This document provides information about electronic devices and circuits, including energy band structures of insulators, semiconductors and metals; PN junction diodes; bipolar junction transistors; field effect transistors; and operational amplifiers. It discusses the construction, operation, characteristics and applications of these components. The key topics covered include intrinsic and extrinsic semiconductors, forward and reverse bias of PN junctions, transistor biasing configurations, JFET and MOSFET operation, and inverting and non-inverting op-amp circuits.

1. 1
Mrs.R.DEEBIKA,
Assistant Professor/EEE,
Kongunadu College of Engineering and Technology
20BE204-BASIC ELECTRICAL, ELECTRONICS AND
INSTRUMENTATION ENGINEERING
Unit IV
Electronic Devices and Circuits

2. Unit IV
Electronic Devices and Circuits
 PN Junction – Forward and Reverse Bias – Zener
Diode – Bipolar Junction Transistor –
Characteristics -Introduction to operational
Amplifier: Inverting Amplifier – Non Inverting
Amplifier.

3. ENERGY BAND STUCTURES AND CONDUCTION IN
INSULATORS, SEMICONDUCTORS AND METALS
 A very poor conductor of electricity is called an
insulator; an excellent conductor is a metal; and a
material whose conductivity lies between these two
extremes is a semiconductor.
 A material may be classified as one of these three
depending upon its energy-band structure.

4. ENERGY BAND STUCTURES AND CONDUCTION IN
INSULATORS, SEMICONDUCTORS AND METALS

5. ENERGY BAND STUCTURES AND CONDUCTION IN
INSULATORS, SEMICONDUCTORS AND METALS
 An INSULATOR is a material having extremely poor
electrical conductivity.
 The forbidden energy gap is large.
 Hence no electrical conduction is possible.
 Number of free electrons in an insulator is very
small, roughly about 107 electrons/m3.

6. ENERGY BAND STUCTURES AND CONDUCTION IN
INSULATORS, SEMICONDUCTORS AND METALS
 The conduction in METALS is only due to the
electrons.
 A metal has overlapping valence and
conduction bands.
 VB is only partially filled and the CB extends
beyond the upper end of filled valence band.

7. Classification of Semiconductor
1. Intrinsic Semiconductor(Pure)
2. Extrinsic Semiconductor(Impure)
Intrinsic Semiconductor
A semiconductor in an extremely pure form is known as Intrinsic
semiconductor.
The silicon(Si) and germanium(Ge) are two important intrinsic
semiconductors.
A missing electron in the valence band leaves a vacant space there,
which is known as a hole. Holes also contribute to electric current.

8. In an intrinsic semiconductor, even at room temperature, electron-
hole pair are created.
When an electric field is applied across an intrinsic semiconductor,
the current conduction takes place due to free electrons and holes.

9. Extrinsic Semiconductor
The current conduction capability of intrinsic
semiconductor should be increased and this can achieved
by adding a small amount of impurity.

10.  This can be achieved by adding a small amount of impurity to the
intrinsic semiconductor, so that it becomes impure or extrinsic
semiconductor. This process of adding impurity is known as
doping.
 Depending upon the type of impurity added, extrinsic
semiconductors can be divided in to two types.
1. N – type semiconductor.
2. P – type semiconductor.
N – Type Semiconductor
 A small amount of pentavalent impurities such as arsenic,
antimony or phosphorus is added to the pure semiconductor
(germanium or silicon crystal) to get N-type semiconductor.

11.  With the addition of pentavalent impurity a large number of free
electrons are made available in the conduction band thereby
increasing the conductivity of N – type semiconductor. As a result
of doping, the number of free electrons far exceeds the number of
holes in N – type semiconductor.

12. P-Type semiconductor
A small amount of trivalent impurity such as aluminium or boron is
added to the pure semiconductor to get the p- type semiconductor.
The addition of trivalent impurity a large number of holes are made
available in the valence band.
These positively charged holes increase the conductivity of P-type
semiconductor.

13.  As the number of holes is very much greater than the number
of free electrons in a P-type material, holes are termed as
majority carriers and electrons as minority carriers.
Conductivity of Semiconductor
 In a pure semiconductor, the number of holes is equal to the
number of electrons. Thermal agitation continues to produce
new electron hole pairs and the electron hole pair disappears
because of recombination.
 With each electron-hole pair created, two charge carrying
particles are formed. One is negative which is free electron
with mobility. The other is positive i.e., the hole with
mobility.

14. PN Junction Diode
In a piece of semiconductor material, if one half is doped by P-type
impurity and the other half is doped by N-type impurity, a PN
junction is formed.
The plane dividing the two halves or zones is called PN junction.
The N-type material has high concentration of free electrons, while
P-type material has high concentration of holes.
At the junction there is a tendency for the free electrons to diffuse
over to the P-side and holes to the N-side. This process is called
diffusion.
Anode Cathode

15. V–I Characteristics of a diode under forward bias
The V–I characteristics of a PN unction diode are shown below

16.  As the forward voltage is increased, for < , the
forward current is almost zero (region OA) because the
potential barrier prevents the holes from P-region and
electrons from N-region to flow across the depletion region
in the opposite direction.
 For > , the potential barrier at the junction completely
disappears and hence, the holes cross the junction from P-
type to N-type and the electrons cross the junction in the
opposite direction, resulting in relatively large current flow
in the external circuit.
Under Reverse Bias Condition
 When the negative terminal of the battery is connected to the
P-type and positive terminal of the battery is connected to
the N-type of the PN junction, the bias applied is known as
reverse bias.

17. Operation
A holes which form the majority carriers of the P-side move
towards the negative terminal of the battery and electrons which form
the majority carrier of the N-side are attracted towards the positive
terminal of the battery.
Electrons forming covalent bonds of the semiconductor atoms in the
P- and N-type regions may absorb sufficient energy from heat and
light to cause breaking of some covalent bonds.

18.  Under the reverse bias condition, the thermally generated holes in
the P-region are attracted towards the negative terminal of the
battery and the electrons in the N-region are attracted towards the
positive terminal of the battery.
 The minority carriers, electrons in the P-region and holes in the N-
region, wander over to the junction and flow towards their majority
carrier side giving rise to a small reverse current. This current is
known as reverse saturation current.
 The V-I characteristics under reverse bias

19.  The magnitude of reverse saturation current mainly depends upon
junction temperature because the major source of minority carriers
is thermally broken covalent bonds.
 The reverse voltage at which the junction breakdown occurs is
known as Breakdown Voltage.

20. PN Diode Applications
Rectifiers in D.C. power supplies.
Switch in digital logic circuits used in computers.
Clamping network used as D.C. restorer in TV receivers and
voltage multipliers.
Clipping circuits used as wave shaping circuits used in computers,
radars, radio and TV receivers.
Demodulation (detector) circuits.

21. Zener Diode
Equivalent Circuit of Zener Diode Model

22. Zener Diode
V-I Characteristics of Zener Diode

23. V-I Characteristics of Zener Diode
 Zener diodes are manufactured to have a very low
reverse bias breakdown voltage
 Since the breakdown at the zener voltage is so sharp,
these devices are often used in voltage regulators to
provide precise voltage references. The actual zener
voltage is device dependent. For example, you can buy
a 6V zener diode.

24. Breakdown in PN Junction Diodes
The diode equation predicts that, under reverse bias conditions, a
small constant current, the saturation current, k , flows due to
minority carriers, which is independent of the magnitude of the bias
voltage.

25. BIPOLAR JUNCTION TRANSISTOR
Introduction
 A Bipolar Junction Transistor (BJT) is a three terminal
semiconductor device in which the operation depends on the
interaction of both majority and minority carriers and hence the
name bipolar.
 BJT is used in amplifier and oscillatory circuits, and as a switch
in digital circuits.
 It has wide application in computers, satellites and other modern
communication systems.

26. CONSTRUCTION
 The BJT consists of a silicon (or germanium) crystal in
which a thin layer of N-type silicon is sandwiched between
two layers of P-type silicon. This transistor is referred to as
PNP.
 Two types of BJT are NPN and PNP transistor.

27.  The three portions of the transistor are Emitter, Base and
Collector, shown as E, B and C, respectively. The arrow on the
emitter specifies the direction of current flow when the EB
junction is forward biased.

28. TRANSISTOR BIASING
 The emitter-base junction is forward biased and collector-base
junction is reverse biased. Due to the forward bias on the
emitter-base junction an emitter current flows through the base
into the collector. Though, the collector-base junction is reverse
biased, almost the entire emitter current flows through the
collector circuit.

29. OPERATION OF NPN TRANSISTOR
 The forward bias applied to the emitter base junction causes a
lot of electrons from the emitter region to crossover to the base
region.
 The base is lightly doped with P-type impurity, the number of
holes in the base region is very small and hence the number of
electrons that combine with holes in the P-type base region is
also very small. Hence a few electrons combine with holes to
constitute a base current.
 The remaining electrons crossover in to collector region to
constitute a collector current.
 Base and collector current summed up i.e.,

30. The magnitude of emitter current related by
OPERATION OF PNP TRANSISTOR
• The forward bias applied to the emitter – base junction of a PNP
transistor causes a lot of holes from the emitter region to crossover
to the base region as the base is lightly doped with N – types
impurity.

31.  A few holes combined with electrons to constitute a base current
and remaining holes crossover in to the collector region to
constitute a collector current.
 Collector and base current summed up i.e.,
 The magnitude of emitter current are related by

32. Types of Configuration
 The transistor has input, output and common to input and
output terminal. A transistor can be connected in three
configurations.
i. CB configuration.
ii. CE configuration.
iii.CC configuration.

33. CB configuration
In this configuration, emitter is the input terminal, collector is the
output terminal and base is the common terminal.
Input Characteristics
To determine the input characteristics, the collector-base voltage is
kept constant at zero.
The emitter current is increased from zero in suitable equal steps
by increasing .

34. • When collector base voltage is equal to zero and the emitter-base
junction is forward biased as shown in the characteristics, the
junction behaves as a forward biased diode so that emitter current
increases rapidly with small increase in emitter-base voltage.
• When collector base voltage is increased keeping emitter base
voltage constant, the width of the base region will decrease.

35. Output characteristics
 To determine the output characteristics, the emitter current is kept
constant at a suitable value by adjusting the emitter-base voltage.
Then collector base voltage is increased in suitable equal steps and
the collector current is noted for each value of emitter current.

36. Saturation Region
 Collector Base voltage VCB is negative.
 CB junction is forward biased and a small change in
VCB results in larger variation in collector current.
Active Region
 Collector current is almost constant
 EB – Forward biased
 CB- Reverse biased
Cut off region
IE =0,
Both junction are reverse biased

37. CE configuration
Input characteristics
To determine the input characteristics, the collector to emitter
voltage is kept constant at zero volt and base current is increased
from zero in equal steps by increasing base emitter voltage in the
below circuit.

38.  The value of base emitter voltage is noted for each setting of
base current. This procedure is repeated for higher fixed
values of and the curves of . are drawn. The
input characteristics thus obtained are shown below.

39. Output characteristics
 To determine the output characteristics, the base current is
kept constant at a suitable value by adjusting base-emitter
voltage,
 The magnitude of collector-emitter voltage is increased in
suitable equal steps from zero and the collector current is noted
for each setting

40. CC configuration

41. Input characteristics
To determine the input characteristics, is kept at a suitable
fixed value. The base-collector voltage is increased in equal
steps and the corresponding increase in is noted.

42. Output characteristics
The output characteristics are the same as those of the common
emitter configuration.

43. FIELD EFFECT TRANSISTORS
Introduction
 The FET is a device in which the flow of current through the
conducting region is controlled by an electric field. Hence the
name Field Effect Transistor (FET). It is also said to be unipolar
device.
The FET can be classified into two types.
a) Junction FET(JFET)
b) Metal Oxide Semiconductor FET(MOSFET) or Insulated Gate
FET (IGFET)or Metal Oxide Silicon Transistor(MOST).
JFET has been classified in to two types
1. N-channel JFET with electrons as the majority carriers.
2. P-Channel JFET with holes as the majority carriers.

44. Construction of N-channel JET
It has N-type bar which is made of silicon. Ohmic contacts are
made at the two ends of the bar are called drain and source.
Source (S)
This terminal is connected to the negative pole of the battery.
Electrons which are the majority carriers in the N-type bar enter
the bar through this terminal.
Drain (D)
This terminal is connected to the positive pole of the battery. The
majority carriers leave the bar through this terminal.

45. Gate (G)
Heavily doped P-type silicon is diffused on both sides of the N-
type silicon bar by which PN junctions are formed. These layers
are joined together called Gate.
Channel
The space between gate through which majority carriers pass.

46. Basic Construction of JFET

47. Circuit Symbols and notations

48. Operation of JFET
GG

49. Transfer Characteristics

50. i. As is increased from zero, increases along OP, and
the rate of increases of with decreases as shown
below
ii. When becomes maximum. When is
increased beyond the length of the pinch-off or
saturation region increases.
Drain Characteristics

51.  When is negative and is increased, the gate is maintained
at a negative voltage less than the negative cut-off voltage, the
reverse voltage across the junction is increased.
 The drain current is controlled by the electric field that extends
into the channel due to reverse biased voltage applied to the gate.
Hence, this device known as Field Effect Transistor.

52. MOSFET

53. MOSFET

54. Enhancement MOSFET
Construction
The construction of an N-channel enhancement MOSFET is
shown below

55.  Two highly doped regions are diffused in a lightly doped
substrate of P-type silicon substrate. One region is called the
source S and the other one is called the drain D.
 The metal area of the gate, in conjunction with the insulating oxide
layer of and the semiconductor channel forms a parallel plate
capacitor. This device is called the insulated gate FET.
Operation
 If the substrate is grounded and a positive voltage is applied at the
gate, the positive charge on G induces an equal negative charge on
the substrate side between the source and drain regions.
 The direction of the electric field is perpendicular to the plates of
the capacitor through the oxide.

56.  The negative charge of electrons which are minority carriers in
the P-type substrate forms an inversion layer.
 The drain current is enhanced by the positive gate voltage as
shown

57. Depletion MOSFET
The construction of an N-channel depletion MOSFET and circuit
symbol for an N-channel and a P-channel depletion MOSFET are
shown below

58. The drain D at a positive potential with respect to the source, the
electrons (majority carriers) flow through the N-channel from S to
D.
The introduction of the positive charge causes depletion of mobile
electrons in the channel. Thus a depletion region is produced in the
channel.

59.  The depletion MOSFET may also be operated in an enhancement
mode. It is only necessary to apply a positive gate voltage so that
negative charges are induced into the N-type channel.
 As the depletion MOSFET can be operated with bipolar input
signals irrespective of doping of the channel, it is also called as
dual mode MOSFET.

60. OPERATIONAL AMPLIFIER

61. Circuit Symbol

62. Equivalent circuit of OP-Amp

63. Characteristics of Ideal OP-Amp

64. Block Diagram of Operational Amplifier

65. Operational Amplifier IC 741

66. Inverting Amplifier

67. Inverting Amplifier

68. Non Inverting Op-Amp

69. Non - Inverting Amplifier

70. Digital to Analog Converters

71. Digital to Analog Converters

72. Binary Weighted Resistor DAC

73. Binary Weighted Resistor DAC

74. R – 2R Ladder

75. R – 2R Ladder

76. R – 2R Ladder

77. Equivalent circuit of practical op-Amp

78. Non inverting Op-Amp