Module II electron devices

1. SRI RAMAKRISHNA ENGINEERING COLLEGE
No. of Credits : 3
20EC201 & ELECTRON DEVICES
Department of Electronics and Communication Engineering
[Educational Service : SNR Sons Charitable Trust]
[Autonomous Institution, Accredited by NAAC with ‘A’ Grade]
[Approved by AICTE and Permanently Affiliated to Anna University, Chennai]
[ISO 9001-2015 Certified and all eligible programmes Accredited by NBA]
VATTAMALAIPALAYAM, N.G.G.O. COLONY POST, COIMBATORE – 641 022.
:

2. COURSE OUTCOMES
• On completion of the course, the students will be able to
• CO1: Interpret and describe the concepts of semiconductor
devices.
• CO2: Apply the electronicfundamental concepts
and determine the device characteristics.
• CO3: Examine the device performance of Semiconductor diodes and
transistors.
• CO4: Design transistor amplifier circuit for different biasing
techniques, clipper & clamper circuits.

3. • BIPOLAR JUNCTION TRANSISTORS
• Basic Concepts, Device Characteristics, Transistor Biasing, Fixed Bias
Circuit, Stability Factors, Different types of Biasing Circuits, CE, CB,
CC Amplifiers, Small Signal –Low Frequency h-parameter model,
Determination of h-parameters from characteristics, Mid-band Analysis
of a BJT single stage amplifiers, Method of Analysis of a Transistor
Circuit, Analysis using simplified hybrid model.
MODULE II

4. BOOKS REFERRED
TEXT BOOKS:
• Jacob Millman, Christos C.Halkias, “Electronic
McGraw HillPublishing Limited, New Delhi,2015.
Devices and Circuits”, Tata
• Robert. L.Boylestad, Louis Nashalskey, “Electronic Devices and Circuits”,
PearsonEducation, India, 11thEdition, 2015.
• Floyd, Electronic Devices conventional current version, Pearson Education, India,
9thEdition, 2015.
REFERENCE BOOKS:
• Donald.A. Neamen, ‘Semiconductor Physics and Devices basic principles’,Tata
McGraw Hill Publishing Limited, 4thEdition, New Delhi,2017.
• Salivahanan.S, Suresh Kumar.N and V
allavaraj.A, “Electronic Devices and
Circuits”, Tata McGraw Hill Publishing Limited, 4thEdition, 2017.
• Simon M.Sze and Kwok K.Ng, “Physics of Semiconductor Devices”, John Wiley &
sons, 3rdEdition, 2008.
• G.K.Mital and Gupta.M, “Electronic Devices and Circuits”, Khanna Publisher,
22ndEdition,1999.

5. Module- 2
BIPOLAR JUNCTION TRANSISTORS

6. Transistor-Introduction
• The transistor is a semiconductor device which
transfers a weak signal from low resistance circuit to
high resistance circuit.
• The words trans mean transfer
property and istor mean resistance property offered
to the junctions.
• In other words, it is a switching device which
regulates and amplify the electrical signal likes
voltage or current.

7. Applications
• Amplification
• Switching
• Regulator
• Buffer or Impedance matching between mis- matched circuits
• Used in building blocks of Integrated circuits

8. BJT - Introduction
• Bipolar transistors are one of the main
‘building-blocks’in electronic systems.
• They are used in both analogue and digital circuits.
• They incorporate two PN junctions and are
sometimes known as Bipolar Junction Transistors
or BJTs.
• Simply called as Bipolar Transistors.

9. Introduction
A bipolar junction transistor (BJT) is a three-layer
active device that consists of two p–n junctions
connected back-to-back.
Although two p–n junctions in a series is not a
transistor since a transistor is an active device
whereas a p–n junction is a passive device.
A BJT is actually a current-amplifying device. In a
BJT, the operation depends on the active participation
of both the majority carrier and the minority carrier;
hence, the name “bipolar” is rightly justified.

10. Introduction
• It has three terminals namely emitter, base and
collector. The base is the middle section which is
made up of thin layers.
• The right part of the diode is called emitter diode
and the left part is called collector-base diode. These
names are given as per the common terminal of the
transistor.
• The emitter based junction of the transistor is
connected to forward biased and the collector-base
junction is connected in reverse bias which offers a
high resistance.

11. Types and Symbol of Transistor
• There are two types of transistor, namely NPN transistor and
PNP transistor.
• The transistor which has two blocks of n-type semiconductor
material and one block of P-type semiconductor material is
known as NPN transistor.
• Similarly, if the material has one layer of N-type material and
two layers of P-type material then it is called PNP transistor.

12. Types and Symbol of Transistor
• The arrow in the symbol indicates the direction of flow of conventional
current in the emitter with forward biasing applied to the emitter-base
junction.
• The only difference between the NPN and PNP transistor is in the
direction of the current.
Fig.2.1 Symbolof Transistor

13. Transistor Terminals
• Emitter – The section that supplies the large section
of majority charge carrier is called emitter.
• The emitter is always connected in forward biased
with respect to the base so that it supplies the
majority charge carrier to the base.
• The emitter-base junction injects a large amount of
majority charge carrier into the base because it is
heavily doped and moderate in size.

14. Transistor Terminals
• Collector The section which collects the major portion of the majority
charge carrier supplied by the emitter is called a collector.
• The collector-base junction is always in reverse bias. Its main function is
to remove the majority of charges from its junction with the base.
• The collector section of the transistor is moderately doped, but larger in
size so that it can collect most of the charge carrier supplied by the emitter.

15. Transistor Terminals
• Base – The middle section of the transistor is known as
the base. The base forms two circuits, the input circuit
with the emitter and the output circuit with the collector.
• The emitter-base circuit is in forward-biased and offered
the low resistance to the circuit. The collector-base
junction is in reverse bias and offers the higher resistance
to the circuit.
• The base of the transistor is lightly doped and very thin
due to which it offers the majority charge carrier to the
base.

16. Transistor Picture
Fig.2.2 TransistorPicture

17. Bipolar Transistor Construction
PNP Transistor NPN Transistor
Fig.2.3 PNPand NPN Transistor

18. Bipolar Transistor Construction
Fig.2.4 TransistorModel

19. Working of Transistor
• Usually, silicon is used for making the transistor
because of their high voltage rating, greater current
and less temperature sensitivity.
• The emitter-base section kept in forward biased
constitutes the base current which flows through the
base region.
• The magnitude of the base current is very small.
The base current causes the electrons to move into
the collector region or create a hole in the base
region.

20. Working of Transistor
Fig.2.5 TransistorWorking

21. Working of Transistor
• The base of the transistor is very thin and lightly
doped because of which it has less number of
electrons as compared to the emitter.
• The few electrons of the emitter are combined with
the hole of the base region and the remaining
electrons are moved towards the collector region
and constitute the collector current.
• Thus we can say that the large collector current is
obtained by varying the base region.

22. Transistor Operating Conditions
• When the emitter junction is forward biased and the
collector junction is in reverse bias, then it is said to
be in the active region. The transistor has two
junctions which can be biased in different ways.

23. Working of Transistor
• FR – In this case, the emitter-base junction is
connected in forward biased and the collector-base
junction is connected in reverse biased. The
transistor is in the active region and the collector
current is depend on the emitter current. The
transistor, which operates in this region is used for
amplification.
• FF – In this condition, both the junction is in
forward biased. The transistor is in saturation and
the collector current becomes independent of the
base current. The transistors act like a closed switch.

24. Working of Transistor
• RR – Both the current are in reverse biased. The
emitter does not supply the majority charge carrier
to the base and carriers current are not collected by
the collector. Thus the transistors act like a closed
switch
• RF – The emitter-base junction is in reverse bias
and the collector-base junction is kept in forward
biased. As the collector is lightly doped as
compared to the emitter junction it does not supply
the majority charge carrier to the base. Hence poor
transistor action is achieved.

25. Bipolar Transistor Configurations
• As the Bipolar Transistor is a three terminal
device, there are basically three possible ways to
connect it within an electronic circuit with one
terminal being common to both the input and output.
• Common Base Configuration
– has V
oltage Gain but no Current Gain.
• Common Emitter Configuration
– has both Current and V
oltage Gain.
• Common Collector Configuration
– has Current Gain but no Voltage Gain.

26. Common Base (CB) Configuration N-P-N BJT
• IE, IC and IB are emitter current, collect current and base current respectively.
• VEB and VCB are emitter base voltage and collector base voltage respectively.

27. • For P-N-P transistors, current enters into the transistor through
the emitter terminal.
• Any BJT, the emitter-base junction is forward biased and the
collector-base junction is reverse biased.
Common Base (CB) Configuration P-N-P BJT

28. Common Base Configuration

29. Common Base Configuration
• In CB configuration, emitter is the input terminal, collector is
the output terminal and base terminal is connected as a common
terminal for both input and output.
• The emitter terminal and common base terminal are known as
input terminals whereas the collector terminal and common
base terminal are known as output terminals.
• In CB configuration, the base terminal is grounded so the
common base configuration is also known as grounded base
configuration, common base amplifier, CB amplifier or CB
configuration.

30. Common Base Configuration
JE is always forward
junction JC is always
• The base-emitter junction
biased and collector-base
reverse biased.
• The input is applied to the emitter and the output is
taken from the collector.

31. Common Base Configuration

32. Input characteristics
• When the output voltage (VCB) is increased
from zero volts to a certain voltage level, the
emitter current flow will be increased which
in turn reduces the depletion region width at
the emitter-base junction.
• As a result, the cut in voltage
will be reduced.
• So, the curves shifted towards the left side for
higher values of output voltage VCB.

33. Output Characteristics
• From the output characteristics, we can see that for
a constant input current IE, when the output
VCB
voltage is increased, the output current
IC remains constant.
• At saturation region, both emitter-base junction
JE and collector-base junction JC are forward
biased. From the above graph, we can see that a
sudden increase in the collector current when the
VCB
output voltage makes the collector-base
junction JC forward biased.

34. Output Characteristics

35. Early Effect
• Due to forward bias, the base-emitter junction JE acts as a forward biased diode and
due to reverse bias, the collector- base junction JC acts as a reverse biased diode.
• The width of the depletion region at the base-emitter junction JE is very small
whereas the width of the depletion region at the collector-base junction JC is very
large.
• If the output voltage VCB applied to the collector-base junction JC is further increased,
the depletion region width further increases. The base region is lightly doped as
compared to the collector region. So the depletion region penetrates more into the
base region and less into the collector region. As a result, the width of the base region
decreases. This dependency of base width on the output voltage (VCB) is known as an
early effect.

36.  Active
Operating range of the amplifier.
Base-Emitter Junction forward biased.
Collector-Base Junction reverse biased
 Cutoff
The amplifier is basically off.
There is voltage but little current.
Both junctions reverse biased
 Saturation
The amplifier is full on.
There is little voltage but lots of current.
Both junctions forward biased
3 Regions of Operation

37. Transistor Parameters
Dynamic input resistance (ri):
• Dynamic input resistance is defined as the ratio of change in input
voltage or emitter voltage (VBE) to the corresponding change in
input current or emitter current (IE), with the output voltage or
collector voltage (VCB) kept at constant.
• The input resistance of common base amplifier is very low.

38. Dynamic output resistance (ro)
• Dynamic output resistance is defined as the ratio
of change in output voltage or collector voltage
(VCB) to the corresponding change in output
current or collector current (IC), with the input
current or emitter current (IE) kept at constant.
• The output resistance of common base amplifier
is very high.

39. Forward Current gain (α)
• The current gain of a transistor in CB configuration
is defined as the ratio of output current or collector
current (IC) to the input current or emitter current
(IE).
• The current gain of a transistor in CB configuration
is less than unity. The typical current gain of a
common base amplifier is 0.98.

40. Common Collector Configuration
• In this configuration, the base terminal of the transistor serves as
the input, the emitter terminal is the output and the collector
terminal is common for both input and output.
• Hence, it is named as common collector configuration.
• The input is applied between the base and collector while the
output is taken from the emitter and collector.
• Common collector configuration is also known as Grounded
Collector Configuration.

41. Common Collector Configuration

42. Input characteristics
input current IB is noted
• The input characteristics describe the relationship between input
current or base current (IB) and input voltage or base- collector
voltage (VBC).
• The output voltage VEC is kept constant at 3V and the input
voltage VBC is increased from zero volts to different voltage levels.
For each level of input voltage VBC, the corresponding input
current IB is noted.
• The output voltage VEC is increased from 3V to 5V and then kept
constant.
• While increasing the output voltage VEC, the input voltage VBC is kept constant at zero volts.
• Then the input voltage VBC is increased from zero volts to different voltage levels. For
each level of input voltage VBC, the corresponding

43. Output characteristics
• The output characteristics describe the relationship between output current or
emitter current (IE) and output voltage or emitter-collector voltage (VEC).
• The input current IB is kept constant at zero micro amperes and the output
voltage VEC is increased from zero volts to different voltage levels. For each
level of output voltage VEC, the corresponding output current IE is noted.
• The input current (IB) is increased from 0 μA to 20 μA and then kept constant at
20 μA. While increasing the input current (IB), the output voltage (VEC) is kept
constant at 0 volts.
• The output voltage (VEC) is increased from zero volts to different voltage levels.
For each level of output voltage (VEC), the corresponding output current (IE) is
recorded.

44. Output characteristics

45. Transistor h-parameters
• Input Impedance (hic):
hic =
• OutputAdmittance (hoc):
hoc =
• Forward Current Gain(hfc):
hfc =
• Reverse Voltage Gain(hrc):
hrc =

46. Common Emitter Configuration
• The configuration in which the emitter is connected
between the collector and base is known as a
common emitter configuration.
• The input circuit is connected between emitter and
base, and the output circuit is taken from the
collector and emitter.
• Thus, the emitter is common to both the input and
the output circuit, and hence the name is the
common emitter configuration.

47. Common Emitter Configuration

48. Base Current Amplification Factor (β)
• The base current amplification factor is defined as the
ratio of the output and input current in a common
emitter configuration.
• In common emitter amplification, the output current is
the collector current IC, and the input current is the base
current IB.
• In other words, the ratio of change in collector current
with respect to base current is known as the base
amplification factor. It is represented by β (beta).

49. Relation Between CurrentAmplification Factor
(α) & BaseAmplification Factor (β)
• The relation between Β and α can be derived as
We Known,
Now,

50. Relation Between Current Amplification
Factor (α) & Base Amplification Factor (β)
• Substituting the value of ΔIE in equation (1), we get,
The above equation shows that the when the α reaches to unity, then the β reaches to
infinity. In other words, the current gain in a common emitter configuration is very
high, and because of this reason, the common emitter arrangement circuit is used in
all the transistor applications.

51. Collector Current
• In CE configuration, the input current IB and the output
current IC are related by the equation shown below.

52. Collector Current
• If the base current is open (i.e.,IB = 0). The collector
current is current to the emitter, and this current is
abbreviated as ICEO that means collector- emitter
current with the base open.
• Substitute the valueΔIB in equations (1), we get,

53. Characteristics of Common emitter (CE) Configuration
The base to emitter voltage varies by adjusting the
potentiometer R1. And the collector to emitter voltage varied by
adjusting the potentiometer R2. For the various setting, the
current and voltage are taken from the milli-ammeters and
voltmeter. On the basis of these readings, the input and output
curve plotted on the curve.

54. Input Characteristic Curve
• The curve plotted between base current IB and the
VEB
base-emitter voltage is called Input
characteristics curve. For drawing the input
characteristic the reading of base currents is taken
through the ammeter on emitter voltage VBE at
constant collector-emitter current.

55. Input Characteristic Curve
• The curve for common base configuration is similar to a
forward diode characteristic. The base current IB increases
with the increases in the emitter-base voltage VBE. Thus the
input resistance of the CE configuration is comparatively higher
that of CB configuration.
• The effect of CE does not cause large deviation on the curves,
and hence the effect of a change in VCE on the input
characteristic is ignored.
• Input Resistance: The ratio of change in base-emitter voltage
VBE to the change in base current ∆IB at constant collector-
emitter voltage VCE is known as input resistance, i.e.,

56. Output Characteristic Curve
• In CE configuration the curve draws between
collector current IC and collector-emitter voltage
VCE at a constant base current IB is called output
characteristic.

57. Output Characteristic Curve
• In the active region, the collector current increases
slightly as collector-emitter VCE current increases. The
slope of the curve is quite more than the output
characteristic of CB configuration. The output
resistance of the common base connection is more than
that of CE connection.
• The value of the collector current IC increases with the
increase in VCE at constant voltage IB, the value β of
also increases.
• When the VCE falls, the IC also decreases rapidly. The
collector-base junction of the transistor always in
forward bias and work saturate. In the saturation region,
the collector current becomes independent and free from
the input current IB
• In the active region IC = βIB, a small current IC is not
zero, and it is equal to reverse leakage current ICEO

58. Output Resistance
• The ratio of the variation in collector-emitter
voltage to the collector-emitter current is known at
collector currents at a constant base current IB is
called output resistance ro.
• The value of output resistance of CE configuration
is more than that of CB

59. Comparison between CB, CE & CC Configuration

60. DC Load Line Analysis

61. DC Load Line Analysis
Applying Kirchhoff’s Voltage Law to output will get
VCC - Ic x RC - VCE = 0 -----------(1)
we can write the above equation as
Ic x RC = - VCE + VCC
Hence Ic = -1/ RC x (VCE - VCC)

62. DC Load Line Analysis
Case 1: If we put Ic = 0, then will get
VCE = VCC
Case 2: If we put VCE = 0,
then, IC = VCC / RC

63. DC Load Line Analysis

64. Transistor Biasing
• Applying external dc voltages to ensure that transistor
operates in the desired region
• Which is the desired region?
– For amplifier application, transistor should operate in
active region
– For switch application, it should operate in cut-off and
sat.

65. Types of biasing:
• Fixed bias
• Self bias
1. Fixed bias:
– Equations to consider are:
Applying KVL In Loop 1 ,
Vcc – IBRB – VBE = 0
Applying KVL In Loop 2 ,
Vcc – IcRc – VCE = 0

66. Voltage divider bias or Self bias
– Resistor RE connected between emitter
and ground
– Voltage-divider resistors R1 & R2
replace RB
– Circuit can be analyzed in two
methods:
• Exact method (using Thevenin’s
theorem)
• Approximation method
(neglecting base current)

67. Exact method:
– Input side of self-bias (Fig. a) transformed into Thevenin’s equivalent
circuit (Fig. b) where, RTH is the resistance looking into the terminals A &
B (Fig. c) and VTH is given by:

68. Self-bias circuit redrawn with input side replaced by
Thevenin’s equivalent :
Since β >> 1 and (β+1)RE >> RTH
Since IC is almost independent of β, Q-point is stable

69. Bias Stabilization and Stability factor
• Stabilization:
The process of making the operating point independent of
temperature changes and variations in transistor parameters is
known as stabilization.
Q-PT = (VCEQ, ICQ)

70. Bias Stabilization and Stability factor
• Causes of UNSTABILIZATION:
Q-PT will shift with an increase or decrease in IC.
Change in β ( No 2 transistors will have same β value)

76. THANK YOU