
2.In N-type semiconductor, the majority charge carriers are free electrons, whereas the holes are in minority.
3.In case of N-type semiconductor, the majority of charge carriers move from low potential to high potential.
4.In case of N-type semiconduc tors, donor energy level is close to the conduction band.
(2)P-type Semiconductor
1.P-type semiconductors are formed by doping group-lll elements (Aluminum, Gallium or Indium) with group-IV ele ments (Ge, Si).
2.In P-type semiconductor, the majority charge carriers are holes, whereas the free electrons are in minority.
3.In case of P-type semiconductor, the majority of charge carriers move from high potential to low potential.
4.In case of P-type semiconductors, ac ceptor energy level is close to the con duction band.
6. A P-type semiconductor has a large member of holes but still it is electrically neutral. Why?
Ans. A P-type semiconductor is electrically neutral because the atom from which this semiconductor is made is electrically neutral as a whole. Explanation: A p-type semiconductor is formed by duping pure germanium or silicon crystals with suitable impurity atoms of group-lll elements. As the impurity atoms take the positions of germanium atoms in germanium crystal its three va lence electrons form covalent bonds by sharing electrons with the neighboring three atoms of germanium whereas the fourth covalent bond is left incomplete.
This creates a hole.
Since the atom on the whole is electrical neutral and no electron is added to or taken out during the formation of P-type semiconductor, therefore we can say that P-type semiconductor is also neutral.
7. Why CE configuration is widely used in amplifier circuits?
Ans. Common Emitter Configuration: When an emitter is common to both input and output circuits, then it is called common emitter configuration. A transistor in common-emitter configuration is preferred because its current and voltage gains are high and power gain is the highest as compare to the other two configurations, i.e. common base configuration and common collector configura tion.
8. Why transistor is called current amplification device?
Ans. A transistor is called current amplification device, because it is used to amplify signals (current, voltage or power)
Explanation: When a BJT transistor is operating in the active region, it amplifies a small signal given at the input several hundred times (up to 400 times) in the out put circuit. Usually common-emitter configuration is used for this purpose. Reason is that it has low input resistance and thus higher current gain is achieved.
Current amplification which shows by how much times a current giyen at the input
is increased at the output, clearly indicates that transistor is a current amplification device. Thus, it can be used to enhance the strength of a given weak signal. The energy required for amplification is taken from power supply.
9. A doped semiconductor has 1010 silicon and 10 trivalent atoms. If the temperature is 25 °C, how many free electrons and holes are there inside the sem iconductor?
Ans. When 10 trivalent atoms are doped in 1010 silicon atoms, there will be 10 holes generated by this doping, while no free electrons will be generated and the material will remain neutral. However, at 25 °C, due to thermal effects some electrons will break away from their covalent bond and can reach the conduction band to become a free carrier, leaving a hole in the valence band. There is no such formula to calculate exactly the number of free electrons and holes inside the semiconductor at a given tem perature. In the present case we can only say that 10 holes will be more in number than the number of free electrons.
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