Diode's and LED's - WolfWikis
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Diode's and LED's
From WolfWikis
Contents
1 Introduction
2 History
3 Analysis
3.1 The p-n junction diode
3.2 Bridge Rectifiers
3.3 Zener Diodes
3.4 Light Emitting Diode (LED)
3.5 Tunnel Diode
4 Conclusion
5 Works Cited
Introduction
A diode, or "rectifier," is any device through which electricity can flow in only one direction [1] while
blocking current in the other direction. Although diodes are made differently and of different materials, they all
have three things in common: they do not obey Ohm’s Law, the current they pass depends upon the voltage
between the leads and they have two leads like a resistor. [2]
Diodes are made of two terminals called the cathode and anode. These are made of semiconducting materials
in which impurities are added (doped) in order to give these useful properties. There are two kinds of
semiconducting diodes: a P-N junction diode and a Schottky diode. [3]
The boundary between the semiconducting materials in a diode has turned out to be crucial for our daily lives.
Diodes change the alternating current that comes from your wall outlet into the direct current that most
appliances need. [1]
LEDs, or Light Emmiting Diodes, are like most diodes, in that current flows through easily, however, when
the electrons moving across the junction come across a hole, or lack of electron, the electrons drop down an
energy level and emmit photons. LEDs harness these photons and use them to provide an effective and longlasting light source.
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History
The first diodes were crystals used as rectifiers in home radio kits. These diodes were discovered by the British
scientist Henry J. Round in 1907 when he fed a weak radio signal into the crystal through a very fine wire
called a cat's whisker. The crystal removed the high frequency radio carrier signal, allowing the part of the
signal with the audio information to come through loud and clear. The crystal was filled with impurities,
making some sections more resistant to electrical flow than others. Using the radio required positioning the
cat's whiskers over the right kind of impurity to get electricity to flow through the crystal to the output below
it.
At the time, though, no one really understood about the
impurities -- then in 1939 Russell Ohl accidentally
discovered that it was the boundary between sections of
different purity that made the crystal work. Now that the
way they work is understood, manufacturers make crystal
diodes that work much more consistently than the ones in
those original radio kits. [1]
Robert Biard [4]
In terms of the LED's first development, the first infrared
LEDs were discovered in 1961 by Bob Biard and Gary
Pittman.[4] Just one year later, Nick Holonyak Jr. developed
the first visible spectrum producing LEDs, however it only
Gary Pittman [4]
produced the color red. [5] New LED technology was soon after developed by many scientists to improve the
range of colors as well as the light's brightness. However, LEDs were very expensive to produce and not very
practical. But, in 1968, the Monsanto Corporation began the first mass-production of LED lights. Ever since,
LED technology has been further developed and are proving to be much more efficient and long-lasting than
conventional light bulbs.
Analysis
The physics of diodes is modern in the sense that it is “derived” from the principles of quantum mechanics. By
using the description of the atom provided by modern physics, the band theory of solids explains how diodes
function. [6]
Diodes are made of semiconductor devices, and band theory helps us understand what makes a semiconductor.
This theory states that allowed energy states for electrons in a solid are nearly continuous over certain ranges,
these are referred to as energy bands. There are also energy gaps, which are forbidden states between energy
bands. When several atoms interact in a solid, energy levels split into nearly continuous bands made up of
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closely spaced energy levels. [6]
When electrons “jump” from the valence band into the conduction band, a vacancy is left behind. This
vacancy is referred to as a hole, and it can be thought of as a positive charge. By adding a small amount of
another element to a semiconductor (this process is known as doping), the properties of the semiconductor can
be changed in order to make it more or less conductive. Depending on which element is added, one can create
an n-type or a p-type semiconductor. An n-type semiconductor has some electrons weekly bonded which can
easily be excited into the conduction band. In a p-type semiconductor, holes are added to the material, which
can easily accept an electron from the valence band. When an electric field is applied to a p-type, the electrons
move into these holes, creating a current. [6]
In a semiconductor, there exists a valence band (filled energy band) followed by a conduction band (unfilled
energy band), which are separated by an energy gap (usually on the order of 1 eV). Because this gap is not
very big, when an electric field is applied, electrons may get excited enough to “jump” across this gap and
produce a current. [6]
The p-n junction diode
A P-n junction diode is made by putting a p-type and an n-type semiconductors together. This allows for
current to easily flow in one direction (forward bias) but not in the other direction (reverse bias). [6] When
there is no potential difference applied to the diode, electrons naturally move from the n-type to the p-type
material in order to fill in the holes at the boundary of the two semiconductor materials. A depletion zone is
formed in which the semiconductor is transformed to its insulating state because all the holes are filled;
therefore there are no free electrons or holes for electrons to move into.[7]
At the junction, free electrons from the N-type
material fill holes from the P-type material. This
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creates an insulating layer in the middle of the diode
called the depletion zone.[7]
In order to get rid of the depletion zone and create a current, electrons must move from the n-type material to
the p-type (and the holes move in the opposite direction). By applying a high enough voltage, and connecting
the positive terminal to the p-type side and the negative terminal to the n-type side, the electrons are repelled
by the negative electrode and attracted towards the positive electrode. The depletion zone therefore disappears
and a charge moves across the diode. [7]
When the negative end of the circuit is hooked up to
the N-type layer and the positive end is hooked up to
P-type layer, electrons and holes start moving and the
depletion zone disappears.[7]
By trying to run a current the opposite way (In the case of a reverse bias) the electrons become attracted to the
positive terminal and the holes are attracted to the negative terminal, therefore increasing the depletion zone
and no charge moves across the diode. [7]
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When the positive end of the circuit is hooked up to
the N-type layer and the negative end is hooked up to
the P-type layer, free electrons collect on one end of
the diode and holes collect on the other. The
depletion zone gets bigger. [7]
The current in the forward bias direction is given by I=Io (e (eV/kt)-1) in which I o indicates the current in the no
bias situation, e is the charge of an electron, V is the forward bias voltage, k boltzman’s constant, and T the
temperature. This is a good approximation of the I-V curve shown below. [6]
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Current versus applied voltage across a diode. [8]
Bridge Rectifiers
Bridge rectifiers are an important application of diodes because many devices use direct current and the
current that we get from our wall sockets is alternating. These are used to convert Alternating Current (AC) to
Direct Current (DC). A bridge rectifier is set up so that it only allows current to flow in one direction through a
resistor when an alternating current is supplied. The resulting current has the form: I=Imax sin(ωt) [6]
Bridge Rectifier diagram. [9]
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Bridge
Rectifiers.[10]
Zener Diodes
Considering a forward bias and looking at the equation I=Io (e (eV/kt)-1), if V is changed by a small amount , I
will change rapidly. Another way to say this is that if the current is changed a great amount, the voltage will
change very little. Therefore diodes can be used to regulate the voltage in a circuit. If the terminals are
reversed (reverse bias), and the voltage is increased, the diode will reach a break down voltage in which the
diode can no longer withstand the reverse-bias voltage impressed across it. Then the diode will similarly
regulate the voltage and keep it fairly constant at that voltage. This breakdown voltage is much greater than
the forward voltage; therefore it is useful to use this reverse bias property when dealing with greater voltages.
As long as the diode operates in the “steep” part of the I-V curve, an increase in current will result in a fairly
constant voltage across the diode. [6] However, most ordinary diodes break down destructively, and so the
Zener diode is a special diode which serves this purpose. These diodes are manufactured to withstand
anywhere from a few volts to hundreds of volts. Zener diodes are commonly used in regulated voltage sources
in electronic instruments. [11]
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[12]
Light Emitting Diode (LED)
An LED works just like a normal p-n junction diode. The LED bulb itself contains a doped chip, which is a
piece of semiconducting material through which electrons flow easily, thus creating a p-n junction. Inside this
doped chip, the current flows from the p-side of the junction to the n-side, while electron holes, which are just
the absence of electrons, flow in the reverse direction. [13]
[14]
When an electron meets one of these holes it drops down an energy level and emits photons.
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[14]
The color of the light produced depends on the band energy, which is the energy range in which an electron
can exist, of the material used as the semiconductor.
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[14]
Obviously it is important to choose proper materials for the p-n junction because only certain materials have a
band energy that corresponds to the visible light spectrum. These types of materials are said to have a direct
band gap. Also a material with a high index of refraction is usually chosen for the bulb as a way of harnessing
the light emitted to provide a larger and more uniform lighted area.[13]
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LED light bulb
Alternatives to the above bulb type LED are various forms of a chip LED. These generally provide a greater
viewing angle with less brightness in the illuminated region and are used as indicator lights such as the power
light on a laptop. Some examples are:
Chip-type LED [15]
Chip-type LED [15]
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Chip-type LED [15]
LED's are also used in Photovoltaic (or Solar) Cells. By considering the behavior of an LED in the reverse
direction, one can see that if a photon is absorbed by the diode, an electron can be "kicked" from the valence
band up to the conduction band. This produces an extra electron (now in the conduction band) and an extra
hole (in the valence band). This results in a current when connected to a circuit. In other words, solar energy is
being converted into electricity.[6]
Tunnel Diode
A tunnel diode differs from a p-n junction diode in that electrons pass from one region into another region
through a junction. These are usually made of heavily doped materials. By thinking of the junction as a
potential barrier, one can relate this behavior to the tunneling effect which results from quantum mechanics.
By applying and changing positive and negative voltages, the height of the potential barrier can be changed to
modify the probability of tunneling. Electron tunneling is faster than the diffusion of electrons and holes which
occurs in a p-n junction diode. These diodes have proven to be very effective when used in switching circuits
and high frequency oscillators[6] . The diagram below compares the currents produced by a typical tunneling
diode and a p-n junction diode.
Tunnel Diode vs. P-n diode [16]
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Conclusion
The future looks extremely 'bright' for diodes, especially LEDs.
AC power is the only type of electricity that can be transmitted any considerable distance currently, so it looks
like we will be converting AC to DC for a long time. This creates a huge need for diodes and improvements
thereof.
There have been many recent improvements in chip type LEDs making them very useful for flashlights as well
as other lights like those used on police cars and ambulances. LEDs consume much less power than typical
incandescent or fluorescent bulbs per unit of light emitted and last much longer, which is very exciting in this
time of concern for the environment. This means flashlights will operate much longer on a set of batteries,
police car lights will last longer and require less energy, and indicator lights for computers and other
electronics will hardly ever need to be replaced. Also if an alternative light energy source isn't soon found
LEDs may become widely used in everything from indoor lighting to car headlights. In fact, there is already a
project in which several universities, including NC State, are implementing LEDs to light classrooms and
parking lots.
Finally, as a result of the current "going green" movement, diodes will have an important role to play in
converting solar energy to electricity. Even though the efficiency of these solar cells is low, recent progress
has been made in improving this efficiency. By finding new ways of improving solar cells, a substantial
amount of electricity could be produced from the Sun's energy as our atmosphere receives about 1400
W/m^2 [6] and about half of this energy makes it to the Earth's surface.
Works Cited
1. ↑ 1.0 1.1 1.2 "Diodes", PBS online, accessed March 26
2009,<http://www.pbs.org/transistor/science/info/diodes.html>.
2. ↑ "The Diode", St. Andrews University,St Andrews, Scotland, accessed March 26 2009,<http://www.standrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/diode.htm>.
3. ↑ "Diode", Encylobeamia, accessed March 25
2009,<http://encyclobeamia.solarbotics.net/articles/diode.html>.
4. ↑ 4.0 4.1 4.2 "The Quart Watch", The Smithsonian Institute, accessed April 23
2009,<http://invention.smithsonian.org/centerpieces/quartz/inventors/biard.html>.
5. ↑ "Nick Holonyak Jr.", Lemelson MIT Program, accessed April 21 2009,<http://web.mit.edu/invent/awinners/a-holonyak.html>.
6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 Thornton,Stephen & Rex, Andrew,Modern Physics for
Scientists and Engineers, Third Edition, Brooks/Cole, 2006.
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7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 "How Light Emitting Diodes Work", HowStuffWorks Inc., accessed April 11
2009,<http://electronics.howstuffworks.com/led1.htm>.
8. ↑ "I-V Curve of Diodes", accessed April 11 2009,<http://www.utc.edu/Faculty/TatianaAllen/ivcurve.html>.
9. ↑ "Bridge Rectifier", accessed April 13 2009,<http://hyperphysics.phyastr.gsu.edu/Hbase/electronic/rectbr.html>.
10. ↑ "Diodes", accessed April 13 2009,<http://www.kpsec.freeuk.com/components/diode.htm>.
11. ↑ "Zener Diodes",All About Circuits, accessed April 15
2009,<http://www.allaboutcircuits.com/vol_3/chpt_3/11.html>.
12. ↑ "Diode",NEC Electronics, accessed April 15 2009,<http://www.necel.com/en/faq/f_diode.html>.
13. ↑ 13.0 13.1 "LEDS: From Indicators to Illumination." Lighting Research Center, accessed April 24 2009,
<http://www.lrc.rpi.edu/programs/Futures/LF-LEDs/index.asp>.
14. ↑ 14.0 14.1 14.2 "How Can a Diode Produce Light", How Stuff Works, accessed April 20
2009,<http://www.angliac.co.uk/NewBookPDF/Optos/288_290.pdf>.
15. ↑ 15.0 15.1 15.2 "LEDs, Surface Mount" accessed April 19
2009,<http://www.angliac.co.uk/NewBookPDF/Optos/288_290.pdf>.
16. ↑ "Bipolar Transistor dan Komponen elektronika lainnya" accessed April 26
2009,<http://203.130.205.68/dosen/aji/embeded/transistor.htm>.
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