This document provides information about BJT transistors, including: 1. It describes the structure of NPN and PNP transistors and their circuit symbols. 2. It explains that transistors are called bipolar because both holes and electrons carry current. 3. It discusses various transistor configurations (CB, CE), characteristics curves, and gain calculations.

1. BJT Transistor
1. Draw a sketch showing the structure of an NPN-junction transistor. Label the emitter, base and
collector regions. Also label the emitter-base and collector-base junctions.
2. Repeat the above for a PNP-junction transistor.
3. Draw the circuit symbol of an NPN transistor and indicate the reference directions, according to
standard convention, for the three currents.
4. Explain why an ordinary junction transistor is called bipolar.
5. Show the biasing arrangement for a PNP transistor in CB configuration so that it works in active
region.
6. Explain the function of the emitter in the operation of a junction transistor.
7. What is done to the base region of a transistor to improve its operation?
8. Though the collector-base junction of a transistor operating in active region is reverse-biased,
the collector current is still quite large. Explain briefly, say, within 10 lines.
9. What do you understand by the collector reverse-saturation current? In which configuration (CB
or CE) does it have a greater value?
10. Besides the active region of operation of a transistor, what are the other possible conditions of
operation of a transistor? Give the biasing conditions of each.
11. What causes collector current to flow when the emitter current is zero? What is this collector
current called?
12. Explain the reason why the base current in a transistor is usually much smaller than IE or le in
active operation.
13. For a PNP transistor in the active region, what is the sign (positive or negative of IE, IB, lC, VEB
and VCE?
14. Draw an NPN transistor in the CB configuration biased for operation in active region.
15. What is considered the input terminal and what is the output terminal in the CB configuration?
16. Sketch typical CB input characteristic curves for an NPN transistor. Label all variables. Explain
how you will calculate the input dynamic resistance of the transistor from these curves.
17. Sketch typical output characteristic curves for a PNP transistor in CB configuration. Label all
variables and indicate active, cut-off and saturation regions.
18. What are the input and output terminals in the CE configuration?
19. An NPN transistor is to be used in common-emitter configuration. Show how you will connect
the external batteries so that the transistor works in the active region.
20. Sketch typical CE input characteristics for an NPN transistor. Label all variables. Outline the
procedure of calculating the input dynamic resistance of the transistor at a given point from
these curves.
21. Sketch typical CE output characteristic curves for an NPN transistor. Label all variables. Explain
in brief how you will compute the beta of the transistor from these characteristic curves?
22. Derive the relationship between the beta and alpha of a transistor.
23. Compare the relative values of input and output resistances for the commonbase
and common-emitter configurations. Give their typical values.
24. Explain why CE configuration is most popular in amplifier circuits.
25. Draw the circuit diagram of a simple transistor amplifier in CE configuration.
Write the equation of a de line.
26. Explain how you will determine the voltage gain of the CE amplifier by plotting the de load line
on the output characteristics of the transistor.
27. Describe briefly the procedure for manufacturing alloy junction transistors.
28. Explain the important steps in making a silicon planar transistor. Why has this technology of
manufacturing transistors become so popular recently?
29. You are given a transistor. Somehow the red dot on its body has been obliterated. Explain how

2. you will determine its three terminals.
30. What is meant by thermal runaway in a transistor? Explain.
31. What is a heat sink? Draw a typical heat sink. List the factors which determine its efficiency.
32. State the order of magnitude of the collector reverse saturation current ICBO.
for (a)germanium transistor, and (b) silicon transistor. How does it vary with temperature in
each case?
ObjectiveType Questions
Below are some incomplete statements. Four alternatives are provided for each.
Choose the alternative that completes the statement correctly.
1. In a PNP transistor with normal bias
(a) only holes cross the collector junction
(b) only majority carriers cross the collector junction
(c) the collector junction has a low resistance
(d) the emitter-base junction is forward biased and the collector-base junction
is reverse biased.
2. In a transistor with normal bias, the emitter junction
(a) has a high resistance
(b) has a low resistance
(c) is reverse biased
(d) emits such carriers into the base region which are in majority (in the base)
3. For transistor action
(a) the collector must be more heavily doped than the emitter region
(b) the collector-base junction must be forward-biased
(c) the base region must be very narrow
(d) the base region must be N-type material
4. The symbol ICBo signifies the current that flows when some de voltage is applied
(a) in the reverse direction to the collector junction with the emitter open circuited
(b) in the forward direction to the collector junction with the emitter open circuited
(c) in the reverse direction to the emitter junction with the collector open circuited
(d) in the forward direction to the emitter junction with the collector open circuited
5. The current ICBO
(a) is generally greater in silicon than in germanium transistors
(b) depends largely on the emitter-base junction bias
(c) depends largely on the emitter doping
(d) increases with an increase in temperature
6. The main current crossing the collector junction in a normally biased NPN transistor is
(a) a diffusion current
(b) a drift current
(c) a hole current
(d) equal to the base current
7. In a PNP transistor, electrons flow
(a) out of the transistor at the collector and base leads
(b) into the transistor at the emitter and base leads
(c) into the transistor at the collector and base leads
(d) out of the transistor at the emitter and base leads
8. The current ICBO flows in
(a) the emitter, base, and collector leads
(b) the emitter and base leads
(c) the emitter and collector leads

3. (d) the collector and base leads
9. The emitter region in the PNP junction transistor is more heavily doped than
the base region so that
(a) the flow across the base region will be mainly because of electrons
(b) the flow across the base region will be mainly because of holes
(c) recombination will be increased in the base region
(d) base current will be high
10. For a given emitter current, the collector current will be higher if
(a) the recombination rate in the base region were decreased
(b) the emitter region were more lightly doped
(c) the minority-carrier mobility in the base region were reduced
(d) the base region were made wider
11. The arrowhead on the transistor symbol always points in the direction of
(a) hole flow in the emitter region
(b) electron flow in the emitter region
(c) minority-carrier flow in the emitter region
(d) majority-carrier flow in the emitter region
12. A small increase in the collector reverse bias will cause
(a) a large increase in emitter current
(b) a large increase in collector current
(c) a large decrease in collector current
(d) very small change in collector reverse saturation current
13. One way in which the operation of an NPN transistor differs from that of a PNP transistor is
that
(a) the emitter junction is reverse biased in the NPN
(b) the emitter injects minority carriers into the base region of the PNP and
majority carriers in the base region of the NPN
(c) the emitter injects holes into the base region of the PNP and electrons into
the base region of the NPN
(d) the emitter injects electrons into the base region of the PNP and holes into
the base region of the NPN
14. The emitter current in a junction transistor with normal bias
(a) may be greatly increased by a small change in collector bias
(b) is equal to the sum of the base current and collector current
(c) is approximately equal to the base current
(d) is designated as Ico
15. In CB configuration, the output volt-ampere characteristics of the transistor
may be shown by plots of
(a) VcB versus ic for constant values of IE
(b) VCB versus iB for constant values of IE
(c) VcE versus iE for constant values of IB
(d) VcE versus ic for constant values of IB
16. A transistor-terminal current is considered positive if
(a) the electrons flow out of the transistor at the terminal
(b) the current is due to the flow of holes only
(c) the current is due to the flow of electrons only
(d) the electrons flow into the transistor at the terminal
17. A transistor-terminal voltage is considered positive if
(a) the terminal is more negative than the common terminal
(b) the terminal is more positive than the common terminal
(c) the terminal is the output terminal
(d) the terminal is connected to P-type material

4. 18. The current ICEO is
(a) the collector current in the common-emitter connected transistor with zero base current
(b) the emitter current in the common-collector connected transistor with zero base current
(c) the collector current in the common-emitter connected transistor with zero emitter current
(d) the same as ICBo
19. The common-emitter input volt-ampere characteristics may be shown by plots of
(a) VCB versus ic for constant values of IE
(b) VCE versus ic for constant values of IB
(c) VCE versus iE for constant values of VEB
(d) VBE versus iB for constant values of VCE
20. In CE configuration, the output volt-ampere characteristics may by shown by plots of
(a) VCB versus ic for constant values of IE
(b) VCE versus ic for constant values of IB
(c) VCE versus iE for constant values of VEB
(d) VBE versus iB for constant values of VCE
21. The beta (/3) of a transistor may be determined directly from the plots of
(a) VCB versus ic for constant values of IE
(b) VEC versus iE for constant values of IB
(c) VCE versus ic for constant values of IB
(d) VBE versus iB for constant values of VCE
22. The most noticeable effect of a small increase in temperature in the common emitter
connected transistor is
(a) the increase in the ac current gain
(b) the decrease in the ac current gain
(c) the increase in output resistance
(d) the increase in ICEO
23. When determining the common-emitter current gain by making small changes in direct
currents, the collector voltage is i1eld constant so that
(a) the output resistance will be high
(b) the transistor will not burn out
(c) the change in emitter current will be due to a change in collector current
(d) the change in collector current will be due to a change in base current
24. The high resistance of the reverse-biased collector junction is due to the fact that
(a) a small change in collector bias voltage causes a large change in collector current
(b) a large change in collector bias voltage causes very little change in collector current
(c) a small change in emitter current causes an almost equal change in collector current
(d) a small change in emitter bias voltage causes a large change in collector current
25. A transistor connected in common-base configuration has
(a) a low input resistance and high output resistance
(b) a high input resistance and a low output resistance
(c) a low input resistance and a low output resistance
(d) a high input resistance and a high output resistance
26. Compared to a CB amplifier, the CE amplifier has
(a) lower input resistance
(b) higher output resistance
(c) lower current amplification
(d) higher c;.1rrent amplification
27. A transistor, when connected in common-emitter mode, has
(a) a high input resistance and a low output resistance
(b) a medium input resistance and a high output resistance
(c) very low input resistance and a low output resistance
(d) a high input resistance and a high output resistance

5. 28. The input and output signals of a common-emitter amplifier are
(a) always equal
(b) out of phase
(c) always negative
(d) in phase
29. A transistor is said to be in a quiescent state when
(a) no signal is applied to the input
(b) it is unbiased
(c) no currents are flowing
(d) emitter-junction bias is just equal to collector-junction bias
30. When a positive voltage signal is applied to the base of a normally biased NPN common-
emitter transistor amplifier
(a) the emitter current decreases
(b) the collector voltage becomes less positive
(c) the base current decreases
(d) the collector current decreases
Indicate which of the following statements pertain to NPN transistors and which pertain to PNP
transistor:
1. The emitter injects holes into the base region.
2. When biased in the active region, current flows into the emitter terminal.
3. The electrons are the minority carriers in the base region.
4. The collector is biased negatively related to the base for active operation.
5. The principal current carriers are electrons.
6. The E-B junction is forward biased for active operation.
7. The base is made by doping the intrinsic semiconductor with indium.