Introduction
A diode is
a specialized electronic component with two electrodes called the anode and the
cathode. Most diodes are made with semiconductor materials such as silicon,
germanium, or selenium.
The
fundamental property of a diode is its tendency to conduct electric current in
only one direction.
In the
below fig a shows the physical structure of diode and b shows illustrate
its terminal structure.
When
placed in a simple battery-lamp circuit, the diode will either allow or prevent
current through the lamp, depending on the polarity of the applied voltage.
Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.
When the
polarity of the battery is such that electrons are allowed to flow through the
diode, the diode is said to be forward-biased. Conversely, when the
battery is “backward” and the diode blocks current, the diode is said to be reverse-biased.
A diode may be thought of as like a switch: “closed” when forward-biased and
“open” when reverse-biased.
Oddly enough, the direction of the diode symbol’s
“arrowhead” points against the direction of electron flow. This is
because the diode symbol was invented by engineers, who predominantly use conventional
flow notation in their schematics, showing current as a flow of charge from
the positive (+) side of the voltage source to the negative (-). This
convention holds true for all semiconductor symbols possessing “arrowheads:”
the arrow points in the permitted direction of conventional flow, and against
the permitted direction of electron flow.
A
forward-biased diode conducts current and drops a small voltage across it,
leaving most of the battery voltage dropped across the lamp. If the battery’s
polarity is reversed, the diode becomes reverse-biased, and drops all of
the battery’s voltage leaving none for the lamp. If we consider the diode to be
a self-actuating switch (closed in the forward-bias mode and open in the
reverse-bias mode), this behavior makes sense. The most substantial difference
is that the diode drops a lot more voltage when conducting than the average
mechanical switch (0.7 volts versus tens of millivolts).
This
forward-bias voltage drop exhibited by the diode is due to the action of the
depletion region formed by the P-N junction under the influence of an applied
voltage. If no voltage applied is across a semiconductor diode, a thin
depletion region exists around the region of the P-N junction, preventing
current flow. (Figure below
(a)) The depletion region is almost devoid of available charge carriers, and
acts as an insulator:
The
schematic symbol of the diode is shown in Figure above
(b) such that the anode (pointing end) corresponds to the P-type semiconductor
at (a). The cathode bar, non-pointing end, at (b) corresponds to the N-type
material at (a). Also note that the cathode stripe on the physical part (c)
corresponds to the cathode on the symbol.
If a
reverse-biasing voltage is applied across the P-N junction, this depletion
region expands, further resisting any current through it. (Figure below)
Conversely,
if a forward-biasing voltage is applied across the P-N junction, the depletion
region collapses becoming thinner. The diode becomes less resistive to current
through it. In order for a sustained current to go through the diode; though,
the depletion region must be fully collapsed by the applied voltage. This takes
a certain minimum voltage to accomplish, called the forward voltage as
illustrated in Figure below.
For
silicon diodes, the typical forward voltage is 0.7 volts, nominal. For
germanium diodes, the forward voltage is only 0.3 volts. The chemical
constituency of the P-N junction comprising the diode accounts for its nominal
forward voltage figure, which is why silicon and germanium diodes have such
different forward voltages. Forward voltage drop remains approximately constant
for a wide range of diode currents, meaning that diode voltage drop is not like
that of a resistor or even a normal (closed) switch. For most simplified
circuit analysis, the voltage drop across a conducting diode may be considered
constant at the nominal figure and not related to the amount of current.
Actually,
forward voltage drop is more complex. An equation describes the exact current
through a diode, given the voltage dropped across the junction, the temperature
of the junction, and several physical constants. It is commonly known as the diode
equation:
You need not be familiar with the “diode equation” to analyze simple diode circuits. Just understand that the voltage dropped across a current-conducting diode does change with the amount of current going through it, but that this change is fairly small over a wide range of currents. This is why many textbooks simply say the voltage drop across a conducting, semiconductor diode remains constant at 0.7 volts for silicon and 0.3 volts for germanium. However, some circuits intentionally make use of the P-N junction’s inherent exponential current/voltage relationship and thus can only be understood in the context of this equation. Also, since temperature is a factor in the diode equation, a forward-biased P-N junction may also be used as a temperature-sensing device, and thus can only be understood if one has a conceptual grasp on this mathematical relationship.
A reverse-biased diode prevents current from going
through it, due to the expanded depletion region. In actuality, a very small
amount of current can and does go through a reverse-biased diode, called the leakage
current, but it can be ignored for most purposes. The ability of a diode to
withstand reverse-bias voltages is limited, as it is for any insulator. If the
applied reverse-bias voltage becomes too great, the diode will experience a
condition known as breakdown (Figure below),
which is usually destructive. A diode’s maximum reverse-bias voltage rating is
known as the Peak Inverse Voltage, or PIV, and may be obtained
from the manufacturer. Like forward voltage, the PIV rating of a diode varies
with temperature, except that PIV increases with increased temperature and
decreases as the diode becomes cooler—exactly opposite that of forward
voltage.
Typically,
the PIV rating of a generic “rectifier” diode is at least 50 volts at room
temperature. Diodes with PIV ratings in the many thousands of volts are
available for modest prices.
Type of
diode
1. Light
Emitting Diode (LED)
2. Avalanche
Diode
3. Laser
Diode
4. Schottky
Diodes
5. Zener
diode
6. Photodiode:
7. Varicap
Diode or Varactor Diode:
8. Rectifier
Diode
9. Small
signal or Small current diode -
10.
Large signal diodes
- Transient voltage supression diodes
- Gold doped diodes
- Super barrier diodes
- Point contact diodes
- Peltier diodes
- Gunn diode
- Crystal diode
- Avalanche diode
- Silicon controlled rectifier
- Vaccum diodes
for more information contact
the below books
- Gupta J.B (2009/10): “An Integrated Course in Electronics Engineering”, S.K Kataria & Sons, 1st Edition.
- Kenneth J. Ayala (1995): “The 8051 Microcontroller Architecture, Programming and application” West publishing company
- Malrino Paul (2000): Electronic Principle, Tata McGraw Hill,India,4th Edition.
- Mehta V.K and Rohit Mehta(1939): Principles of Electronics, S-Chand publishing, New Delhi,India,11th Edition.
- Paul Horowitz and Winfield Hill (1989): “The Art of Electronics” Cambridge University Press, Second Edition.
- Robert L.Boylestat and Louis Nashlelsky (2006): Electronic Devices and Circuit theory, Pearson Educational Inc, USA,9th Edition.
- Ronald J. Tocci, Neals S. Windner and Gregory .L. Moss (2007): “Digital Systems Principles and Applications” Pearson International, Tenth Edition.
- Tom Floyd (2010): “Digital Fundamentals”, Pearson International Edition.
- Websites: www.google.com, www.instructables.com www.wikipedia.com, www.alldatasheet.com
More rectifier diode definition content will be great
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