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Light Emitting Diode LED
LED Light Emitting Diode What is an LED A light emitting diode LED is essentially a PN
junction optosemiconductor that emits a monochromatic single color light when operated in a
forward biased direction. LEDs convert electrical energy into light energy. They are
frequently used as quotpilotquot lights in electronic appliances to indicate whether the circuit
is closed or not.
LED Light Emitting Diode LED How it works
When sufficient voltage is applied across the the LED, electrons can move easily in only one
direction across the junction between the p and n regions. In the p region there are many
more positive than negative charges. When a voltage is applied and the current starts to
flow, electrons in the n region have sufficient energy to move across the junction into the p
region.
LED Light Emitting Diode LED How it works When current flows across a diode Negative
electrons move one way and positive holes move the other way .
LED Light Emitting Diode LED How it works The wholes exist at a lower energy level than
the free electrons Therefore when a free electrons falls it losses energy .
LED Light Emitting Diode LED How it works This energy is emitted in a form of a photon.
which causes light The color of the light is determined by the fall of the electron and hence
energy level of the photon .
LED Light Emitting Diode Inside a Light Emitting Diode . Diode . Terminal Pins . Transparent
Plastic Case .
LED Light Emitting Diode Kinds of LEDs .
Remember to connect the LED the correct way round . for quick testing purposes a k
resistor is suitable for most LEDs if your supply voltage is V or less.LED Light Emitting Diode
Testing LEDs Never connect an LED directly to a battery or power supply It will be destroyed
almost instantly because too much current will pass through and burn it out. LEDs must have
a resistor in series to limit the current to a safe value.
LED Light Emitting Diode How Much Energy Does an LED Emit The energy E of the light
emitted by an LED is related to the electric charge q of an electron and the voltage V
required to light the LED by the expression E qV Joules. This expression simply says that the
voltage is proportional to the electric energy. . . The constant q is the electric charge of a
single electron. x Coulomb. and is a general statement which applies to any circuit. as well
as to LEDs.
x Joule. Joule. and you wished to find the corresponding energy required to light the LED. x .
Multiplication of these numbers then gives E . So the Energy required to light the LED is E
qV or E . and the voltage measured between the leads of is . Let us say that you have a red
LED. Volts. .LED Light Emitting Diode Finding the Energy from the Voltage Suppose you
measured the voltage across the leads of an LED. since a CoulombVolt is a Joule.
Different LED chip technologies emit light in specific regions of the visible light spectrum and
produce different intensity levels. Blue and white LEDs are much more expensive than the
other colours. amber. The colour of an LED is determined by the semiconductor material.
The coloured packages are also available as diffused the standard type or transparent. not
by the colouring of the package the plastic body. yellow. green. LEDs of all colours are
available in uncoloured packages which may be diffused milky or clear often described as
water clear.LED Light Emitting Diode Colours of LEDs LEDs are made from galliumbased
crystals that contain one or more additional materials such as phosphorous to produce a
distinct color. LEDs are available in red. orange. blue and white. .
.colour LED. or both together to give the third colour. Note the different lengths of the three
leads. the outer leads a and a are the anodes to the LEDs allowing each one to be lit
separately.LED Light Emitting Diode Colours of LEDs Tricolour LEDs The most popular type
of tricolour LED has a red and a green LED combined in one package with three leads. They
are called tricolour because mixed red and green light appears to be yellow and this is
produced when both the red and green LEDs are on. The diagram shows the construction of
a tri . The centre lead k is the common cathode for both LEDs.
Only one of the LEDs can be lit at one time and they are less useful than the tricolour LEDs
described above. .LED Light Emitting Diode Colours of LEDs / Bicolour LEDs A bicolour LED
has two LEDs wired in inverse parallel one forwards. one backwards combined in one
package with two leads.
NonDiffused LEDs .LED Light Emitting Diode Comparison Of Chip Technologies For
WideAngle.
LED Light Emitting Diode LED Performance LED performance is based on a few primary
characteristics Color White light Intensity Eye safety information Visibility Operating Life
Voltage/Design Current .
Although process variations are NM.LED Light Emitting Diode LED Performance Colour
Peak wavelength is a function of the LED chip material. the to NM wavelength spectral
region is where the sensitivity level of the human eye is highest. . it is easier to perceive color
variations in yellow and amber LEDs than other colors. Therefore.
middle. and lower parts of the spectrum red. when combined. the eye does not require a
mixture of all the colors of the spectrum to perceive white light. . the additive mixture of colors
appears white.LED Light Emitting Diode LED Performance White Light When light from all
parts of the visible spectrum overlap one another. However. green. Primary colors from the
upper. appear white. and blue.
encapsulation.LED Light Emitting Diode LED Performance Intensity LED light output varies
with the type of chip. .quot and quotultrabright to describe LED intensity. Such terminology is
entirely subjective. Several LED manufacturers use terms such as quotsuperbright. efficiency
of individual wafer lots and other variables. as there is no industry standard for LED
brightness.
Only a few LEDs produce sufficient intensity to require eye safety labeling. However.LED
Light Emitting Diode LED Performanc Eye Safety The need to place eye safety labeling on
LED products is dependent upon the product design and the application. for eye safety. do
not stare into the light beam of any LED at close range .
Both the luminous intensity and the spatial radiation pattern viewing angle must be taken
into account.LED Light Emitting Diode LED Performance Visibility Luminous intensity Iv does
not represent the total light output from an LED. . If two LEDs have the same luminous
intensity value. the lamp with the larger viewing angle will have the higher total light output.
Unlike standard incandescent bulbs. When the LED degrades to half of its original intensity
after .LED Light Emitting Diode LED Performance Operating Life Because LEDs are
solidstate devices they are not subject to catastrophic failure when operated within design
parameters. hours it is at the end of its useful life although the LED will continue to operate
as output diminishes. DDP LEDs are designed to operate upwards of . . DDP LEDs resist
shock and vibration and can be cycled on and off without excessive degradation. Operating
life is characterized by the degradation of LED intensity over time. hours at C ambient
temperature.
The result will be reduced light output and reduced operating life. not voltage driven.LED
Light Emitting Diode LED Performance Voltage/Design Current LEDs are currentdriven
devices. LEDs that are designed to operate at a specific voltage contain a builtin
currentlimiting resistor. . The operating current for a particular voltage is designed to maintain
LED reliability over its operating life. exceeding the maximum current rating will produce
excessive heat within the LED chip due to excessive power dissipation. Additional circuitry
may include a protection diode for AC operation or fullbridge rectifier for bipolar operation.
Although drive current and light output are directly related.
LED Light Emitting Diode Some Types of LEDs Bargraph segment Starburst Dot matrix .
Organic Light Emitting Diodes OLEDs .
not versatile OLEDs may be better on all counts Displays Significant advantages over liquid
crystals Faster Brighter Lower power Cost and design LEDs are crystals. Easier to fabricate
In general. can be bent. OLEDs are not Malleable. etc. LCDs are highly structured. OLED
research proceeds on many fronts . rolled up.OLEDs Why OLEDs Lighting efficiency
Incandescent bulbs are inefficient Fluorescent bulbs give off ugly light LEDs ordinary light
emitting diodes are bright points.
OLEDs Plan of talk LightEmitting Diode Bands and Conduction Semiconductor Standard
Diode Light Emission Organic LightEmitting Diode Organic Semiconductors Organic Diode
Light Emission .
OLEDs Electrons in a Lattice Atom has bound states Discrete energy levels Partially filled by
electrons Vr E r Periodic array of atoms cf. QM textbook Effectively continuous bands of
energy levels Also partially filled Vx E r .
Photodiode .
Photodiode What is a Photodiode Photodiodes transform a basic physical indicator. . light.
the magnitude of the photogenerated current corresponds directly to the light intensity.
Semiconductor junctions convert the photon energy of light into an electrical signal by
releasing and accelerating currentconducting carriers within the semiconductor. In every
case. Avalanche photodiodes increase efficiency through an internal current gain developed
by avalanche multiplication. into the electrical form commonly used to monitor physical
conditions. PIN photodiodes expand the response range through an added intrinsic layer
formed between the normal P and N regions.
Photodiode The Photo Effect Light entering a semiconductor material produces an electrical
current by releasing holeelectron pairs al shown in the figure. . The construction of the
photodiode and the wavelength of the light strongly influence the efficiency of the
lighttocurrent conversion. There. the semiconductor doping levels and the junction depth
become key parameters.
a photodiode responsivity expresses the resulting efficiency through. This doping produces
atoms with potential hole or electron carriers residing near the conduction band
energy.Photodiode The Photo Effect Lightly doped material expand the depletion region by
reducing the number of doped atoms per unit volume. The depth and extent of the junction
determines the location of the depletion region and the light wavelengths that produce an
efficient response. For a given photodiode and a given wavelength. shallow junctions
efficiently convert short wavelengths with a depletion region that easily encompasses the
majority of the photo generation. i p rJJe Where r is the diodes flux responsivity e is the
radiant flux energy in Watts . Thus.
which is the resistance of the zerobiased diode junction. Resistance RD represents the
diodes dark resistance. CD C D VR NB Where CD is the photodiode capacitance at zero
bias B is the builtin voltage of the diode junction VR is the reverse bias voltage .Photodiode
The Photodiode Model Current source ip represents the photodiode signal. and the diode
replicates voltage conditions for the forwardbiased state. Resistance RS models the series
resistance of the semiconductor material. CD represents the stored charge effect of the
photodiode junction and varies with the diodes area and voltage.
Photodiode The Photodiode Model .
Photodiode PIN Photodiode Adding an intrinsic layer to the photodiode structure increases
the spectral range of response by expanding the depletion region. .
. known as photocurrent. This give rise to a current Flow in an external circuit.Photodiode
pin Photodetector w The high electric field present in the depletion region causes
photogenerated carriers to Separate and be collected across the reverse biased junction.
Photodiode EnergyBand diagram for a pin photodiode .
Photodiode Photocurrent Optical power absorbed. the absorbed power in the width of
depletion region. Pc Qm E g eV Taking entrance face reflectivity into consideration. w.
becomes Rf Pw P eEs Pw Rf . Pxin the depletion region can be written in terms of incident
optical power. P P x P e E s P x Absorption coefficient Es P strongly depends on wavelength.
The upper wavelength cutoff for any semiconductor can be determined by its energy gap as
follows .
Photodiode
Optical Absorption Coefficient
Photodiode
Responsivity
The primary photocurrent resulting from absorption is
q I p P eEs P R f hR
Quantum Efficiency
of electron hole photogener ated pairs L of incident photons IP / q L P / h R
Responsivity
IP Lq P hR
A/W
Photodiode
Responsivity vs. wavelength
In this region. The newly created carriers in the presence of high electric field result in more
ionization called avalanche effect. Optical radiation ReachThrough APD structure RAPD
showing the electric fields in depletion region and multiplication region. photogenerated
electrons and holes gain enough energy to ionize bound electrons in VB upon colliding with
them. In order to carrier multiplication take place. the photogenerated carriers must traverse
along a high field region.Photodiode Avalanche Photodiode APD APDs internally multiply the
primary photocurrent before it enters to following circuitry. . This multiplication is known as
impact ionization.
Photodiode Responsivity of APD The multiplication factor current gain M for all carriers
generated in the photodiode is defined as IM M Ip IP Where I M is the average value of the
total multiplied output current amp is the primary photocurrent. The responsivity of APD can
be calculated by considering the current gain as APD q M M hR .
Voltage for different optical wavelengths .Photodiode Current gain M vs.
Photransistors .
Phototransistors Photodiode with amplifier B/E exposed to radiation. IC versus VCE is
plotted based on steps of irradiance. Responstivity of a phototransistor RE is for a specified
black body radiation usually at a colour temp. Collector current is a linear function of
irradiance. Linearity is over a much narrower range than a photodiode or photoconductor.
assuming a constant beta. of K . Irradiance is the EM power flow per unit area.
It is a figure of merit. Dark current is the main factor in limiting detection
sensitivity.Phototransistor Responsivity is the ratio of electrical output to the applied
radiation. the lifetimes of the carriers in the depletion region of the junction. VCE. Dark
current is a function of a. . ambient temperature deg. the combined capacitance of the B/E
and C/E junctions and b. C causes a decade increase in dark current and . Rise time is poor
due to a.
Phototransistors VCC VCC C Q Q Vout E .
Phototransistors Emitter Window Base n ptype ntype Collector .
gtCapacitance lt pf.Optocouplers Optotisolator. . gtInsulation voltage strength of several KV.
gtElectrically Insulated insulation resistance ohms. Eliminates ground effects from systems
with different ground potentials.
Differential modes desired versus Common mode undesired. CTR forward current transfer
ratio I CTR v I . B BIT O F . Baud rate. Operating speed. BW typically refers to NRZ f t b.
Isolation ability to separate desired and stray input signals.Optocouplers Specifications . . .
Propagation delay. a.
Aging characteristics Ex. .C. . .same as typical semiconductor data. CTR deterioration due
to LED source of optocoupler. pulse c. Signal Sources a. voltage ratings. A.C. current.
D.Optocouplers . Insulation. b. Limiting parameters Pmax.
UUAKCEQ.
Photoconductor Photocells. photoresistors. . P max E longest absorbed wavelength nm
Below lambda max RE decreases linearly with wavelength. Semiconductors that release
electrons when radiated therefore the semiconductor conductivity increases. Similar action
as photodiode in terms of EH pair action.
Close to a log function over to decades. .Photoconductors G of PC increases with irradiance
or illumination. R of PC decreases with same.
VCC R R U To switching device. R D .
VCC V R RC R M MO T U Q R D K .
R R U U b C E A K VCC V iF mA Ub ALSBN ALSAN .
Q .Optocoupler with phototriac receiver.
R VCC Q U a R V A C Hz Q ALSBN .
L R R R b BN . C M .