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Intrinsic and extrinsic semiconductors.
Principle of operation of PN Junction diode.
V-I characteristics of PN Junction diode. Specifications of diode.
Principle of working of Zener diode & LED.
Principle of working of Photo diode & Solar cell.
Bipolar Junction Transistors: PNP and NPN structures-Principle of
operation.
 Input and output characteristics of common
emitter
configuration.
 Specifications of transistors.
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Conductor:
 If the number of valance electrons is less than 4, the material is
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generally called conductor. Instead of accepting electrons, it is easier to
donate electrons to fill the outer sub shell as 8.
Insulator:
If the number of valance electrons is more than 4, the material is
generally called insulator. Instead of donating electrons, it is easier to
accept lesser electrons to fill the outer sub shell.
Semiconductor:
If the number of valance electrons is equal to 4, the material is
generally called semi conductor. Here the probability of donating and
accepting electrons is equal.
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The electrical behavior of solid can be explained with the help of energy bands.
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Insulators
e.g. polythene
electrons are tightly bound to atoms so few can break
free to conduct electricity
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Here the valance band is full while the conduction band is empty. The energy gap between valance
band and conduction band is very large (15 eV).Therefore a very high electric field is required to lift
the valance electrons to the conduction band
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Conductors
 e.g. copper or aluminium
 have a cloud of free electrons (at all
temperatures above absolute zero).
 If an electric field is applied electrons will
flow causing an electric current
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In the energy band diagram, there is no forbidden energy gap
between the valance band and the conduction band .The two
bands actually overlap as shown in fig.
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Semiconductors
e.g. silicon or germanium
at very low temperatures these have the properties of
insulators
as the material warms up some electrons break free
and can move
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In the case of semi conductors, the valance band is almost filled and conduction band is empty.
But the forbidden energy gap is very small (1 eV) as shown in fig. There fore comparatively a
smaller electric field is required to lift the valance electrons to the conduction band. Thus the
conductivity of semiconductor lies between a conductor and insulator.
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A Copper atom has only 1 electron
in it’s valence ring. This makes it a
good conductor. It takes 2n2
electrons or in this case 32
electrons to fill the valence shell.
A Silicon atom has 4 electrons in it’s
valence ring. This makes it a
semiconductor. It takes 2n2 electrons
or in this case or 18 electrons to fill
the valence shell.
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A semi conductor in its purest form is known as intrinsic semi conductor.
Eg: Ge or Si crystal
Covalent bonding is a bonding of two
or more atoms by the interaction of
their valence electrons.
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The conductivity of the intrinsic semiconductor can be increased
by adding small amount of impurities. The process of adding
impurities to the intrinsic (pure) semiconductor is called doping.
The doped semiconductor is then called extrinsic (impure) semi
conductor.
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Depending on the dopant (impurity) used, extrinsic semi
conductor can be divided in to two classes.
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N-type Semi conductor.
P-type Semi conductor.
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N-type semi conductor is an extrinsic semi conductor doped
with a pentavalent impurity like Antimony, Phosphorus and
Arsenic etc.
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The N-type semi conductor can be represented as shown
in fig.
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It consists of
Free electrons (Majority carriers).
Holes (Minority Carriers).
Immobile positive ions.
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P-type semi conductor is an extrinsic semi conductor doped
with a trivalent impurity like Gallium, indium and Boron etc.
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The P-type semi conductor can be represented as shown in fig.
It consists of
 Holes (Majority carriers).
 Free electrons (Minority Carriers).
 Immobile negative ions.
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When an external voltage is applied to the PN junction in such a
way that positive terminal of the battery is connected to the Ptype and negative terminal of the battery is connected to the Ntype. This arrangement is called forward biased.
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When an external voltage is applied to the PN junction in such a way
that positive terminal of the battery is connected to the N-type and
negative terminal of the battery is connected to the P-type. This
arrangement is called reverse biased.
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If the reverse bias voltage is increased beyond a certain limit,
a new phenomenon called break down occurs.
In this region high current may be passed through the
junction.
This high current may generate large amount of heat to
destroy the junction.
The two processes are responsible for junction break down in
reverse biased condition namely,
 Avalanche break down
 Zener break down
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The increased reverse voltage increases the amount of energy
impaled to minority carriers.
As the reverse voltage is increased further the minority carriers
acquire a large amount of energy.
When these carriers collide with atoms, within the crystal structure
they impact sufficient energy to break a covalent bond and
generate additional carriers (electron hole pairs).
These additional carriers pick up energy from the applied voltage
and generate more carriers, and reverse current increased rapidly.
This cumulative process of carrier generation (or multiplication) is
known as Avalanche breakdown.
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It occurs when diode is heavily doped. Due to heavy doping,
depletion layer is narrow.
When the reverse voltage across the diode is increased, electric
field is developed across depletion layer.
Electric field is strong enough to generate large number of
electron-hole pair by breaking covalent bonds. Because of large
number of these carriers reverse current increases sharply and
breakdown occurs which is known as Zener Breakdown.
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Diode junctions that breakdown below 5 V are caused by Zener effect
whereas Junctions that experience breakdown above 5 V are caused by
Avalanche effect.
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The Zener breakdown occurs in heavily doped junctions, which produce
Narrow depletion layers, whereas Avalanche breakdown occurs in lightly
doped junctions, which produce wide depletion layers.
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With the increase in junction temperature, Zener breakdown voltage is
reduced while the Avalanche breakdown voltage is increases.
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The zener diodes have a negative temperature coefficient while
Avalanche diodes have a positive temperature coefficient.
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Diode is a two terminal device consisting of a PN junction formed either in
Ge or Si crystal. Here the terminal on the P-side is called the anode and the
terminal on the N-side is called the cathode. The PN junction conducts the
current only when it is in forward biased and no current flows through it
when it is in reverse biased(i.e. ,current flows in only one direction). Thus
the diode is called uni directional device.
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The following are the different diode parameters.
 Semiconductor material
 Forward voltage drop (Vf)
 Peak Inverse Voltage (PIV)
 Maximum forward current
 Junction capacitance:
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Zener diodes are also called breakdown diodes.
Specially doped PN junction diodes to produce controlled
break down characteristics without damage and are
operated in the break down region.
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Break down in zener diode is influenced by two
phenomenon, zener effect and avalanche effect.
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Zener effect is predominant for break down voltages less
than about 4V and avalanche break down is predominant
for voltages greater than 6V. Between 4V and 6V, both
effects are present.
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Because of high temperature and current capability,
Silicon is usually preferred for the manufacture of zener
diodes.
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Applications
 Voltage regulator
 Fixed reference voltage source
 Over voltage protection circuit.
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 formed by the junction between a layer of metal
(e.g. aluminium) and a semiconductor
 action relies only on majority charge carriers
 much faster in operation than a pn junction diode
 has a low forward voltage drop of about 0.25 V
 used in the design of high-speed logic gates
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Light emitting diode is a PN junction that emits
optical radiation generated by the recombination of
electrons and holes, when the junction is forward
biased. Most of the commercial LEDs are realized
using a highly doped N and a P Junction.
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Fig:The energy band diagram of a pn+ junction
under unbiased and biased conditions
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Advantages:
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High reliability
Fast response
Low cost
Low power consumption
Disadvantages:
 Temperature dependence of radiation
 Sensitivity to over voltage damage
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Applications:
 Indicator lamp and displays in equipments such as digital watches, calculators
etc.
 Optical communication system
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A photo-diode is a reverse-biased silicon or germanium pn
junction in which reverse current increases when the junction
is exposed to light. The reverse current in a photo-diode is
directly proportional to the intensity of light falling on its pn
junction. This means that greater the intensity of light falling
on the pn junction of photo-diode, the greater will be the
reverse current.
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When light (photons) falls on the pn junction, the energy is imparted by
the photons to the atoms in the junction. This will create more free
electrons (and more holes). These additional free electrons will increase
the reverse current. As the intensity of light incident on the pn junction
increases, the reverse current also increases. In other words, as the
incident light intensity increases, the resistance of the device (photodiode) decreases.
When no light is incident on the pn junction of photo-diode, the reverse current Ir is
extremely small. This is called dark current. As the intensity of light increases, the
reverse current IR goes on increasing till it becomes maximum. This is called saturation
current.
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Photodiodes can be operated in different modes, which are as follows:
Photovoltaic mode – It is also known as zero bias mode, in which a voltage is
generated by the illuminated photodiode.
Photoconductive mode - The diode used in this mode is more commonly reverse
biased.
Avalanche diode mode - Avalanche photodiodes are operated in a high reverse
bias condition, which allow multiplication of an avalanche breakdown to each
photo-generated electron-hole pair. This results in internal gain within the
photodiode, which gradually increases the responsivity of the device.
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Photodiodes find application in the following:
 Cameras
 Medical devices
 Optical communication devices
 Automotive devices
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A solar cell is a solid-state electrical device (p-n
junction) that converts the energy of light directly into
electricity (DC) using the photovoltaic effect.
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Diode Circuits
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Half-wave rectifier
 peak output voltage is equal to
the peak input voltage minus
the conduction voltage of the
diode
 reservoir capacitor used to
produce a steadier output
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Full-wave rectifier
 use of a diode
bridge reduces
the time for which
the capacitor has
to maintain the
output voltage
and thus reduced
the ripple voltage
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Signal rectifier
 used to demodulate
full amplitude
modulated signals
(full-AM)
 also known as an
envelope detector
 found in a wide range
of radio receivers from
crystal sets to
superheterodynes
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Signal clamping
 a simple form of
signal conditioning
 circuits limit the
excursion of the
voltage waveform
 can use a
combination of
signal and Zener
diodes
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Catch diode
 used when switching
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inductive loads
the large back e.m.f.
can cause problems
such as arcing in switches
catch diodes provide a low impedance path across
the inductor to dissipate the stored energy
the applied voltage reverse-biases the diode which
therefore has no effect
when the voltage is removed the back e.m.f.
forward biases the diode which then conducts
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EIA/JEDEC
 A standardized 1N-series numbering system was introduced in the US by
EIA/JEDEC (Joint Electron Device Engineering Council) about 1960.
Among the most popular in this series were: 1N4001-1N4007 (Silicon 1A
power rectifier)
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Pro Electron
 The European Pro Electron coding system for active components was
introduced in 1966 and comprises two letters followed by the part code.
The first letter represents the semiconductor material used for the
component (A = Germanium and B = Silicon) and the second letter
represents the general function of the part (for diodes: A = lowpower/signal,Y = Rectifier and Z = Voltage reference)
 e.g.: BY127
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Three terminal active device which transforms
current flow from low resistance path to high
resistance path. This transfer of current through
resistance path, given the name to the device
‘transfer resistor’ as transistor.
Transistors consists of junctions within it, are called
junction transistors.
Current carries inside is by two opposite polarities
of charge carriers (electrons and holes), hence the
name bipolar junction transistor.
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If a P-type material is sandwiched between two N-type materials ,the
resulting structure is called NPN transistor. Similarly when N-type
material is sandwiched between the two P-type materials , the
resulting structure is called PNP transistor.
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In both cases, the first layer where the emission or injection of the
carriers starts is called emitter. The second layer through which
carriers passes is called the base and the third layer which collects the
injected carriers is called collector.
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Emitter junction
Collector junction
Region of operation
Reverse biased
Reverse biased
Cut-off region
Forward biased
Reverse biased
Active region
Forward biased
Forward biased
Saturation region
Reverse biased
Forward biased
Inverse action
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Common Base (CB).
Common Emitter (CE).
Common Collector (CC).
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Input is given between base and emitter, while output
is taken across the collector and emitter. Here emitter
is common to both input and output.
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Circuit Arrangement
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Properties
Input
Impedance
Output
Impedance
Voltage
Gain
Current
Gain
Power Gain
Common
Base
Low
Common
Emitter
Medium
Common
Collector
High
Very High
High
Low
High
Medium
Low
Low
Medium
High
Low
Very High
Medium
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Type number
 The type number of the device is a unique identifier given to
each type of transistor. There are three international schemes
that are widely used: European Pro-Electron scheme; US
JEDEC (numbers start with 2N for transistors); and the
Japanese system (numbers start with 2S).
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Polarity
 There are two types of transistor: NPN and PNP. It is
important to choose the correct type otherwise all the circuit
polarities will be wrong.
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Material
 The two main types of material used for transistors are
germanium and silicon. Other materials are used, but in very
specialised transistors.
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Joint Electron Device Engineering Council (JEDEC)
 These part numbers take the form: digit, letter, sequential
number, [suffix]
 The letter is always 'N', and the first digit is 1 for diodes, 2 for
transistors, 3 for four-leaded devices, and so forth. The
sequential numbers run from 100 to 9999 and indicate the
approximate time the device was first made. If present, a
suffix could indicate various things. For example, a 2N2222A is
an enhanced version of a 2N2222. It has higher gain,
frequency, and voltage ratings. Always check the data sheet.
 Examples: 1N914 (diode), 2N2222, 2N2222A, 2N904
(transistors).
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Pro-Electron
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These part numbers take the form: two letters, [letter], sequential number, [suffix]
The first letter indicates the material:
A = Ge
B = Si
C = GaAs
The second letter indicates the device type and intended application:
A: diode, RF
C: transistor, AF, small signal
D: transistor, AF, power
F: transistor, HF, small signal
L: Transistor, HF, power
U: Transistor, power, switching
Y: Rectifier
Z: Zener, or voltage regulator diode
The third letter indicates if the device is intended for industrial or commercial applications. It's
usually a W, X, Y, or Z. The sequential numbers run from 100-9999.
Examples: BC108A, BAW68, BF239, BFY51.
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