Different
diodes are used for different purposes. However, the basic working of a
diode still remains the same. It is a p-n junction semiconductor, that
allows current to flow only in one direction. In this article, just how a
diode works, is explained.
Before answering the question, 'How
does a diode work?', let us first understand, what is a diode? It is
also important to understand the practical application of a diode. Let's
start answering all these questions one by one.
What is a Diode?
A
diode is a p-n junction semiconductor, that allows current to flow in
only one direction. A p-type semiconductor is a semiconductor, which
only has positively charged holes. The n-type semiconductor, is one
which has electrons or negatively charged carriers. The p-type and
n-type semiconductors are diffused into each other to form a p-n
junction. Mostly, silicon is used for the semiconductor material, while
in many cases, germanium is also used. Diodes are made, based on the
purpose for which they are going to be used. For example, a varactor
diode is used as a variable capacitor and a zener diode is operated in
the reverse biased mode, hence their manufacturing process is also
different. An LED (Light Emitting Diode) is constructed, such that the
holes and electrons on recombination, release energy in the form of
light. Hence, they are manufactured from materials like gallium
arsenide, gallium phosphide etc. instead of silicon, so as to have a
higher potential barrier.
How do Diodes Work?
To answer the
question, 'How does a diode work?', we will need to understand its V-I
characteristics. V-I characteristics is the graph plotted between the
voltage and the current, at which the diode is working. A diode is a
voltage controlled device. In a diode, current flows in the forward
biased mode, while there is no flow of charge, when the diode is reverse
biased. A diode is said to be in the forward biased mode when the
positive terminal of the battery is connected to the p-terminal and
negative side of the diode is connected to the n-terminal. Once the
voltage is applied to the diode in the forward biased mode, the diode
immediately does not allow the charge to flow. On increasing the
voltage, such that it reaches the breakdown voltage, the current flow
starts increasing and reaches its maximum. This breakdown voltage is
different for different semiconductor materials. For silicon, the
breakdown voltage is 0.7 volt. On applying the voltage, the positively
charged holes are repelled by the positive terminal of the battery and
the negatively charged electrons are repelled by the negative terminal
of the battery and start flowing in the opposite directions. This causes
flow of charge in the positive to negative direction.
Recombination
of the electrons and holes takes place at the junction and a small
region is developed at the junction. It consists of minority carriers,
electrons in the p-layer and majority carriers, holes in the n layer.
This limited region on both sides of the junctions is known as the
depletion region. Once the depletion region is formed, the current flow
becomes practically constant. Further increase in voltage can destroy
the depletion region and hence the diode. Most diodes when operated in
the reverse biased mode, get destroyed on increasing the voltage to a
large extent. When a diode is operated in the reverse biased mode, there
is practically no flow of charge initially. When the voltage is
increased and reaches the reverse threshold voltage, current increases
indefinitely and flows in the reverse direction, destroying the diode.
However, zener diode is operated in the reverse biased mode and finds a
wide range of applications.
Applications of Diodes
Though all
diodes are not used for the same purpose, the basic function is the
same. It allows the flow of charge in only one direction. Varactor
diodes are used in places where they need to serve the purpose of a
variable capacitor. Tunnel diodes are used in applications where we need
the current to increase and decrease alternately. LEDs compulsorily
need to operate in the forward biased mode. Zener diodes, on the other
hand, are operated in the reverse biased mode and are used as voltage
regulators.
Diodes are one of the most fundamental devices that
are used in any electronic kits. Even if you are using a microcontroller
or microprocessor to prepare your project, it is most likely that you
will need a diode somewhere in the circuit. Knowing answers to questions
like 'What is a diode?' and 'How does a diode work?' is extremely
necessary, before undertaking any electronic project or designing any
circuit.
By Shah Newaz Alam
