SEMICONDUCTOR DEVICES
Band theory of metals:
In an atom, there are fixed energy level. If two atoms are brought closer due to repulsion
between the valence electron, their energy levels are changed. Similarly, if three atoms are
brought closer due to mutual repulsion between valence electrons, the energy levels are
further changed. Thus, due to mutual between valence electron, in a small space a range of
energy levels are seen forming continuous structure called energy bond. It is divided into
three regions:
(i) Valence band (VB): The region of energy band where valence electrons are present is
called valence bond.
(ii) Conduction bond (C.B): The region of energy bond where free electrons are present is
called conduction bond.
(iii) Energy gap (Eg): The region of energy bond which separates valence bond and
conduction bond is called energy gas.
Difference between conductor, semi conductor and Bad conductor:
Conductor
Semi conductor
Bad conductor
Those substances whose
Those substance whose
1. Those substance whose
energy gap of energy bond is energy gap of energy gap of energy gap of energy bond is
energy band is small is called wide are called bad
zero are called conductor.
semi – conductor. Eg: Silicon, conductor. Eg: Diamond,
Eg: Copper, Iron, etc.
germanium, etc.
glass, etc.
2. It has positive temperature It has – ve temperature
coefficient. Ie. αt = + ve
coefficient. Ie. αt = -ve
3. At normal condition V.B.
and C.B. are partially filled.
It has – ve temperature
coefficient. Αt = - ve.
At normal condition V.B. is
At normal condition V.B. and
completely filled but C.B. is
C.B are partially filled.
completely empty.
Hole and electron current:-In semiconductor, the valence electron moves from the end of
negative potential to the end of positive potential and the hole moves from the end at
positive potential to the end at negative potential is called hole current.
The electric current set up in the semi – conductor due to the flow of the free electron is called
electron current. Now, let us consider the semiconductor is connected to a battery. The free
electron in the conduction bond will be attracted by the anode of battery, it neutralizes a
positive charge on it. At the same time, an electron leaves the cathode of the battery and
enters into the semi conductor to take the place of the previous electron. In this way, if a
large number of electrons an electric current is set up in the circuit.
Consider atoms (Say A, B, C, D, etc) in the semi conductor. Also consider the electron of A
has jumped from valence bond to conduction bond. As a result a hole has been created in
A. The valence electron in B and C are still in valence bond. If the battery is connected, the
valence electron of atom C is pushed towards atom B and that of B is pushes towards the
atom A. The result is that the valence electron of the atom B moves to the hole in the atom
A and a hole is created in B. The valence electron in C jumps into the hole in B and a hole is
created in atom C.
Page |1
This way is a hole is created of the extreme and of the semiconductor which is connected to
the cathode of the battery; an electron from the cathode enters into the hole neutralizes the
atom. At the same time, the anode of the battery detaches an electron from an atom which
lies at the other extreme end of the semiconductor. As a result, a new hole is created which
moves towards the cathode.
Semiconductor:-A semiconductor is a material whose electrical conductivity lies between
those of conductors and insulator. The semi – conductor having tetravalent atoms and
found in nature is called Intrinsic or pure semi conductors. E.g: Silicon, Germanium, etc.
The semi conductor formed by addition of impurities like trivalent or pentavalent atoms to
the intrinsic semi conductor is called extrinsic semi conductor. Eg: SiB, GeP, etc. Doping
(semiconductor), intentionally introducing impurities into an extremely pure
semiconductor to change its electrical properties. The process of adding a measured
quantity of a trivalent or a pentavalent impurity to a pure semi conductor is called doping
in a semi – conductor.
In semiconductor production, doping intentionally introduces impurities into an extremely
pure (also referred to as intrinsic) semiconductor for the purpose of modulating its
electrical properties. The impurities are dependent upon the type of semiconductor. Lightly
and moderately doped semiconductors are referred to as extrinsic. A semiconductor doped
to such high levels that it acts more like a conductor than a semiconductor is referred to
as degenerate. In the context of phosphors and scintillates, doping is better known
as activation.
P-N JUNCTION: - When a P-type crystal is joined with a N-type crystal in such a manner
that crystal structure remains continuous then this structure is called as P-N Junction.
Formation of P-N junction: - Diffusion method is used to form a P-N Junction. In this
method an element of III group (like Boron) is coated on a slice of N-type semiconductor
called wafer or an element of V group (like phosphorus) is coated on P-type semiconductor.
When this semiconductor is heated at a high temperature (about 500°C) the impurity is
diffused inside the semiconductor. Diffusion is more at surface and decreases as the depth
increases. The depth up to which the diffusion takes place, a junction is formed which is
called P-N Junction. On the one side of this junction there is P-type semiconductor and on
the other side there is N-type semiconductor.
What happens at the time of formation of P-N Junction (formation of depletion region
and potential barrier): As soon as a junction is formed the holes from p-region diffuse
towards n- region and electron from n- region diffuse towards p-region due to the high
concentration of holes and electron into two different regions. In the vicinity of junction the
Potential Barrier
P-Type
N-Type
Electrons
majority carrier
Holes
majority carrier
Depletion region
Immobile +ve ions
Immobile - ve ions
Page |2
electrons and holes recombines with each other and vanishes, due to which there is a excess
of immobile +ve ions in n-region and –ve ions in p-region. Thus an electric field and hence
a potential difference called potential barrier is developed across the junction which stops
the further diffusion of holes and electrons. The region free form charge-carriers on both
side of junction is called depletion region or space charge region.
The thickness of the depletion region is of the order of 10 -6 meter while the potential barrier
is about 0.7 volt. Therefore
P
N
0.7
ElectricField  6  7  10 5 Vm 1
10
Biasing of p-n junction: B
A
(I)
Forward Bias: - When p-region of a p-n
junction is joined to the (+) ve pole of a battery and nregion to -ve pole then the junction is said to be
forward biased.
Action of p-n junction: - When the p-n junction is
Depletion layer
made forward bias the (+) ve pole of the battery repels
the holes towards n-region and the (-) ve pole repels
FORWARD BIASING
the electron towards p-region. Due to which the
electrons and holes enter the depletion region and the thickness of depletion region
decreases. If the external potential is greater than the potential barrier then near the
junction electrons recombine with holes. For each electron-hole combination that takes
place near the junction, a covalent bond breaks in p-region near the positive pole of battery.
Due to which electrons and holes are produced in pair, the electron is captured by the (+)
ve terminal, while the hole moves towards the junction. At the same time an electrons
enters the n-region from the –ve terminal of the battery, thus a forward current flows in the
circuit due to the flow of electrons and hole.
During the forward bias the applied D.C. voltage opposes the potential barrier due to
which the thickness of the depletion layer decreases. Thus p-n junction offers low resistance
in forward bias.
(II)
Reverse Bias: - When p-region of a p-n
junction is joined to the (-) ve pole of a battery and nP
N
region to +ve pole then the junction is said to be
reversed biased
Action of p-n junction: - when p-n junction is
reversed biased, the –ve pole of the battery attracts
the holes present in P-region, while the +ve pole of
the battery attracts the electrons present in the nregion. Thus the electrons and holes get away from
the junction and the thickness of depletion region
increases. But a very small current flows through the
REVERSE BIASING
junction due to the recombination of minority
carriers. This current is called as reverse current. If the reverse bias voltage is made very
high, all the covalent bonds near the junction break and a large number of electron-hole
Page |3
pairs are created due to which reverse current increases abruptly. This phenomenon is
called avalanche breakdown and the reverse voltage at which this phenomenon occurs is
called as reverse break down voltage or zener voltage which depends upon the density of
impurity atoms. Due to the over heating at this voltage, the p-n junction may be damaged.
During the reverse bias the applied D.C. voltage aids the potential barrier due to
which the thickness of the depletion layer increases and hence it offers the high resistance
in reverse bias.
p
n
Symbol of p-n junction diode:Characteristics of p-n junction: -There is two type of characteristics(I) forward bias Characteristics- First of all makes the connection according
to the circuit shown in fig.1. By changing the forward voltage with the help of potential
divider note down the corresponding forward current and plot the graph between forward
voltage and forward current. The graphs so obtained are called as forward characteristic
curve of p-n junction.
If
Vf
R
h
Forward Current If
N
BATTER
Y
P
+
Knee Voltage
0.2 0.4 0.6 0.8 1.0
FORWARD VOLTAGE Vf
From the graph it is clear that initially there is no current. When the applied voltage is less
than the potential barrier, the current flow through the junction is very small. As the
forward voltage increases above the potential barrier, current increases approximately
linearly. When the forward voltage is equal to voltage of potential barrier then the curve
becomes like a knee and called as knee voltage. At this voltage the thickness of depletion
layer becomes negligibility and the diffusion of electrons and holes across the junction take
place easily i.e. the p-n junction offer low resistance when it is forward bias and the
resistance is of the order of 100 ohm.
(II) Reverse Characteristic Curve: -Make the connection according to the circuit shown in
the following figure. Change the reverse voltage and note the corresponding reverse
current. The graph plotted between reverse voltages and reverse current is called as reverse
bias curve. Practically in reverse bias there is no current if the applied voltage is low but a
very small flow due to minority carriers. On increasing the reverse
REVERSE VOLTAGE
Page |4
Reverse Current Ir (
Zener
Voltage
Voltage to a very high value, the current increases abruptly, which is clear from graph. It is
due to the fact that at very high voltage, the entire covalent bond near the junction is
broken. Due to which a large number of holes & electrons are liberate and the
corresponding voltage is called as Zener voltage. In reverse bias the thickness of depletion
layer increases due to which the further diffusion of charge carriers stops and no current
flows through the junction. Thus in reverse bias the junction offers very high resistance.
DYNAMIC RESISTANCE: - The ratio of the small change in voltage to the small change in
the current is called as dynamic or a.c. resistance of the junction diode. It is represented by Vd.
Vd 
V
I
The region of the characteristic curve where dynamic resistance is almost independent of
the applied voltage is called the linear region of junction diode.
Junction diode as Rectifier: - An electronic device, which converts a.c. power in to
D.C. power, is called rectifier.
Half – wave Rectifier: - A rectifier, which rectifies only one half of each a.c. input supply
cycle is called half wave rectifier.
Principle: - It is based on the principle that the diode offers low resistance when it is
forward bias and high resistance when it is reversed bias i.e. current can flow through the
diode when it is forward biased.
A.C.
Input
Voltage
D.C.
0utput
Voltage
Arrangement:- The p-region of the junction diode is joined to the one terminal of the
secondary coil of a step down transformer and the load resistance is joined between nregion and the IInd terminal of the secondary coil.
Working:- Let during the first half cycle of the input a.c. upper end i.e. point S1 of
secondary is at +ve potential and the lower end i.e. point S2 is at –ve potential. Thus the
diode is forward bias. During first half cycle and current flows through diode in
loadresistance from C to D.
During the next half cycle the upper end becomes –ve and lower end becomes +ve
and thus the diode gets reverse biased and no current fows through it. In the next half cycle
diode gets forward biased and current flows through it from C to D and this process
repeated again and again. The current obtain in output is discontinuous and pulsating d.c.
due to which there is a huge loss of energy.
Page |5
Full-wave Rectifier:- A rectifier which rectifies both halves of the a.c. input is called a full
wave rectifier.
Principle:- It is based on the principle that the diode offers low resistance when it is
forward bias and offers high resistance when it is reverse biased.
A.C.
Input
Voltage
D.C.
0utput
Voltage
Arrangement:- The a.c. supply is fed across the primary coil P of a step down transformer.
Two two ends of the secondary coil S of the transformer are connected to the p- regions of
the junction diodes D1 and D2 . A load resistance RL is connected beteen the n-regions of
the two diodes and the ncentral tapping of the secondary coil. The out put d.c. is obtained
across the load reistance.
Working:-Suppose that during first half of the input, the upper end S1 of the secondary is
at + ve pot. and lower end S2 is at (–) ve pot. So the diode D1 gets forward bias and D2 gets
reverse bias hence current flows through D1 in load resistance from C to D. During the next
half cycle S1 becomes –ve and S2 becomes +ve and hence D1 gets reverse bias and D2 gets
forward bias. Thus the current flows through D2 from C to D in load resistance.
Hence the full wave rectifier, rectifies the both halves of a.c. The output d.c. is
continuous but pulsating. To reduce the fluctuations, filter circits are used in output circits.
Electrolytic condenser and zener diodes are use to reduce the fluctuations of d.c.
Different types of junction diode :(I) Zener diode:- A specially designed diode in which P and N region are heavily dopped
due to which the depelation layer junctioin width is small and the junction field ie potential
barrier is high and it can operate continuously, with out being damaged in the region of
reverse breakdown voltage, is called zener diode.
An important application of zener diode is that it can be used as voltage regulator. The
regulating action takes place because of the fact that in reverse breakdown region, a very
small change in voltage produces large change in current. This causes a sufficient increase
Page |6
Output Voltage (VO)
in voltage drop across the resistance to lower voltage back to normal. Similarly, when the
voltage across the diode tends to decrease, the current through diode goes down out of
proportion so that voltage drop across the resistor is much less and it raises voltage back to
normal.
Hence the output voltage remains constant.
Regulated Output Voltage
VZ
VZ
Input Voltage (Vi)
Question-What is a photo diode? Explain its working principle. Also give some uses.
Photo diode: - A junction diode made from light sensitive semiconductor is called a photo
diode.
mA
LIGHT
LIGHT
Reverse bias
P
RL
N
Volts
I1
I2
I3
I4
I4 > I3 > I2 > I1
µA
Photo diode is always reverse bias. When no light falls on it, a small reverse current flows
through the junction. This current is due to the thermally generated electron-hole pairs and
is called as dark current. When the photodiode is illuminated with light photons of energy
hν>Eg then it ejects the valence electrons due to which the current increases and becomes
maximum. This current is called as saturation current. On increasing the light intensity, the
saturation current increases.
A photodiode can turn its current ON and OFF in nanoseconds. So it can be used as a
fastest photo detector.
Uses:
1. In detection of optical signals.
2. In demodulation of optical signals
3. In light-operated switches
4. In speed reading of computer punched cards.
5. In electronic counters.
Light Emitting Diode (LED): - A light emitting diode is simply a forward biased p-n
junction made of gallium arsenide or indium phosphide and emits spontaneous light
radiation. When a LED is made forward bias then the energy is released due to the
recombination of electrons and holes, falls in visible region or infrared region of EM
spectrum.
Page |7
Advantages over conventional incandescent lamps:
LIGHT
1. Low operational voltage and less power
consumption.
2. Fast action and no warm up time required.
3. Long life and ruggedness.
4. Light emitted is nearly monochromatic
Uses:
1. Infrared LED’s are used in burglar alarm systems.
2. In optical communication system.
3. LED’s are used in numeric displays (in watches and calculators).
4. In optical mouses for the computers.
5. In remote controls
Solar cell: - It is a junction diode which converts solar energy into electrical energy and is
based on photovoltaic effect (generation of voltage due to bombardment of photons).
It consists of a p-n junction made of Si or GaAs. A very thin layer of n-type semiconductor
is grown over a p-type semiconductor by using diffusion method. (So that the energy
falling on the diode not greatly absorbed before reaching to junction)
Working: When light is incident on p-n junction each photon absorbed creates an electron
and a hole. If is because the electron acquires sufficient energy to move from valence to the
conduction band. Due to barrier voltage electrons moves towards n region and holes
towards the p region. As a result the two regions gets opposite potential and emf is
developed across the terminals of the diode.
This photovoltaic emf can be used as ordinary cell in the electrical circuits.
Applications: [1] Solar cells are used in wrist watches and calculators.
[2] They are used to produce power in artificial satellites and space craft.
Transistor: - When a thin layer of one type of semiconductor is sandwiched between the two thick
blocks of another type of semi conductor then obtained structure is called a transistor. These are used
as an amplifier as well as an oscillator. These are of two types: (I)
NPN transistor: - A junction transistor in which a thin layer of p-type semiconductor
is sandwiched between two layers of n-type semiconductor is known as NPN
transistor.
N
P
N
C
E
E
C
Emitter
Collector
B
(II)
B
PNP Transistor: - A junction transistor in which a thin layer of N-type
semiconductor is sandwiched between two layers of P-type semiconductors is
known as PNP transistor.
E
C
P
N
P
C
E
Emitter B
Collector
B
Page |8
In a transistor base is lightly doped and very thin. The region, which is lightly doped and
very thin, is called as Base. The region, which is highly doped, is called emitter while the
remaining one is called collector. When a transistor is used in a circuit, base emitter junction
is always forward bias while the collector base junction is reverse bias.
Action of Transistor: (a)
Action of n-p-n Transistor: - The emitter base junction is made forward bias by
N
P
N
E
C
IE
Emitter
IB
IC
Collector
B
VEE
VCC
using a battery VEE while the collector base junction is made reversed bias by using the VCC.
The –ve pole of battery VEE repels the electrons in emitter region (as majority carrier in nregion) towards base. Since the base is very thin and lightly doped, hence about 95%
electrons cross over the base region and entered the collection region where they are
attracted by the +ve pole of the battery VCC. As soon as an electron enters the +ve pole of
the battery VCC, at the same time an electron enters the emitter region from the –ve pole of
the battery VEE and this process is carried out continuously. About 5% electrons
recombined with holes in base region. For each recombination a covalent bond breaks
which creates the hole and electron in pair. Electron enters +ve pole of V EE through B and
hence base current IB flows which is very small.
If IE, IC and IB are the emitter, collector and base current then (According to Kirchhoff’s 1st
law)
IE = I B + I C
It may note that in n-p-n transistor current flows due to the flow of electrons in and outside
of transistor.
Action of P-N-P transistors:P
N
Legends: -
P
E
HoleElectron-
C
IE
Emitter
IB
B
Collector
IC
Holes
VEE
VCC
Electrons
Page |9
Characteristics of n-p-n transistor in Common Emitter configuration: - Common Emitter
characteristics of a transistor are the graph plotted between the voltage and the current
when emitter is earthed, base is used as input terminal and the collector as output terminal.
Ic
C
mA
IB
n-p-n
µA
B
E
VCC
VBB
+
+
VBE
VCE
_
_
N-P-N Transistor: -The base emitter circuit is made forward biased by using a battery VBB
while the emitter, collector circuit is made reversed bias by using battery VCC. To draw the
characteristic the circuit arrangement is shown in the above figure in which a n-p-n
transistor is used.
A transistor has two types of characteristics.
D. C. Input characteristics: - Keeping VCE at constant voltage, charge VBE (Base emitter
voltage) and note down the corresponding values of base current. Now for some other
value of VCE , find out the change in base current for the corresponding change in
VBE. Now plot the graph between VBE and IB at different constant value of VCE. The graphs
so obtained are called as input characteristics.
A.C.I input resistance:- The ratio of the change in the emitter base voltage (Δ VBE) to the
change in base current (Δ IB) at the constant VCE is called as a.c. input resistance. It is
 VBE
Rin  
 I B


VCE
denoted by Rin.
(2) Output characteristics:- The graphs plotted between emitter collector voltage and the
collector current (IC) at different constant values of base current (IB). Following result may
be drawn from the output characteristic curves(I) The collector current changes rapidly in beginning but soon it becomes saturated.
(II) The saturation current increases on increasing the base current.
(III) In audio frequency amplifiers the linear part of the output characteristics is used in
order to obtain undistorted output.
P a g e | 10
 VCE
Rout  
 I C


 IB
Output resistance :-The radio of the change in emitter collector voltage to the change in
collector current at the constant base current. It is denoted by Rout.
Transfer characteristics:-The graph plotted between collector current (IC) and the base
current (IB) at different constant values of collector voltages (VCE).
Current gain :- The ratio of change in collector current to the change in base current at
constant collector – emitter voltage is called as current gain. It is also called as current
 I

C
 
 I 

B V

CE
transfer ratio. It is denoted by: (VCE) =3v
(VCE) = 3V
IC (mA)
IB = 250
IB = 200
IB = 150
IB = 100
IB = 50
IC (m A)
IB (mA)
(VCE) = 2v
(VBE)
IB (mA)
TRANSFER
(VCE)
INPUT CHARACTERISTICS
0UTPUT CHARACTERISTICS
CHARACTERISTICS
TRANSISTOR AS AN AMPLIFIER:- An amplifier is a device which is used for increasing the
amplitude of variation of alternating voltage or current or power.
A transistor can be used as an amplifier. There are three configurations1. Common base amplifier
2. Common emitter amplifier
3. Common Collector Amplifier
Common Emitter Amplifier: - In common emitter configuration emitter is common to both
the base and collector.
Amplifier circuit using n-p-n transistor: - The emitter is common to both the input and
output. The emitter is made forward bias by the battery VBB and collector emitter circuit is
IC
C
Amplified
Voltage Signal
IB
Input Voltage
Signal
n-p-n
B
RL
E
IE
ICRL
VCE
VBB
VCC
P a g e | 11
made
reversed bias by the battery VCC Thus the input resistance is low and the output resistance
is high. The low input voltage signal is plied across emitter base circuit and amplified
output voltage is obtained across collector emitter circuit.
Let IE , IB and IC are the emitter base and collector current so according to Kirchoff’s lawIE = IB + IC ---------------------- (1)
If RL is the load resistance then ICRL will be voltage drop across it. If VCE is the voltage
across emitter collector then
VCE =VCC – IC RL ---------------------(2)
The variation in input signal voltage cause the variation in emitter current which produce
the variation in collector current and hence in collector voltage. These variations in collector
voltage appear as amplified output-voltage. The input signal and output signal are in
opposite phase.
Phase relation between input and output signals: - The input signal and the output signal
are in opposite phase, which can be explained as belowWhen an a.c. signal is fed to the input circuit, the forward bias increases during
positive half cycle of the input. This results in increase in IC and consequent decrease in VCE
, thus during positive half cycle of the input, the collector becomes less positive.
During negative half cycle of the input, forward bias decreases, therefore, the value of
IE and IC also decreases and VCE would increase making the collector more positive. In
common emitter amplifier, thus there is 180ºout of phase amplification.
Current Gain: - It is defined as the ratio of the change in collector current to the change in


I C
I B
base current at constant emitter base voltage. It is denoted β.
Voltage Gain: - It is defined the ratio of the change in the output voltage to the change in
input voltage. It is denoted by AV.
Since β > α so the voltages
amplifier is very large as
common base amplifier.
AV 
Vout
I C  Rout

Vin
I B  Rin
AV 
I C  Rout
I B  Rin
A V   ac  Resistance Gain
P a g e | 12
gain in common emitter
compared to that in
A.C. Power Gain:-It is defined as the ratio of change in output power to change in the input
Change in output power
Change in input power
PO
AP 
Pi
AP 
AP 
I c 2  RO
I B 2  Ri
AP   2  resistance gain
power. It is denoted by AP i.e.β> α so the power gain in common emitter amplifier is very large as compared to that in
common base amplifier.
Trans conductance:- It is defined as the ratio of the change in the collector current (ΔIC ) to
the change in emitter base voltage
(ΔVBE) at constant collector voltage. It is


I c
denoted by gm i.e.
g 


 VBE  VC E
m
gm 
I C
I B

I B
VBE
g m   ac 
1
Rin
Relation Between α and β: We know that
I E  I B  IC
or I E  I B  I C
divide by I C on both sides
I E
I B

1
I C
I C
1

1

1




1

1

1
1
1

  
1
1
Transistor as an oscillator: -An oscillator is a device which converts direct current into
alternating current and produces high frequency undamped. A transistor can be used to
produce undamped oscillations.
The base oscillatory circuit consists of an inductance and capacitance called tank circuit.
Due to resistance of circuit, a part of energy is dissipated, therefore, amplitude of
oscillations goes on decreasing with time and damped oscillations are produced.
P a g e | 13
In order to maintain these oscillations, energy is supplied to circuit at the right moment
and in the right direction using a feedback arrangement. The feedback arrangement
consists of primary P and secondary with variable
capacitor C of suitable range. The secondary coil of
inductance L. The inductance L and capacitance C
constitute tank circuit.
Working: - When the tapping key K is pressed for a
moment, a small current starts flowing through the
coil L1 due to the change of current, an emf is
induced in inductor L. Due this induced voltage
the emitter current and hence the collector current
increases. Due to the increase in collector current
the magnetic flux linked with L & L1 increases; thus
the voltage induced in L also increases and hence
forward bias is further increased which increases IC and IE. This process continues until the
induced emf across the inductor attains a saturation value. During this process the upper
plate of the capacitor gets +ve charge. When induced emf attains saturation value the
induced emf becomes zero. Now the capacitor discharges through L; as a result emitter
current decreases and hence collector current also decreases. The decreasing collector
current will induced emf in inductor L in the reverse direction, which decrease the emitter
current and hence collector current. This process continues till the collector current reduces
to zero. Now the mutual induction stops playing its role. At this stage the lower plate of
the capacitor C will get + ve charge and discharges through L. Thus the emitter current
and hence the collector current again start to increasing i.e. the process gets repeated and
the collector current oscillates between a maximum and zero value. The repeated process
generates oscillations of constant amplitude and the relation gives freq.
ʋ = 1/ 2∏√ LC
By changing the value of C the freq. of the oscillations can be changed
TRANSISTOR AS A SWITCH
A transistor can be used as a switch; the following fig (1) shows the circuit diagram of a
base biased n-p-n transistor in CE configuration states Here RB is a resistor in the input
circuit and Rc in the output circuit.
Applying Kirchhoff’s rule to the input and output circuits separately, we get
VBB = IBRB + VBE = Vi -----------------------------(1)
VCE=VCC—ICRC = Vo ------------------------------(2)
The voltage VBB has been regarded as the dc input voltage Vi and VCE as the dc output
voltage V0.
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Fig. 2 shows typical output voltage (V0) — input voltage (Vi) characteristic, called the
transfer characteristic of the base biased transistor. It has three well-defined regions as
follows:
1. Cutoff region: When Vi increases from zero to a low value (less than 0.6 V in case of a Si
transistor), the forward bias of the emitter-base junction is insufficient to start a forward
current i.e. IB = 0 and hence Ic = 0. The transistor is said to be in the cutoff region. From
equation (1), the output voltage Vo = Vcc.
2. Active region: When Vi increases slightly above 0.6 V. a current Ic flows in the output
circuit and the transistor said to be in the active state.
3. Saturation region: When Vi becomes very high , a large collector current Ic flows which
produces such a large potential drop across load resistance Rc that the emitter-collector
junction also gets forward biased and output voltage V0 decreases to almost zero. Now the
transistor is said to be in the saturation state because it cannot pass any more collector
current Ic.
Switching action of a transistor: A transistor can be used as a switch if it is operated in its
cutoff and saturation states only. A switch circuit is designed in such a manner that the
transistor does not remain in the active state. As long as the input voltage is low and unable
to forward-bias the transistor, the output voltage V0 (at Vcc) is high. If Vi is high enough to
drive the transistor into saturation, then V0 is low, nearly zero. Thus when the transistor is
not conducting (in cutoff region), it is said to be switched off and when it is driven into
saturation, it is said to be switched on.
DIGITAL ELECTRONICS
In these electronics circuits, the current or voltages will have only two values, High (1)
and Low (0). In digital circuits, the electrical pulses of two levels only are used as signal
voltages.
Logic Gates:
A gate is a digital circuit which is used to perform certain specific function. The three
basic logic gates are:
a.
OR gate
b.
AND gate
c.
NOT gate
All other logic gates can be formed by combination of these three gates.
Truth Table:-It is table that indicate all possible combinations of input signals and their
output.
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Boolean Algebra:- This is the algebra which can be applied to logic gates based on Binary
number system.
OR Gate:- It is a two input single output gate. The output is one if any of the two inputs or
both the inputs are one.
The truth table and symbol of OR gate are:
A
B
Y
0
0
0
0
1
1
1
0
1
1
1
1
The circuit diagram for OR gate is:
The diodes used are considered to be ideal diodes ie. during forward bias they offer
zero resistance and during reverse bias they offer infinite resistance.
Case I: When A = 0, B = 0: Here both the diodes are in off state. There is no current that
flows through R, thus output voltage is Y=0.
Case II: When A = 0, B = 1: In this case D1 is in off state and D2 is forward biased. The
current flows through D2 and sets up a potential difference of 5V across it, so Y = 1.
Case III: When A = 1, B = 0: in this case diode D1 is forward biased and D2 is reverse
biased. Diode D1 conducts and Y=1.
Case IV: When A=1, B=1: Here both the diodes are forward biased and hence conduct
perfectly. A potential difference of 5V appears across resistance. Thus, Y = 1.
AND Gate:
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It is also a two input single output gate. The output is one if both the inputs are one.
(a) Suppose A=0 and B=0: The potentials at A and B are forward biased and offers no
resistance. The diode D1 conducts and net potential difference appears across R and Y=0.
(b) Suppose A = 0 and B = 1: In this case also A is forward biased and B is in off state. The
diode D1 conducts and net potential difference appears across R and Y = 0.
(c) When A = 1 and B = 0: In this case also A is in off state and B is forward biased. The
diode D2 conducts and Y = 0
(d) When A = 1 and B = 1: Here both diodes are in off state, hence no potential drop
occurs across R and Y = 1.
NOT Gate:
It is a single input single output gate. The truth table and symbol is
A
Y A
0
1
1
0
It is realised with the help of a transistor. Consider an pnp transistor to be used as NOT
gate.
If A = 0, the emitter base junction is reverse biased and no current flows through it.
Correspondingly current through RC is also equal to zero. The potential Y = 1.
On the other hand, if A is 5V i.e. A =1, the emitter base junction is forward biased. Potential
drop occurs R and Y = 0
NAND Gate:
It is AND gate followed by a NOT Gate.
It is two input single output gate. The truth table and symbol are,
P a g e | 17
X
Y X
A
B
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
NOR Gate:
It is OR gate followed by a NOT gate. It is a two input single output gate. The truth
table and symbol are,
Y X
A
B
X
0
0
0
1
0
1
1
0
1
0
1
0
1
1
1
0
Exclusive OR (XOR) Gate:
It is also two input, single output gate. The output is one iff one of the inputs is one.
The truth table and symbol are;
Y  AB  AB
A
B
0
0
0
0
1
1
1
0
1
1
1
0
Exclusive NOR Gate:-It is an exclusive OR gate followed by a NOT gate. Output is one
either both the inputs are one or zero. The truth table is,
P a g e | 18
Y  AB  A B
A
B
0
0
1
0
1
0
1
0
0
1
1
1
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