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.
The term kT/q describes the voltage produced within the P-N junction due to the action of temperature, and is called the thermal voltage, or Vt of
the junction. At room temperature, this is about 26 millivolts. Knowing
this, and assuming a “nonideality” coefficient of 1, we may simplify
the diode equation and re-write it as such:
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
The below pics shows different types of diode
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
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