SEMICONDUCTOR DIODES UNIT-1

SEMICONDUCTOR
A semiconductor is a material which has electrical conductivity to a degree between that of a
metal (such as copper) and that of an insulator (such as glass). Semiconductors are the foundation of
modern electronics, including transistors, solar cells, light-emitting diodes (LEDs), quantum dots and
digital and analog integrated circuits.

DIODE
Diode – Di + ode Di means two and ode means electrode. So physical contact of two electrodes is
known as diode and its important function is alternative current to direct curren

PN JUNCTION
When the N and P-type semiconductor materials are first joined together a very large
density gradient exists between both sides of the junction so some of the free electrons from the donor
impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type
material producing negative ions.

However, because the electrons have moved across the junction from the N-type silicon to
the P-type silicon, they leave behind positively charged donor ions (ND) on the negative side and now the
holes from the acceptor impurity migrate across the junction in the opposite direction into the region are
there are large numbers of free electrons. As a result, the charge density of the P-type along the junction is
filled with negatively charged acceptor ions (NA), and the charge density of the N-type along the junction
becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.

This process continues back and forth until the number of electrons which have crossed the
junction have a large enough electrical charge to repel or prevent any more carriers from crossing the
junction. The regions on both sides of the junction become depleted of any free carriers in comparison to
the N and P type materials away from the junction. Eventually a state of equilibrium (electrically neutral
situation) will occur producing a "potential barrier" zone around the area of the junction as the donor
atoms repel the holes and the acceptor atoms repel the electrons. Since no free charge carriers can rest in a
position where there is a potential barrier the regions on both sides of the junction become depleted of any
more free carriers in comparison to the N and P type materials away from the junction. This area around
the junction is now called the Depletion Layer.

THE PN JUNCTION
The total charge on each side of the junction must be equal and opposite to maintain a neutral charge
condition around the junction. If the depletion layer region has a distance D, it therefore must therefore
penetrate into the silicon by a distance of Dp for the positive side, and a distance of Dn for the negative
side giving a relationship between the two of Dp.NA = Dn.ND in order to maintain charge neutrality also
called equilibrium.

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PN JUNCTION DIODE:

As the N-type material has lost electrons and the P-type has lost holes, the N-type material has become
positive with respect to the P-type.

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Then the presence of impurity ions on both sides of the junction cause an electric field to be established
across this region with the N- side at a positive voltage relative to the P-side. The problem now is that a
free charge requires some extra energy to overcome the barrier that now exists for it to be able to cross the
depletion region junction. This electric field created by the diffusion process has created a "built-in
potential difference" across the junction with an opencircuit (zero bias) potential of:

Where: Eo is the zero bias junction voltage, VT the thermal voltage of 26mV at room
temperature, ND and NA are the impurity concentrations and ni is the intrinsic concentration.

A suitable positive voltage (forward bias) applied between the two ends of the PN junction can the free
electrons and holes with the extra energy. The external voltage required to overcome this potential barrier
that now exists is very much dependent upon the type of semiconductor material used and its actual
temperature.

Typically at room temperature the voltage across the depletion layer for silicon is about 0.6 - 0.7 volts and
for germanium is about 0.3 - 0.35 volts. This potential barrier will always exist even if the device is not
connected to any external power source.

The significance of this built-in potential across the junction, is that it opposes both the flow of holes and
electrons across the junction and is why it is called the potential barrier. In practice, a PN junction is
formed within a single crystal of material rather than just simply joining or fusing together two separate
pieces.

Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be
made to an external circuit. Then the resulting device that has been made is called a PN junction Diode or
Signal Diode.

DEPLETION LAYER PN JUNCTION

If one side of crystal pure semiconductor Si(silicon) or Ge(Germanium) is doped with acceptor
impurity atoms and the other side is doped with donor impurity atoms , a PN junction is formed as
shown in figure.P region has high concentration of holes and N region contains large number of electrons

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As soon as the junction is formed, free electrons and holes cross through the junction by the
process of diffusion. During this process , the electrons crossing the junction from N- region into
P-region , recombine with holes in the P-region very close to the junction. Similarly holes
crossing the junction from the P-region into the N-region, recombine with electrons in the
Nregion very close to the junction. Thus a region is formed, which does not have any mobile
charge very close to the junction. This region is called the depletion layer of pn junction.

In this region, on the left side of the junction, the acceptor atoms become negative ions and on the right
side of the junction, the donor atoms become positive ions as shown in figure.

CURRENT EQUATION:
To derive the expression for the total current as function of applied voltage (neglect the barrier width)
When diode is forward biased, holes injected from the p to n material. The concentration pn of holes in
the n-side is increased above equilibrium value pno

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If V >> V T then the term e (-V/ Ƞ VT) << 1 therefore I=Io termed as reverse saturation current, which is
valid as long as the external voltage is below the breakdown value.

DRIFT AND DIFFUSION CURRENTS

→ The flow of charge (ie) current through a semiconductor material are of two types namely drift &
diffusion.

→ (ie) The net current that flows through a (PN junction diode) semiconductor material has two
components

(i) Drift current

(ii) Diffusion current

DRIFT CURRENT
→ When an electric field is applied across the semiconductor material, the charge carriers attain a certain
drift velocity Vd , which is equal to the product of the mobility of the charge carriers and the applied
Electric Field intensity E ;

Drift velocity Vd = mobility of the charge carriers X Applied Electric field intensity

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→ Holes move towards the negative terminal of the battery and electrons move towards the positive
terminal of the battery. This combined effect of movement of the charge carriers constitutes a current
known as ― the drift current ― .

→ Thus the drift current is defined as the flow of electric current due to the motion of the charge carriers
under the influence of an external electric field.

→ Drift current due to the charge carriers such as free electrons and holes are the current passing through
a square centimeter perpendicular to the direction of flow.

(i) Drift current density Jn , due to free electrons is given by Jn = q n μn E A / cm2

(ii) (ii) Drift current density JP, due to holes is given by JP = q p μp E A / cm

DIFFUSION CURRENT

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(a) Exess hole concentration varying along the axis in an N-type semiconductor bar

(b) The resulting diffusion current

→ In a semiconductor material the change carriers have the tendency to move from the region of higher
concentration to that of lower concentration of the same type of charge carriers. Thus the movement of
charge carriers takes place resulting in a current called diffusion current.

As indicated in fig a, the hole concentration p(x) in semiconductor bar varies from a high value to a low
value along the x-axis and is constant in the y and z directions.

Diffusion current density due to holes Jp is given by

Since the hole density p(x) decreases with increasing x as shown in fig b, dp/dx is negative and the minus
sign in equation is needed in order that Jp has positive sign in the positive x direction.

Diffusion current density due to the free electrons is given by

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Where dn/dx – concentration gradient for electrons

Dp/dx - concentration gradient for holes

Dn and Dp – diffusion coefficient for electrons and holes

Total Current
The total current in a semiconductor is the sum of both drift and diffusion currents that is
given by

Similarly the total current density for an N type semiconductor is given by

FORWARD BIAS CONDITION

When positive terminal of the battery is connected to the P-type and negative terminal to N-
type of the PN junction diode that is known as forward bias condition.

Operation

The applied potential in external battery acts in opposition to the internal potential barrier
which disturbs the equilibrium.

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As soon as equilibrium is disturbed by the application of an external voltage, the Fermi level is no
longer continuous across the junction.

Under the forward bias condition the applied positive potential repels the holes in P type region so
that the holes move towards the junction and the applied positive potential repels the electrons in N type
region so that the electrons move towards the junction.

When the applied potential is more than the internal barrier potential the depletion region and
internal potential barrier disappear

V-I Characteristics
As the forward voltage increased for VF < Vo, the forward current IF almost zero because the
potential barrier prevents the holes from P region and electrons from N region to flow across the
depletion region in opposite direction.

For VF > Vo, the potential barrier at the junction completely disappears and hence, the holes cross the
junction from P to N type and electrons cross the junction to opposite direction, resulting large current
flow in external circuit.

A feature noted here is the cut in voltage or threshold voltage VF below which the current is very
small.

At this voltage the potential barrier is overcome and the current through the junction starts to increase
rapidly.

Cut in voltage is 0.3V for germanium and 0.7 for silicon.

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UNDER REVERSE BIAS CONDITION

When the negative terminal of the battery is connected to the P-type and positive terminal to
N-type of the PN junction diode that is known as forward bias condition.

Operation
The holes from the majority carriers of the P side move towards the negative terminal of the battery
and electrons which from the majority carrier of the N side are attracted towards the positive terminal of
the battery.

Hence, the width of the depletion region which is depleted of mobile charge carriers increases.
Thus, the electric field produced by applied reverse bias, is in the same direction as the electric field of
the potential barrier.

Hence the resultant potential barrier is increased which prevents the flow of majority carriers in
both directions. The depletion width W is proportional to under reverse bias.

V-I characteristics
The oretically no current flow in the external circuit. But in practice a very small amount of
current of the order of few microamperes flows under reverse bias.

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Electrons forming covalent bonds of semiconductor atoms in the P and N type regions may absorb
sufficient energy from heat and light to cause breaking covalent bonds. So electron hole pairs
continuously produced.

Consequently 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 a small reverse current. This
current is known as reverse saturation current Io.

The magnitude of this current is depends on the temperature because minority carrier is thermally
broken covalent bonds.

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Half-wave rectifiers
Half-wave rectifiers transform AC voltage to DC voltage. A halfwave rectifier circuit
uses only one diode for the transformation. A halfwave rectifier is defined as a type of rectifier
that allows only one-half cycle of an AC voltage waveform to pass while blocking the other half
cycle. In this session, let us know in detail about the half-wave rectifier.

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Half Wave Rectifier Circuit

A half-wave rectifier is the simplest form of the rectifier and requires only one diode for
the construction of a halfwave rectifier circuit.

A halfwave rectifier circuit consists of three main components as follows:

Working of Half Wave Rectifier

In this section, let us understand how a half-wave rectifier transforms AC into DC.

1. A high AC voltage is applied to the primary side of the step-down transformer. The obtained
secondary low voltage is applied to the diode.
2. The diode is forward biased during the positive half cycle of the AC voltage and reverse
biased during the negative half cycle.
3. The final output voltage waveform is as shown in the figure below:

For better understanding, let us simplify the half-wave circuit by replacing the secondary
transformer coils with a voltage source as shown below:

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For the positive half cycle of the AC source voltage, the circuit effectively becomes as shown
below in the diagram:

When the diode is forward biased, it acts as a closed switch. But, during the negative half cycle
of the AC source voltage, the equivalent circuit becomes as shown in the figure below

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When a diode is reverse biased, it acts as an open switch. Since no current can flow to
the load, the output voltage is equal to zero.

Half Wave Rectifier Waveform

The halfwave rectifier waveform before and after rectification is shown below in the
figure.

Half Wave Rectifier Capacitor Filter

The output waveform of a halfwave rectifier is a pulsating DC waveform. Filters in

halfwave rectifiers are used to transform the pulsating waveform into constant DC
waveforms. A capacitor or an inductor can be used as a filter.

The circuit diagram below shows how a capacitive filter is used with halfwave rectifier to
smoothen out a pulsating DC waveform into a constant DC waveform.

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Similar Articles
 Full Wave Rectifiers
 Bridge Rectifiers

Half Wave Rectifier Formula

Ripple Factor of Half Wave Rectifier

Ripple factor determines how well a halfwave rectifier can convert AC voltage to DC
voltage.

Ripple factor can be quantified using the following formula:

The ripple factor of a halfwave rectifier is 1.21.

Efficiency of Halfwave Rectifier

The efficiency of a halfwave rectifier is the ratio of output DC power to the input AC
power.

The efficiency formula for halfwave rectifier is given as follows;

RMS value of Half Wave Rectifier

The RMS value of the load current for a half-wave rectifier is given by the formula:

Form factor of a Halfwave Rectifier

The form factor is the ratio between RMS value and average value and is given by the
formula:

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Applications of Half Wave Rectifier

Here are a few common applications of half wave rectifiers:

 They are used for signal demodulation purpose

 They are used for rectification applications
 They are used for signal peak applications

Disadvantages of Half Wave Rectifier

 Power loss
 Low output voltage
 The output contains a lot of ripples

Bridge Rectifie
Construction

The construction of a bridge rectifier is shown in the figure below. The bridge rectifier
circuit is made of four diodes D1, D2, D3, D4, and a load resistor RL. The four diodes are
connected in a closed-loop configuration to efficiently convert the alternating current
(AC) into Direct Current (DC). The main advantage of this configuration is the absence
of the expensive centre-tapped transformer. Therefore, the size and cost are reduced.

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The input signal is applied across terminals A and B, and the output DC signal is
obtained across the load resistor RL connected between terminals C and D. The four
diodes are arranged in such a way that only two diodes conduct electricity during each
half cycle. D1 and D3 are pairs that conduct electric current during the positive half
cycle/. Likewise, diodes D2 and D4 conduct electric current during a negative half cycle.

Working
When an AC signal is applied across the bridge rectifier, terminal A becomes positive
during the positive half cycle while terminal B becomes negative. This results in diodes
D1 and D3 becoming forward biased while D2 and D4 becoming reverse biased.

The current flow during the positive half-cycle is shown in the figure below:

During the negative half-cycle, terminal B becomes positive while terminal A becomes
negative. This causes diodes D2 and D4 to become forward biased and diode D1 and
D3 to be reverse biased.

The current flow during the negative half cycle is shown in the figure below:

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From the figures given above, we notice that the current flow across load resistor R L is
the same during the positive and negative half-cycles. The output DC signal polarity
may be either completely positive or negative. In our case, it is completely positive. If
the diodes’ direction is reversed, we get a complete negative DC voltage.

Thus, a bridge rectifier allows electric current during both positive and negative half
cycles of the input AC signal.

The output waveforms of the bridge rectifier are shown in the below figure.

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Characteristics of Bridge Rectifier

Ripple Factor
The smoothness of the output DC signal is measured by a factor known as the ripple
factor. The output DC signal with fewer ripples is considered a smooth DC signal while
the output with high ripples is considered a high pulsating DC signal.

Mathematically, the ripple factor is defined as the ratio of ripple voltage to pure DC
voltage.

The ripple factor for a bridge rectifier is given by

For bridge rectifiers, the ripple factor is 0.48.

Peak Inverse Voltage

The maximum voltage that a diode can withstand in the reverse bias condition is known
as a peak inverse voltage. During the positive half cycle, the diodes D 1 and D3 are in the
conducting state while D2 and D4 are in the non-conducting state. Similarly, during the
negative half cycle, diodes D2 and D4 are in the conducting state, and diodes D1 and
D3 are in the non-conducting state.

Efficiency
The rectifier efficiency determines how efficiently the rectifier converts Alternating
Current (AC) into Direct Current (DC). Rectifier efficiency is defined as the ratio of the
DC output power to the AC input power. The maximum efficiency of a bridge rectifier is
81.2%.

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Advantages

 The efficiency of the bridge rectifier is higher than the efficiency of a half-wave rectifier.
However, the rectifier efficiency of the bridge rectifier and the centre-tapped full-wave rectifier
is the same.
 The DC output signal of the bridge rectifier is smoother than the output DC signal of a half-
wave rectifier.
 In a half-wave rectifier, only half of the input AC signal is used, and the other half is blocked.
Half of the input signal is wasted in a half-wave rectifier. However, in a bridge rectifier, the
electric current is allowed during both positive and negative half cycles of the input AC
signal. Hence, the output DC signal is almost equal to the input AC signal.

Disadvantages

 The circuit of a bridge rectifier is complex when compared to a half-wave rectifier and centre-
tapped full-wave rectifier. Bridge rectifiers use 4 diodes while half-wave rectifiers and centre-
tapped full wave rectifiers use only two diodes.
 When more diodes are used more power loss occurs. In a centre-tapped full-wave rectifier,
only one diode conducts during each half cycle. But in a bridge rectifier, two diodes
connected in series conduct during each half cycle. Hence, the voltage drop is higher in a
bridge rectifier.

Efficiency
Efficiency is the ratio of the work performed by a machine or in a process to the total
energy expended or heat consumed.

Efficiency refers to how close we can get to a particular outcome of the given input with
as much less wastage as possible. Efficiency is the ability to minimise wasting
materials, efforts, energy and time in performing something or producing the desired
result.

Efficiency can be determined quantitatively by the ratio of useful output to total input.
The ratio of energy transferred to a useful form compared to the total energy supplied
initially is called the efficiency of the device.

Efficiency is denoted by η.

Efficiency formula regarding Work is given as

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What Is a Filter?
A filter is a circuit capable of passing (or amplifying) certain frequencies while
attenuating other frequencies. Thus, a filter can extract important frequencies
from signals that also contain undesirable or irrelevant frequencies.

Filter Capacitor
The capacitor is a reactive component, used in analog electronic filters because the
capacitor impedance is a function of frequency. The capacitor that affects a signal can
be frequency-dependent. So this property is widely used in designing the filter.
Analog electronic filters like LPF can be used to execute a function of predefined
signal processing. The main function of this filter is to allow low frequencies and
block high frequencies. Similarly, an HPF allows high frequencies and blocks low
frequencies. The electronic filter can be made with the help of analog components like
resistors, capacitors, transistors, op-amps, and inductors. This article discusses an
overview of the filter capacitor and it’s working.

What is a Filter Capacitor?

A capacitor that is used to filter out a certain frequency otherwise series of

frequencies from an electronic circuit is known as the filter capacitor.
Generally, a capacitor filters out the signals which have a low frequency. The
frequency value of these signals is near to 0Hz, these are also known as DC
signals. So this capacitor is used to filter unwanted frequencies. These are
very common in different types of equipment like electronics as well as
electrical and applicable in different applications.

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Working of Filter Capacitor

The working of this capacitor mainly depends on the capacitive reactance
principle. It is nothing but how the impedance of a capacitor alters with a
signal frequency that is flowing through it. A nonreactive component like a
resistor offers similar resistance to a signal apart from the frequency of the
signal. This means 1Hz & 100KHZ signals flow throughout a resistor with
equal resistance.
But, a capacitor is different because its impedance
or resistance will change based on the signal frequency which is flowing
through. These are reactive devices that offer high resistance to low-
frequency signals and low-resistance to high-frequency signals using the
formula like XC= 1/2πfc. A capacitor gives dissimilar impedance values for
dissimilar frequency signal. In a circuit, it can operate as a resistor.

Filter Capacitor Formula

In power supply circuits, this capacitor can be calculated to ensure the
least ripple at the output. The formula is C = I / 2f Vpp
From the equation above, ‘I’ is load current, ‘f’ is i/p frequency of AC and ‘Vpp’
is the minimum ripple that may be acceptable because almost it’s never
possible to make this ‘0’

Filter Capacitor Circuit

The circuit diagram of the filter capacitor is shown below. In this circuit, the
capacitor works like a high pass filter that allows high frequencies and blocks

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direct current. Similarly, they can also work as a low pass filter to allow DC and
block AC.
Here the capacitor is connected in parallel with the component instead of
connecting in series. This circuit is a high-frequency capacitive filter. Here, the
flow of current will be in the least resistance direction.

Filter Capacitor Circuit

Because a capacitor gives extremely low resistance for high-frequency

signals, so these signals will supply through the capacitor. Like this, the circuit
in this arrangement, it is a high-frequency filter. The signals like low-frequency
current will not supply throughout the capacitor, as it gives high resistance for
low-frequency signals.

Filter Capacitor Circuit to Block DC and Pass AC

For low-frequency signals, the capacitor offers extremely high resistance and
for high-frequency signals, it proves less resistance. So it acts as a high pass
filter to allow high-frequency signals and block low-frequency signals.

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In a circuit, both AC and DC signals can be used several times. But, in some
cases, we need only AC signals & the DC signals will be taken out. The best
example of this is a microphone circuit. In this, as an input, DC is given to the
microphone. We need DC as input to the microphone to power on & we
require AC as input to represent the music, voice signals, etc

Filter Out the DC Component from the Signal

A capacitor is used to filter out the DC signal. This can be done by connecting
the capacitor in series in the circuit. The following circuit is the capacitive high-
pass filter. In this, signals like DC or low frequency will be blocked.

Generally, a ceramic capacitor with 0.1µF value can be placed following the
signal that includes both the AC and DC signals. This capacitor allows AC and
filters out the Dc component.

Filter Capacitor Applications

The applications of this include the following.

 The line filter capacitor is applicable in several industrial loads as well as

appliances in order to defend the appliance from the noise of line voltage
noise and to defend other devices on a similar line from the generated
noise within the circuit.
 These capacitors can be used in all types of filters which are used in signal
processing. The best example of this application is like an audio equalizer.
It uses different frequency bands to permit amplification for low, high, and
midrange frequency tones.
 It is used for glitch removal on DC power rails
 It is used for RFI removal (radio frequency interference) for power or signal
lines to come in or exit equipment.
 This capacitor can be connected after the voltage regulator to get a smooth
DC power supply.
 This capacitor is used in audio, IF or RF filters

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Choke Filter
Definition: Choke filter consists of an inductor connected in series with rectifier
output circuit and a capacitor connected in parallel with the load resistor. It is also
called L-section filter because the inductor and capacitor are connected in the
shape of inverted L. The output pulsating DC voltage from a rectifier circuit
passes through the inductor or choke coil.

The inductor has low DC resistance and extremely high AC reactance. Thus,
ripples get filtered through choke coil. Some of the residual ripples if present in
filtered signal from inductor coil will get bypassed through the capacitor. The
reason behind this is that capacitor allow AC and block DC

Choke Filter

Definition: Choke filter consists of an inductor connected in series with rectifier

output circuit and a capacitor connected in parallel with the load resistor. It is also
called L-section filter because the inductor and capacitor are connected in the
shape of inverted L. The output pulsating DC voltage from a rectifier circuit
passes through the inductor or choke coil.

The inductor has low DC resistance and extremely high AC reactance. Thus,
ripples get filtered through choke coil. Some of the residual ripples if present in
filtered signal from inductor coil will get bypassed through the capacitor. The
reason behind this is that capacitor allow AC and block DC.

Significance of Choke Filter or L-section filter

Choke filter came into existence due to shortcomings of the series inductor and
shunt capacitor filter. A series inductor filter filters the output current but reduces
the output current (RMS value and Peak value) up to a large extent. And the
shunt capacitor filter performs filtering efficiently but increases the diode current.
The excess of current in a diode may lead to its destruction.

Moreover, the ripple factor of series inductor filter is directly proportional to the
load resistance it means as the load resistance increases, ripple factor also starts
increasing. And in the case of shunt capacitor, the ripple factor is inversely
proportional to the value of load resistance. It implies that in shunt capacitor filter
the ripple factor decreases with increase in load resistance and increases with
the decrease in load resistance.

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Thus, for better performance, we need a filter circuit in which ripple factor is low
and do not vary with the variation in load resistance. This can be achieved by
using the combination of series inductor filter and shunt capacitor filter. The
voltage stabilization property of shunt capacitor filter and current smoothing
property of series inductor filter is utilized for the formation of choke filter or L-
section filter.

The combination of series inductor filter and shunt capacitor filter is generally
used for most of the applications. The combination results in two types, i.e. L-
section filter and Pi filter. In this article, we will discuss the working of L-section or
choke filter and in next article, we will discuss Pi filter in detail.

Working of Choke Filter or L-section filter

When the pulsating DC signal from the output of the rectifier circuit is feed into
choke filter, the AC ripples present in the output DC voltage gets filtered by
choke coil. The inductor has the property to block AC and pass DC. This is
because DC resistance of an inductor is low and AC impedance of inductor coil is
high. Thus, the AC ripples get blocked by inductor coil.

Although the inductor efficiently removes AC ripples, a small percentage of AC

ripples is still present in the filtered signal. These ripples are then removed by the
capacitor connected in parallel to the load resistor. Now, the DC output signal is
free from AC components, and this regulated DC can be used in any application.

If the inductor of high inductive reactance (XL), greater than the capacitive
reactance at ripple frequency is used than filtering efficiency gets improved.

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Waveform of Choke Filter or L-section Filter

The waveform of DC output signal with a filter and without filter is shown in the
below diagram.

Characteristics of Choke filter or L-section filter

1. Regulation: The variation of DC output voltage from rectifier with respect to

the DC flowing through load resistor of the rectifier circuit is termed as
regulation.

2. Ripple factor: This is one of the significant characteristics of the filter. A

filter is considered efficient if it can remove ripples effectively. The ripple factor is
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the ratio of the AC present in the output signal and the DC. More the ripple factor
poor will be the performance of the device.
The most effective way to minimize the ripple factor is to increase the value of
inductive reactance. The combined reactance of load resistor and capacitor can
be minimized up to a large extent by using the capacitor of low reactance so that
the complete AC signal get bypassed through the capacitor and regulated DC
voltage can be obtained across the load resistor. In these conditions, the net
impedance will be due to inductor coil and that will be approximately 2ωL i.e. XL=
2ωL.

3. Critical Inductance: The value of inductance can be increased up

to a limit and this value of inductance is called critical inductance.
Advantages of Choke Filter or L-section Filter

1. It provides better voltage regulation.

2. The ripple factor can be varied according to the need.
Disadvantages of Choke Filter or L-Section filter

1. Bulky Size: These kinds of filters were popular in ancient time but it has
become obsolete now due to bulky size of inductors and capacitors.
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2. Not suitable for low voltage power Supplies: These are not suitable for
low voltage power supplies. IC regulators or active filters are used in such
devices.
It is the combination of series inductor filter and shunt capacitor filter. The
advantages of both these filters are utilized to form Choke input filters. And the
disadvantages of both of these filters are removed in choke filter.

Pi filter
A Pi filter is an electronic filter circuit consisting of a series connection of a capacitor and
an inductor, followed by another capacitor, arranged in the shape of the Greek letter “Pi”
(π). It is used to reduce high-frequency noise and ripple from a power supply, typically
in order to provide a clean and stable DC voltage to sensitive electronic components.

The main use of a Pi filter is to reduce high-frequency noise and ripple from a power supply, in
order to provide a clean and stable DC voltage to sensitive electronic components. Pi filters are
commonly used in power supplies for audio equipment, computer components, and other
electronic devices where stable and noise-free DC power is required. They are also used in radio
frequency (RF) applications to remove unwanted harmonics and spurious signals from RF
signals.
Function of the Pi filter (-filter)
AC components are also present in the rectifier’s output voltage. Therefore, getting rid
of these AC ripples is absolutely necessary to boost the device’s performance. The result
from the rectifier is straightforwardly applied to the info capacitor. The capacitor has a
high resistance to DC voltage and a low impedance to AC ripples in the output voltage.
As a result, only the capacitor in the input stage is used to get around the majority of
the AC ripples.

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The inductor coil and the capacitor connected parallel across the load filter the residual
AC components that are still present in the filtered DC signal. The effectiveness of
filtering thus increases multiple times.

Because there was only one inductor and capacitor in the L-section filter, even if 1% of
the AC ripples remained after filtering, the Pi-filter could remove them. As a result, the Pi
filter is deemed more effective.

Features of the Pi filter (-filter)

At low current drains, the Pi filter has the characteristics to produce a high output
voltage. The main filtering action in pi-filters is performed by the capacitor at input C1.
The inductor coil L and capacitor C2 filter the residual AC ripples.

Waveform of the Pi filter’s output voltage The reason for the high voltage at the Pi
filter’s output is that the entire input voltage appears across the input capacitor C1. The
voltage drop across gag curl and capacitor C2 is tiny.

Subsequently, this is the upside of Pi capacitor that it gives high voltage gain. However,
in addition to the high output voltage, the Pi filter’s voltage regulation is extremely
poor. This is because as the load’s current increases, the output voltage rapidly
decreases.

Aside from the previously mentioned impediment, its most pivotal benefit is low wave
factor.

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Benefits of the Pi filter (-filter)

High Voltage Output: This is the filter you should use if the application you are
working with requires a high output voltage after filtering. The significance of the pi
filter lies in its ability to maintain a high output voltage at its output terminals by
providing a low voltage drop across the choke coil and capacitor C2.
Low Wave factor: It provides improved filtering action due to the addition of two
capacitors and one inductor. This prompts decrement in swell element. A low ripple
factor indicates a low ratio of direct Current to rippled AC current. As a result, a DC
voltage that is regulated and free of ripples has a low ripples factor.
Strong PIV: When compared to an L-section filter, the peak inverse voltage of Pi filters
is higher.

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Negative aspect of the Pi filter (-filter)

Voltage regulation issues: As was mentioned earlier, the load current has an effect on
the output voltage. As a result, this capacitor cannot handle a variety of loads. Pi filters
are not appropriate for use in an application where the load current varies. As a result, L-
section filters can be used in this situation because their output voltage does not
significantly change with load current.

Use of Pi channel (π-channel)

In communication devices, these are used to retrieve the particular modulated signal.
The signal is modulated into high-frequency multiples during transmission. The specific
frequency range is demodulated using filters on the receiver side.

What is the Advantages of Pi filter?

The main advantage of the Pi filter is the extremely small voltage ripple using only
simple passive components. The concepts behind the working principle are easy to
grasp, even for non-engineers. Because only passive components are used, it withstands
high voltages, which is fundamental for power systems applications.

What is the Disadvantages of Pi filter?

Pi filter requires higher value capacitors which make them bulky. This is a major concern
for space constraint designs.

➨Pi filters are not suitable when load current varies as they offer poor voltage
regulation.

➨Pi filter is not suitable for high current flow through it.

Zener Diode
Pronounced zee·nr dai·owd

Zener diodes are semiconductor devices that allow current to flow in both
directions but specialize in current flowing in reverse. Also known as breakdown
diodes, Zener diodes are the most common electronic components used as
stable voltage references for electronic circuits.

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Zener Diodes Functionality

Current going across the terminals in reverse bias (backward) is called the Zener
effect. When voltage potential is met, this causes the Zener voltage
(Vz)/breakdown voltage. Zener diodes uniquely consist of a heavily doped P-N
junction that allows current to flow in reverse when reaching Vz. A well-defined
Vz can conduct current continuously in reverse bias without getting damaged.
Current then increases to a maximum level determined by a series resistor and
stabilizes, remaining constant over a range of applied voltages. Therefore, Zener
diodes are applicable for use as voltage regulators.

A Zener diode operates within the normal range of forward bias, with a turn-on
voltage between 0.3V and 0.7V. When connected in reverse bias, the current
flows backward, thus causing a small leakage of current to flow. As the reverse
voltage increases to the arranged Vz, current flows throughout the diode. Current

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increases to a maximum decided by the series resistor and then stabilizes to

remain constant over a range of applied voltages.

Zener diodes have two breakdown effects. Knowing these two effects help with
understanding their functionality. First is the Zener effect, prevalent in voltages
below 5.5V and involves a tunneling effect to cause the breakdown. The second
is the avalanche breakdown or impact ionization which occurs in voltages above
5.5V. These breakdowns both result in the same behavior, and they do not
require different circuitry, but each does have a different temperature coefficient.
The Zener effect has an anode (negative) terminal temperature coefficient, while
the avalanche has a cathode (positive) terminal temperature coefficient. Both
temperature effects are almost equal to 5.5V. They cancel each other out,
making Zener diodes rated at around 5.5V and stable over a wide range of
temperature requirements.

Think of a Zener diode like two diodes in parallel, facing opposite directions. The
voltage that's forward bias in a Zener diode has a voltage drop of 1V needed for
the diode to turn on for the current to flow. The forward voltage is forward biased.
The current flowing backward is considered the Zener voltage or reversed biased
due to its properties. An example of the Zener voltage is 3.3V. For the current to
flow across the diode, the current must maintain at least a minimum of this
voltage. Having a predictable voltage drop makes Zener diodes not only useful
as voltage regulators, but a correctly set Zener diode may limit the voltage of
other devices.

Common Zener Diode Specifications

 Current Iz (max): Maximum current at the rated Zener Voltage (Vz – 200μA to 200A).

 Current Iz (min): Minimum current value required for a diode to break down.

 Forward Bias: Voltage going across a diode that allows the current to flow easily in one
direction.

 Power Rating: Denotes the maximum power the Zener diode can dissipate and is a product
of the diode voltage/current flowing through it.

 Reverse Bias/Zener Effect: Voltage going across a diode in the opposite direction of
forward bias, this particular voltage doesn't cause any detectable current to flow, changes
AC to DC current, and manipulates other electronic signals.

 Temperature Stability: Diodes around 5V have the best stability.

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 Voltage Tolerance: Typically ±5%

 Zener/Breakdown Voltage (Vz): This is the minimum voltage that causes a portion of an insulator to
experience a breakdown and become electrically conductive. Voltage ranges from 2.4V to 200V and
can go up to 1kV, while the maximum for the surface-mounted device is 47V.

 Zener Resistance (Rz): The resistance exhibited to the Zener diode.

Equivalent circuit
When analyzing electrical circuits — including those for AC induction motors and DC motors — if the
circuit contains two or more of the same passive elements (such as resistors) and is wired exclusively in
series or exclusively in parallel, the circuit can be drawn with a simpler representation that contains the
voltage source and a single, equivalent passive element. This simplified version retains the electrical
characteristics of the original circuit and is referred to as an equivalent circuit.

Passive elements are those that dissipate, store, or release energy in the form of voltage or
current. Examples of passive elements are resistors, capacitors, and coils (aka inductors).

Active elements deliver or produce energy in the form of voltage or current. They include
semiconductor components such as diodes, transistors (field-effect transistors, or FETs, and
metal-oxide-semiconductor FETs, or MOSFETs).

The rules for combining resistors to create an equivalent circuit are based on Ohm’s law together
with Kirchhoff’s circuit laws.

The first of Kirchhoff’s laws, referred to as Kirchhoff’s current law (KCL), states that the
amount of current flowing into any node (junction) in a closed circuit equals the amount of
current flowing out of that node, thus ensuring the conservation of charge in a closed circuit.

Kirchhoff’s second law, referred to as Kirchhoff’s voltage law (KVL), states that for a closed
circuit, the algebraic sum of all the voltages around the circuit equals zero. Kirchhoff’s
voltage law ensures conservation of energy in a closed circuit.

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Finding equivalent resistance for series circuits

For a circuit wired in series, Kirchhoff’s voltage law, KVL, tells us that the voltage
around the circuit will equal zero. This means the sum of the voltage drops across each
resistor will equal the supply voltage. For a series circuit with three resistors, the supply
voltage, Vs, equals the sum of the voltages across the three resistors (V R1, VR2, and VR3):

We know from Kirchhoff’s current law, KCL, that the charge flowing into any node
equals the charge flowing out of that node. Series circuits have only one node
(junction), so current is the same at all points in the circuit. This means the same current
flows across each resistor. Using Ohm’s law, V = IR, to express the voltage across each
resistor, we can rewrite the equation above:

Now we can see that the equivalent resistance is simply the sum of all the resistances
in the series circuit.

And voltage can now be written in terms of the equivalent resistance.

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The example below shows a series circuit with three resistors.

For this circuit, the equivalent resistance is:

So the equivalent circuit will have one, 10-ohm equivalent resistor (Req).

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We can check our equivalent circuit by calculating the voltage across the equivalent
resistor and ensuring it equals the supply voltage:

Finding equivalent resistance for parallel circuits

For circuits wired in parallel, we can treat each loop as a separate circuit that is wired in
series. Kirchhoff’s voltage law, KVL, tells us that in each separate loops (series circuit),
the voltage drop across the resistor equals the supply voltage. For a parallel circuit with
three resistors, the voltage drop across each resistor equals the supply volta

According to Kirchhoff’s current law, KCL, the current is divided at each node, or
junction, such that:

Expressing current as voltage divided by resistance (V/R), according to Ohm’s law:

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Kirchhoff’s circuit laws applied to series circuits: For a circuit wired in series, the
voltage flowing through the circuit is divided among the passive elements, but the
current is the same through each passive element.

Kirchhoff’s circuit laws applied to parallel circuits: For a circuit wired in parallel, the
voltage flowing through the circuit is the same across each passive element, but the
current is divided among the passive elements.

voltage regulator
A voltage regulator is an electronic device or circuit that maintains a constant output voltage
irrespective of changes in input voltage, load current, or ambient temperature conditions. It
ensures that electronic devices receive a stable and consistent voltage level, which is crucial for
their proper operation and longevity.

There are different types of voltage regulators:

1. Linear Regulators: These regulators use an active (usually a transistor) and a passive
element (like a resistor) to regulate the voltage. They are simple and inexpensive but less
efficient, especially when there is a significant difference between input and output
voltages.
2. Switching Regulators: Also known as switching mode power supplies (SMPS), these are
more complex and efficient than linear regulators. They operate by rapidly switching a

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transistor on and off to control the amount of power delivered to the load, thus regulating
the output voltage. They are capable of converting high voltages to low voltages
efficiently.
3. Switched Capacitor Regulators: These are used in low-power applications and operate
by using switches and capacitors to regulate voltage. They are efficient for low current
applications but have limited power handling capabilities.

Voltage regulators are used in a wide range of applications including power supplies for
electronic devices, battery chargers, and in automotive and industrial electronics to ensure stable
voltage supply under varying conditions.

Light-emitting diode

Light-emitting diode (LED) is a widely used standard source of light in electrical

equipment. It has a wide range of applications ranging from your mobile phone to large
advertising billboards. They mostly find applications in devices that show the time and
display different types of data.

What is LED?

A light-emitting diode (LED) is a semiconductor device that emits light when an electric
current flows through it. When current passes through an LED, the electrons recombine
with holes emitting light in the process. LEDs allow the current to flow in the forward
direction and blocks the current in the reverse direction.

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Light-emitting diodes are heavily doped p-n junctions. Based on the semiconductor
material used and the amount of doping, an LED will emit coloured light at a particular
spectral wavelength when forward biased. As shown in the figure, an LED is
encapsulated with a transparent cover so that emitted light can come out.

Read More: Diodes

LED Symbol
The LED symbol is the standard symbol for a diode, with the addition of two small
arrows denoting the emission of light.

Simple LED Circuit

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The figure below shows a simple LED circuit.

The circuit consists of an LED, a voltage supply and a resistor to regulate the current
and voltage.

How does an LED work?

When the diode is forward biased, the minority electrons are sent from p → n while the
minority holes are sent from n → p. At the junction boundary, the concentration of
minority carriers increases. The excess minority carriers at the junction recombine with
the majority charges carriers.

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The energy is released in the form of photons on recombination. In standard diodes, the
energy is released in the form of heat. But in light-emitting diodes, the energy is
released in the form of photons. We call this phenomenon electroluminescence.
Electroluminescence is an optical phenomenon, and electrical phenomenon where a
material emits light in response to an electric current passed through it. As the forward
voltage increases, the intensity of the light increases and reaches a maximum.

Uses of LED
LEDs find applications in various fields, including optical communication, alarm and
security systems, remote-controlled operations, robotics, etc. It finds usage in many
areas because of its long-lasting capability, low power requirements, swift response
time, and fast switching capabilities. Below are a few standards LED uses:

 Used for TV back-lighting

 Used in displays
 Used in Automotives

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 LEDs used in the dimming of lights

Types of LED
Below is the list of different types of LED that are designed using semiconductors:

 Miniature LEDs
 High-Power LEDs
 Flash LED
 Bi and Tri-Colour
 Red Green Blue LEDs
 Alphanumeric LED
 Lighting LED

Advantages of LEDs over Incandescent Power Lamps

Some advantages of LEDs over Incandescent Power Lamps are:

 LEDs consume less power, and they require low operational voltage.
 No warm-up time is needed for LEDs.
 The emitted light is monochromatic.
 They exhibit long life and ruggedness.

V-I characteristics
V-I characteristics stand for voltage-current characteristics of an electrical component or
device. The V-I graph yields valuable information about the resistance and breaks down
an electronic component. It also provides the operating region of a component. By
studying these characteristics, we can understand where and how to use a component
in an electric circuit.

The voltage-ampere characteristics of an electronic component are its behaviour for

various values of an applied voltage. Put simply, it is the graph between Voltage and
Current obtained when current is measured through an electronic component as a
voltage is applied across it.

In V-I characteristics, the Voltage, V is on the x-axis, and the current, I, is on the y-axis
because it is easier to control the applied voltage than the current. This makes the
voltage the independent variable and is traditionally placed on the x-axis.

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Types of V-I Characteristics

Linear VI Characteristics
A linear VI curve has a constant slope and hence a constant resistance. Carbon
resistors and metals obey Ohm’s law and have constant resistance. The V-I curve is a
straight line passing through the origin. An electronic component may exhibit linear
characteristics only in a particular region. For example, a diode has a mostly linear
behaviour in its operating region.

Non-linear VI Characteristics
A circuit component has a non-linear characteristic if the resistance is not constant
throughout and is some function of voltage or current. The diode, for example, has
varying resistance for different voltage values. However, it has a linear characteristic for
a narrow operating region. In the graph below, we can also see the maximum forward
and reverse voltage in which the diode can be operated without causing breakdown and
burning up of the diode.

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Advantages of VI characteristics:

The VI characteristics (Voltage-Current characteristics) describe the relationship between

voltage (V) across a component and the current (I) flowing through it. Here are some advantages
of VI characteristics:

1. Visualization of Operating Behavior: VI characteristics provide a clear graphical

representation of how a component behaves under different voltage and current
conditions. This helps engineers and designers understand the behavior of components
such as diodes, transistors, and resistors.
2. Identification of Operating Points: By analyzing VI characteristics, engineers can
determine the operating points of components. This includes points like cutoff, saturation,
and active regions in transistors, or breakdown and forward bias regions in diodes.
3. Performance Evaluation: VI characteristics allow for the evaluation of a component's
performance, such as its efficiency, linearity, and range of operation. For example, in
power electronics, understanding the VI characteristics of a semiconductor device helps
in designing efficient power circuits.
4. Design and Simulation: Engineers use VI characteristics extensively in circuit design
and simulation. By incorporating these characteristics into simulations, they can predict
how components will perform in different circuit configurations before physically
building them.
5. Troubleshooting: When diagnosing faults in electronic circuits, VI characteristics
provide valuable insights into whether a component is functioning correctly. Deviations
from expected VI curves can indicate issues such as component failure or incorrect
circuit operation.

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6. Education and Training: VI characteristics are fundamental concepts taught in

electronics education. They provide a practical understanding of electronic components
and circuits, helping students grasp the relationship between voltage and current in
various applications.

Overall, VI characteristics serve as essential tools in electronics for analysis, design,

troubleshooting, and education, enabling engineers and students to comprehend and manipulate
the behavior of electronic components effectively.

V-I characteristics applications

The term "vi characteristics" typically refers to the current-voltage (I-V) characteristics of
electronic components or devices, especially semiconductor devices like diodes and transistors.
These characteristics describe how current (I) through the device varies with the applied voltage
(V).

Applications of VI Characteristics:

1. Device Characterization:
o Diodes: Vi characteristics help in understanding how a diode behaves under
forward bias (allowing current to flow easily) and reverse bias (blocking current
flow).
o Transistors: For bipolar junction transistors (BJTs) and field-effect transistors
(FETs), vi characteristics show how the current between different terminals (base-
emitter, collector-emitter for BJTs; drain-source for FETs) changes with the
applied voltages.
2. Design and Analysis:
o Engineers use vi characteristics extensively in the design and analysis of
electronic circuits. For example, understanding the vi characteristics of a diode is
crucial when designing rectifier circuits or voltage regulators.
o In amplifier design, vi characteristics of transistors help determine the operating
points that ensure proper amplification without distortion.
3. Quality Control and Testing:
o Vi characteristics are used in quality control during semiconductor device
manufacturing. Devices are tested to ensure they meet specified vi characteristics
under various operating conditions.
o Testing vi characteristics can reveal deviations from expected behavior, indicating
faulty or degraded components.
4. Simulation and Modeling:
o In circuit simulation software, vi characteristics are modeled to simulate the
behavior of electronic circuits accurately.
o Models based on vi characteristics help predict how a circuit will behave before it
is physically built, saving time and resources.
5. Education and Learning:

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o Vi characteristics are fundamental concepts taught in electronics courses at

universities and technical schools. Students learn how different types of devices
(diodes, transistors) operate and how to analyze their behavior using vi
characteristics.
6. Device Parameter Extraction:
o Vi characteristics provide essential data for determining key parameters of
electronic components, such as diode forward voltage drop, reverse leakage
current, transistor gain (hFE for BJTs), and channel resistance (Rds(on) for
FETs).

Overall, vi characteristics play a crucial role in both the theoretical understanding and practical
application of semiconductor devices in various electronic systems and circuits.

Photo diodes
Photodiodes are a class of diodes that converts light energy to electricity. Their
working is exactly the opposite of LEDs which are also diodes but they convert
electricity to light energy. Photodiodes can also be used in detecting the brightness of
the light. Let’s explore more about the photodiodes in this article.

What is Photodiode?
A photodiode is a PN-junction diode that consumes light energy to produce an electric
current. They are also called a photo-detector, a light detector, and a photo-sensor.
Photodiodes are designed to work in reverse bias condition. Typical photodiode
materials are Silicon, Germanium and Indium gallium arsenide.

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Symbol of Photodiode
The following image shows the symbol of the photodiode:

The symbol of the photodiode is similar to that of an LED, but here the arrow points
inwards.

Photodiode Working
A photodiode is subjected to photons in the form of light which affects the generation
of electron-hole pairs. If the energy of the falling photons (hv) is greater than the energy
gap (Eg) of the semiconductor material, electron-hole pairs are created near the
depletion region of the diode. The electron-hole pairs created are separated from each
other before recombining due to the electric field of the junction. The direction of the
electric field in the diode forces the electrons to move towards the n-side and
consequently the holes move towards the p-side. As a result of the increase in the
number of electrons on the n-side and holes on the p-side, a rise in the electromotive
force is observed. Now when an external load is connected to the system, a current flow
is observed through it.

The more the electromotive force created, the greater the current flow. The magnitude
of the electromotive force created depends directly upon the intensity of the incident
light. This effect of the proportional change in photocurrent with the change in light
intensity can be easily observed by applying a reverse bias.

Since photodiodes generate current flow directly depending upon the light intensity
received, they can be used as photodetectors to detect optical signals. Built-in lenses
and optical filters may be used to enhance the power and productivity of a photodiode.

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Applications of Photodiode
 Photodiodes are used in simple day-to-day applications. The reason for their prominent use
is their linear response of photodiode to light illumination.
 Photodiodes with the help of optocouplers provide electric isolation. When two isolated
circuits are illuminated by light, optocouplers are used to couple the circuit optically.
Optocouplers are faster compared to conventional devices.
 Photodiodes are used in safety electronics such as fire and smoke detectors.
 Photodiodes are used in numerous medical applications. They are used in instruments that
analyze samples, detectors for computed tomography and also used in blood gas monitors.
 Photodiodes are used in solar cell panels.
 Photodiodes are used in logic circuits.
 Photodiodes are used in the detection circuits.
 Photodiodes are used in character recognition circuits.
 Photodiodes are used for the exact measurement of the intensity of light in science and
industry.
 Photodiodes are faster and more complex than normal PN junction diodes and hence are
frequently used for lighting regulation and optical communication.

characteristics of photodiodes
A photodiode is a semiconductor device that converts light into electrical current. Here are some
key characteristics of photodiodes:

1. Photosensitivity: Photodiodes are sensitive to light and can generate a photocurrent

proportional to the incident light intensity.
2. Reverse Bias Operation: Typically, photodiodes are operated in reverse bias mode to
maximize their sensitivity. In this mode, a small reverse voltage is applied across the
photodiode.
3. Responsivity: This refers to the ratio of the generated photocurrent to the incident light
power. It is usually expressed in amps per watt (A/W).
4. Spectral Response: Photodiodes have specific wavelength ranges where they are most
sensitive. This spectral response depends on the material used in the photodiode.
5. Speed: Photodiodes can respond to changes in light intensity very quickly, making them
suitable for applications requiring fast detection or modulation.
6. Dark Current: Even in the absence of light, photodiodes produce a small amount of
current called dark current. This current increases with temperature.
7. Noise: Photodiodes can exhibit various types of noise (thermal noise, shot noise) which
can limit their performance in low-light conditions.
8. Linearity: Ideally, the photocurrent generated by a photodiode is linearly proportional to
the incident light intensity over a certain range.
9. Packaging: Photodiodes are often packaged to protect them from environmental factors
and to facilitate integration into larger systems.

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10. Applications: Photodiodes are used in a wide range of applications including optical
communication systems, light meters, position sensors, and imaging devices (such as in
cameras).

These characteristics make photodiodes versatile devices for detecting and measuring light
across various wavelengths and intensities.

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