What is a diode?
A diode is a specialised electronic component with two
electrodes called the anode and the cathode. The fundamental property of a
diode is its tendency to conduct electric
current in only one direction.
Vacuum Tube Diode
Originally the diode was in the form of a vacuum tube and we
have what is called vacuum tube diodes. I will briefly introduce the construction and the operation of a vacuum tube diode. Figure 1 depicts a simplified diagram of a Vacuum Tube Diode.
Figure 2.1: Vacuum Tube Diode
In the diagram, the vacuum tube is represented by a circle; the
anode A, the Cathode C, the anode and cathode plate, and the filament F. The
cathode plate is heated by the filament when supplied with an AC current from another
source. When the cathode plate gets heated up it starts to emit electrons i.e. thermionic
emission, these electrons are collected by the anode plate A. In the external
circuit, the anode is maintained positive and cathode negative, the electron
will flow from the negative cathode to the positive plate A, thus current flows.
If on the other hand, the external circuit has made plate A
negative and C positive, the electrons emitted by the cathode plate will be
attracted by the positive C and repeled by negative A, electron will not
migrate to the anode A and thus, current will not flow in the circuit. The is
the unique property of a diode, i.e. only allow current to flow in one
direction and not in the other direction, that is why it is called vacuum tube diode.
However, there are some limitation of this vacuum diodes
which are enumerate bellow;
i.
It requires high voltage between plate (A) and cathode
(C) up to about 100 to 200V for its operation.
ii.
The filament requires separate power supply (6V) to
generate electrons by Thermoionic emission.
iii.
It is too bulky and produces a lot of heat.
These limitations necessitate the search for another alternative
which brought about the solid state device called Semiconductor Diodes.
Semiconductor Diodes
These diodes are
semiconductor devices which might be described as passing current in one
direction only. The have two leads like a resistor and allow current to flow
through them depending upon the voltage between the leads. Though, they do not
obey ohms law, meaning that the current flow is not directly proportional to
the applied voltage.
Figure 2.2: Diode Symbol
The diode symbol in figure 2.2 above show the anode A, an
arrow, a bar and cathode C, the arrow depicts the direction of current flow, that
is the current flow from the anode to the cathode.
Applications of
Diodes
-
Diodes are used as rectifiers to convert AC to DC.
-
They are used as Switches in some applications.
-
They are used as Voltage regulators
The ordinary diodes used in electronics can be categorized
into two types;
Signal diodes and rectifier diodes. Both diodes work the
same way by allowing current to flow in one direction, the difference in the
two diode is that;
Signal diodes have
much lower power and current ratting, around 150mA, 500mW maximum compared to rectifier diodes, they function better
in high frequency applications or in clipping and switching applications with
short-duration pulse waveforms.
Rectifier diodes in
the other hand are designed to handle much higher voltage and current and are
typically found in power supplies.
In addition to the two diodes mentioned above, we have other
diodes like the light emitting diodes LEDs
and the Zener Diodes.
P-N Junction Diode
Understanding the operation of the semiconductor diode is
the basis for an understanding of all semiconductor devices. One of the
fundamental structures within semiconductor technology is the PN
junction, it is the fundamental building block of semiconductor diodes
and transistors and a number of other electronic components. PN junction is in essence the basic form a semiconductor
diode.
The diode is actually manufactured as a single piece of
material but to better understand its construction, we will imagine producing two
separate pieces of N-type and P-type material (see figure 1) as discussed in
our Lecture 1 and then “sticking” then together.
Figure 2.3:
Separate Pieces of N-type and P-type Semiconductor material
Now imagine fusing these N-type and P-type semiconductor
material together as in figure 2.4.
Figure 2.4: Fused
N- and P- type Semiconductor material
When the fusing is done, the electron in the N-type will tend to migrate into the P-type and the holes in the P-type into the N-type (due to attraction between unlike charges) these phenomenon tend to cause an even distribution of electron and holes throughout the
semiconductor. As the electron from the N-type migrates across the
junction into the P-type they depletes the holes that are close
to the junction through recombination, likewise the holes in the P-type depletes the electron near the junction in
the N-type by recombining with them.
If we recall the process of producing the N-type and P-type
material, for N-type the donor atom that is introduced into the intrinsic
silicon crystal lattice has the same number of proton and electron in its atom.
After doping, it combines with the silicon atom in a covalent bond to donate a
free electron, this does not mean an extra electron is been produced the
electron is just made to be free due to covalent bonding structure with the
Silicon atoms. Mechanically, the total number of protons (positive charge) in
the material is still equal to the number of electron (negative charge). Ditto
the P-type material.
Now, back to our discussion on P-N junction, as the holes from the P-type depletes the electron in the N-type it deposits immobile positive charge near the junction of the N-type , likewise the electron form the N-type material moving into the P-type depletes the holes near the junction and deposits immobile negative charges near the junction in the P-type side. (see figure 2.5).
Figure 2.5
The area near the junction of both the P- and N- type, where
the mobile charge carrier has been depleted is known as the depletion
layer. When this has occur, the positive charge deposit in the N-type near
the junction will prevent further migration of the holes from the P-type into the
N-type, since like charges repel, it will also attracts the electrons and keep
them in the N-type.
Also the electron deposit in the P-type near the junction
will debar the electron from the N-type material from further migration into it
by repulsion; the electrons attract the holes and keep them in the material.
When this barrier imposed by the charge deposits at the
depletion layer builds up to a certain level, it will completely halt the
migration of charge carriers totally, this is barrier known as barrier voltage (i.e. the one sufficient
to prevent any further migration). The voltage is about 0.6 to 0.7 for silicon
and 0.2 to 0.3 for Germanium.
Forward bias
Connection of a P-N junction diode
Figure 2.6: Forward
bias connection
When we apply an external voltage source to a P-N junction,
as in the connection in figure 2.6, the positive electrode of the voltage source is
connected to the P-type side of the P-N junction and the negative electrode is
connected to the N-type side of the P-N junction. The effect of this is that, the electrons in the N-type will be repelled by the negative electrode
and the holes in the P-type will also be repelled by the positive electrode of
the voltage source, both towards the junction thus narrowing the width of the depletion layer. When the
applied voltage is more than the barrier
voltage at the junction, the holes and electron will migrate across the
junction and current will flow. This is known as forward biasing of the P-N junction diode.
Reverse biased connection of a P-N Junction
Figure 2.7: Reverse
bias connection of a P-N junction
Here, the polarity of the voltage source is reversed
compared to the forward bias connection
described above. The positive electrode of the source is connected to the
N-side of the P-N junction and the negative electrode is connected to the
P-type. With this connection, the holes in the P-type will be attracted by the negative
electrode while the electrons in the N-type are attracted by the positive
electrode, this effect will make the depletion
layer to be widened and thus disallow flow of electron and holes across the junction i.e. current will
not flow across the P-N junction and in the circuit. This connection is called reverse biasing of the diode. This phenomenon makes the P-N junction to function
as a diode as it allow current to flow in only one direction.
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