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A diode is a very basic semiconductor
component containing junction of P-Type and N-Type material. It consists of one
block of P-Type material and one block of N-Type material. Base material of
almost all semiconductors is Silicon and sometimes Germanium is also used.
P-Type material is made by doping silicon with trivalent impurities such as
Boron, Gallium, Indium etc are called acceptor impurity, because they have
empty spaces to accept one more electron as silicon has four electrons in its
outer shell and these impurities have only three in outer shell to make bond.
So we say P-Type material consist of holes or these vacant spaces.
On the other hand N-Type material
is made by doping base silicon material with impurities like phosphorus, arsenic, antimony, bismuth
which are pentavalent materials, hence they make bond with four electrons in
outer shell of Silicon and have one extra electron to conduct.
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| Diode Symbol |
The symbol show a triangle with a
vertical bar, triangular side is Anode and bar side is cathode. That means
current is allowed to flow from anode (A) to cathode (K), the triangle is
actually resemble an arrow saying current can flow from A to K, but bar
indicates a barrier saying current cannot flow from K to A. In this animated
simulation video below, you can see that when I press switch S1, the lamp doesn’t glow
as the diode is not allowing current from positive terminal of battery till
lamp as it encounters the bar (i.e. cathode)
of diode, but when I press switch
S2, it allows the current from positive terminal of battery to flow from anode
of diode to cathode and to the lamp hence the lamp glow up. This feature of
conducting current in one direct helps us to use diode to convert AC to DC
which we will see further.
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| Diode |
A diode physically looks like a
small cylindrical part with two wires coming out from both side with black body
and a white/silver color ring made on one side of body indicating that is cathode
(K).
So the condition when positive supply is applied
to anode and negative toward cathode, the electrons from negative terminal of
battery pushes the electrons in N-type material and they cross the barrier and
move to hole or vacant spaces in P-type material, this causes a current
opposite to flow of electron i.e. from A to K and hence this condition is
called forward biasing the diode as we can see for D2 in video.
Hence a diode conducts current in only one
direction when it is forward biased. The reverse condition of applying positive
supply to K and negative to A is called reverse biasing and a diode does not
conduct electricity in reverse biased condition as in case of D1 in video. But
yes there is a breakdown voltage of the barrier and if the reverse voltage of
more than that barrier voltage is applied to diode, barrier will break down and
off course it will start conducting in reverse direction, but normally this
reverse breakdown voltage is very high, hence that never reaches in a circuit.
In the second video I have
removed one diode and connected a voltmeter to measure voltage drop across
diode when it conducts in forward biased condition. We can see that the meter
shows 0.77V when the current flow through it under forward biased condition. But
in reverse bias condition, when there is no current through it, there is no
voltage drop across it. So, whenever we are using one diode in any circuit, we
are losing 0.7volt across it, sometimes it is useful and sometimes it is a loss
to us. This is called forward voltage drop and different diodes have different
diodes have slightly different voltage drop. Mostly silicon diodes have voltage
drop starting from 0.7V can go to 1V also and for Germanium diodes it starts
from around 0.3V. Watch the simulation video for more clarity.
Zener Diode
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| Zener Diode |
Construction of zener diode is
also similar to diode with small modification due to which it has a lower
breakdown voltage; this is achieved by heavily doping the silicon base with
P-type and N-Type impurities. It can work as a normal diode when it is
connected in forward biased mode, but in reverse mode when the supplied voltage
exceeds the breakdown voltage, the diode works as a voltage regulator because
the voltage across it remains same over the breakdown voltage. Normal diodes
designed to work in forward biased mode but not to work in reverse biased mode
in breakdown voltage region as compared to zener diodes
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| Symbol |
which are designed to
work specially in breakdown voltage region. We can get zener diode operating in
range of 2.4V to 200V. Physically looks similar to diode except transparent body
with red color internal structure visible. Symbol is similar to diode with broken/bent
bar showing it works in breakdown voltage range where the current flow from cathode
to anode breaking this barrier. The black ring on body indicates the cathode.
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| Zener Diode Circuit |
A typical zener diode based
voltage regulator circuit consists of a DC voltage source greater than zener
voltage, a current limiting resistor ‘R’ and a zener diode of required output
voltage. As we can in the circuit we have used three cell of 1.5V in
series to get a battery of 4.5V feeding supply to zener diode through resistor ‘R’. Here the anode of zener is connected to negative of battery and cathode
is connected to positive terminal through resistor ‘R’, hence here the
zener is working in reverse biased condition in the breakdown voltage which is
3.3V here as we are using 3.3V zener diode.
Now a days many simple voltage
regulators are available in for of chip or IC which have internally this basic
circuit using zener and some more components, hence these zener based voltage
regulator circuits are rarely used now a days.
Further since we have learnt
basics of transformer, diode, capacitor and zener diode, we will
next learn basic transformer based AC to DC circuit and regulated DC source using
zener to the AC to DC converter.
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