Definition:
A Diode is an electrical
device that allows the current to flow in one direction only and shows maximum
resistance for the current to flow in the opposite direction. A diode carries
two terminals called anode and cathode. The anode is a positive terminal and
the cathode is a negative terminal and the current will only flow from the
anode terminal to the cathode terminal.
Symbol:
The following figure shows the electrical
symbol of the diode.
Working:
The working of the diode depends
on the interaction between the P and N junction. The P junction is a region
that contains a high concentration of holes while the N junction is a region
that contains a high concentration of electrons.
To understand the working of the
diode we’ll take three following conditions.
A: Forward
Biased Diode:
Forward biased condition will
occur when the P-type material of the diode is connected with the positive
terminal of the source and the N-type material is connected with the negative
terminal of the source.
At first, when we increase
the voltage from zero, no current will flow through the diode due to the
presence of a potential barrier. However, when the applied voltage exceeds the
forward potential barrier, the diode will behave as a short-circuited path and
the current flow will be resisted by the external resistors.
Reverse Biased
Diode:
This condition will occur when
the P-type material of the diode is connected to the negative terminal of the
source and the N-type material is connected to the positive terminal of the
source.
In this condition, the
holes present in the P region will shift further away from the depletion region
due to electrostatic attraction. As a result, more uncovered negative ions will
be left behind. In this scenario, there will be no current flow in the circuit.
Unbiased
PN-Junction Diode:
In unbiased conditions, there
will be no voltage applied from the external energy source. When the P and N
junctions are attached, it results in the flow of electrons from the n-type
material to the p-type material, and the flow of holes from p-type material to
the n-type material.
This flow of charge
carriers will generate the third region where no charge carriers are present,
this third region is called the depletion region.
Characteristics:
The characteristics of the diodes
can be demonstrated by the current-voltage curve. This means, for a certain
amount of current we’ll measure the respective voltage across it. The resistors
show the linear V-I relationship, however, in the case of diodes this relationship
is different. The following figure shows the V-I curve of the diode.
The diode operates in
three different regions based on the voltage applied across it.
· Forward Bias Region: When
the positive voltage is applied across the diode, the diode will be turned ON
and the current will pass through it. To flow the current through the diode in
the forward bias region, the positive voltage should exceed the forward voltage
Vf.
· Reverse Bias Region: In
this region, the diode will be turned OFF and the applied voltage will be less
than the forward voltage Vf and more than the breakdown voltage Vbr. In this
condition, the device shows the maximum resistance for the current, however, a
very small amount of current will flow through the diode called reverse
saturation current.
· Breakdown Region: When
a very large and negative voltage is applied across the diode, it will allow
the current to flow in a reverse direction from cathode to anode. This region
is called the breakdown region.
Types:
The diodes are divided into the
following different types.
Zener Diodes:
Zener diodes are heavily doped
semiconductor devices that conduct in reverse bias conditions. They are also
known as reverse breakdown diodes and come with breakdown voltage below 5V.
Because of the presence of heavily doped semiconductor material, the Zener
diode constitutes a very thin depletion region to increase the electric field
intensity.
Photodiodes:
Photodiodes are the right match
for solar cells and optical communication applications
because they can sense light and are mostly packaged in a material that allows
the light to pass through it. A range of photodiodes can be incorporated in a
single device either as a two-dimensional array or as a linear array.
Avalanche Diodes:
Avalanche diodes are similar to
Zener diodes with one difference i.e. both come with a temperature coefficient
of different polarities. These diodes start conducting in the reverse direction
when the reverse-biased voltage surpasses the breakdown voltage. At a certain
reverse voltage, these diodes break down without being destroyed.
Crystal Diodes:
These diodes are point contact
diodes. They contain a semiconductor crystal material for cathode, and the
anode is made up of thin metal. These diodes are also called Cat’s Whisker
Diode and are not easily available in the market.
LED Diodes:
LED diodes contain a crystalline
substance that can emit light in different colours including orange, red,
green, and blue, based on the crystalline substance used in the diode. These
diodes are widely used in signal applications and are called low-efficiency
devices.
PIN Diodes:
PIN diodes are widely used in
power electronics because they can bear high voltages. A PIN diode contains a
p-type/intrinsic/n-type structure because of an un-doped central layer. They
are frequently employed as attenuators and frequency switches.
Applications:
The diodes are used in the
following applications.
· Used
as a waveform clipper
· Used
to control the flow of current
· Incorporated
for demodulation of the amplitude signal
· Employed
for temperature measuring applications
· Used
in the construction of rectifiers to convert AC signal to DC signal
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