The LED
Development of LEDs
V G Harris
10/19/2022
Fundamentals
A light-emitting diode (LED) is a
semiconductor light source that emits
light when current flows through it.
Electrons in the semiconductor
recombine with electron holes,
releasing energy in the form of
photons.
The color of the light (corresponding
to the energy of the photons) is
determined by the energy required for
electrons to cross the band gap of the
semiconductor.
White light is obtained by using
multiple semiconductors or a layer of
light-emitting phosphor on the
semiconductor device
https://en.wikipedia.org/
wiki/Light-emitting_diode
https://warwick.ac.uk/fac/sci/physics/current/postgraduate/re
gs/mpagswarwick/ex5/devices/led/
1st Generation
Appearing as practical electronic components in
1962, the earliest LEDs emitted low-intensity
infrared (IR) light. Infrared LEDs are used in
remote-control circuits, such as those used with
a wide variety of consumer electronics.
The first visible-light LEDs were of low intensity
and limited to red. Early LEDs were often used as
indicator lamps, replacing small incandescent
bulbs, and in seven-segment displays. Later
developments produced LEDs available in visible,
ultraviolet (UV), and infrared wavelengths, with
high, low, or intermediate light output, for
instance white LEDs suitable for room and
outdoor area lighting.
LEDs have also given rise to new types of displays
and sensors, while their high switching rates are
useful in advanced communications technology.
https://en.wikipedia.org/wiki/Light-emitting_diode
Blue LED (controversy)
In the late 1980s, key breakthroughs in GaN epitaxial growth
and p-type doping ushered in the modern era of GaN-based
optoelectronic devices. Building upon this foundation, Theodore
Moustakas at Boston University.
Two years later, in 1993, high-brightness blue LEDs were
demonstrated by Shuji Nakamura of Nichia Corporation using a
gallium nitride growth process.
In parallel, Isamu Akasaki and Hiroshi Amano of Nagoya
University were working on developing the important GaN
deposition on sapphire substrates and the demonstration of ptype doping of GaN. This new development revolutionized LED
lighting, making high-power blue light sources practical, leading
to the development of technologies like Blu-ray.
Nakamura, Hiroshi Amano and Isamu Akasaki were awarded
the Nobel Prize in Physics in 2014 for the invention of the blue
LED.
In 2015, a US court ruled that three companies had infringed
Moustakas's prior patent, and ordered them to pay licensing
fees of not less than US$13 million.
https://en.wikipedia.org/wiki/Light-emitting_diode
1.6 x 1.6 x 0.35 mm3
The Edison light bulb
converts electrical
energy into heat!
(thousands of
filaments!)
However, electrical
energy can also be
converted to light!
The LED!
• Electrical energy flowing through it is directly
converted into light energy!
• Light is made up of many small particles
called photons. Photons have energy and
momentum but no mass.
• What is Light Emitting Diode (LED)?
• Recall:
• Photon is a Boson - Like the God-like Higgs Boson
• Fermion is a particle that obeys Fermi exclusion
principle – like and electron….etc
LEDS are the most popular of semiconductor
pn diodes….
• LEDS emit visible light or IR in forward bias
(consider laser pointers versus IR remote
controls)
• LEDS are an optical semiconductor pn diode
that converts electrical energy to visible light
• In pn forward bias – electrons recombine with
holes and emit a photon- called
electroluminescence
Normal pn junction
• To create an LED, the n-type material should be connected to
the negative terminal of the battery and p-type material
should be connected to the positive terminal of the battery.
In other words, the n-type material should be negatively
charged and the p-type material should be positively
charged.
• The construction of LED is similar to the normal p-n junction
diode except that gallium, phosphorus and arsenic materials
are used for construction instead of silicon or germanium
materials
Heat generation vs light generation
• In normal p-n junction diodes, silicon is most widely
used because it is less sensitive to temperature. Also,
it allows electric current efficiently without any
damage. In some cases, germanium is used for
constructing diodes.
• However, silicon or germanium diodes do not emit
energy in the form of light. Instead, they emit energy
in the form of heat. Thus, silicon or germanium is not
used for constructing LEDs.
Layers of LED
• A Light Emitting Diode (LED) consists of three layers: p-type semiconductor, n-type semiconductor and
depletion layer. The p-type semiconductor and the n-type semiconductor are separated by a depletion
region or depletion layer.
P-type semiconductor
• When trivalent impurities are added to the intrinsic or pure semiconductor, a p-type semiconductor is
formed.
• In p-type semiconductor, holes are the majority charge carriers and free electrons are the minority
charge carriers. Thus, holes carry most of the electric current in p-type semiconductor.
N-type semiconductor
• When pentavalent impurities are added to the intrinsic semiconductor, an n-type semiconductor is
formed.
• In n-type semiconductor, free electrons are the majority charge carriers and holes are the minority
charge carriers. Thus, free electrons carry most of the electric current in n-type semiconductor.
Depletion layer or region
• Depletion region is a region present between the p-type and n-type semiconductor where no mobile
charge carriers (free electrons and holes) are present. This region acts as barrier to the electric current.
It opposes flow of electrons from n-type semiconductor and flow of holes from p-type semiconductor.
How Light Emitting Diode (LED) works?
• Light Emitting Diode (LED) works only in forward
bias condition. When Light Emitting Diode (LED) is
forward biased, the free electrons from n-side and
the holes from p-side are pushed towards the
junction.
• When free electrons reach the junction or
depletion region, some of the free electrons
recombine with the holes in the positive ions. We
know that positive ions have less number of
electrons than protons. Therefore, they are ready
to accept electrons. Thus, free electrons recombine
with holes in the depletion region. In the similar
way, holes from p-side recombine with electrons in
the depletion region.
Recombination….the devil is at play!
• Because of the recombination of free electrons and holes in the depletion region,
the width of depletion region decreases. As a result, more charge carriers will cross
the p-n junction.
• Some of the charge carriers from p-side and n-side will cross the p-n junction before
they recombine in the depletion region. For example, some free electrons from n-type
semiconductor cross the p-n junction and recombines with holes in p-type
semiconductor. In the similar way, holes from p-type semiconductor cross the p-n
junction and recombines with free electrons in the n-type semiconductor.
• Thus, recombination takes place in depletion region as well as in p-type and n-type
semiconductor.
• The free electrons in the conduction band releases energy in the form of light before
they recombine with holes in the valence band.
• In silicon and germanium diodes, most of the energy is released in the form of heat and
emitted light is too small.
• However, in materials like gallium arsenide and gallium phosphide the emitted photons
have sufficient energy to produce intense visible light.
How LED emits light?
• When external voltage is applied to the valence electrons, they gain sufficient
energy and breaks the bonding with the parent atom. The valence electrons
which breaks bonding with the parent atom are called free electrons.
• The energy level of all the valence electrons is almost same.
• The energy level of free electrons in the conduction band is high compared to the
energy level of valence electrons or holes in the valence band. Therefore, free
electrons in the conduction band need to lose energy in order to recombine with
the holes in the valence band.
• The free electrons in the conduction band do not stay for long period. After a
short period, the free electrons lose energy in the form of light and recombine
with the holes in the valence band. Each recombination of charge carrier will emit
some light energy
• The brightness of the emitted light depends on the material used for constructing
LED and forward current flow through the LED.
• In normal silicon diodes, the energy gap between conduction band and valence
band is less. Hence, the electrons fall only a short distance. As a result, low
energy photons are released. These low energy photons have low frequency
which is invisible to human eye.
• In LEDs, the energy gap between conduction band and valence band is very large
so the free electrons in LEDs have greater energy than the free electrons in silicon
diodes. Hence, the free electrons fall to a large distance. As a result, high energy
photons are released. These high energy photons have high frequency which is
visible to human eye.
The efficiency of generation of light in LED
• The efficiency of generation of light in LED increases with increase in injected current
• In light emitting diodes, light is produced due to recombination process. Recombination
of charge carriers takes place only under forward bias condition. Hence, LEDs operate
only in forward bias condition.
• When light emitting diode is reverse biased, the free electrons (majority carriers) from
n-side and holes (majority carriers) from p-side moves away from the junction. As a
result, the width of depletion region increases and no recombination of charge carriers
occur. Thus, no light is produced.
• All diodes emit photons or light but not all diodes emit visible light. The material in an
LED is selected in such a way that the wavelength of the released photons falls within
the visible portion of the light spectrum.
• Light emitting diodes can be switched ON and OFF at a very fast speed of 1 ns.
Direct-bandgap materials make better LEDs
than indirect bandgap materials.
Direct-bandgap materials make better LEDs
than indirect bandgap materials.
LEDs
Solar cells
Performance: Haitz's law
Liu, Zongyuan & Liu, Sheng & Wang, Kai & Luo, Xiaobing.
(2009). Status and prospects for phosphor-based white LED
packaging. Frontiers of Optoelectronics in China. 2. 119-140.
10.1007/s12200-009-0011-2.
Different colors!
• LEDs are available in different colors. The most common colors of
LEDs are orange, yellow, green and red.
Indium gallium nitride (InGaN, InxGa1−xN) is
a semiconductor material made of a mix of gallium
nitride (GaN) and indium nitride (InN). It is a ternary group
III/group V direct bandgap semiconductor.
Its bandgap can be tuned by varying the amount of indium
in the alloy. InxGa1−xN has a direct bandgap span from the
infrared (0.69 eV) for InN to the ultraviolet (3.4 eV) of
GaN. The ratio of In/Ga is usually between 0.02/0.98 and
0.3/0.7.
White LEDs: RGB vs Phosphors
• There are two primary ways of producing white light-emitting
diodes.
• One is to use individual LEDs that emit three primary colors—red,
green and blue—and then mix all the colors to form white light.
• The other is to use a phosphor material to convert monochromatic
light from a blue or UV LED to broad-spectrum white light, similar
to a fluorescent lamp. The yellow phosphor is cerium-doped YAG
crystals suspended in the package or coated on the LED.
• This YAG phosphor causes white LEDs to appear yellow when off,
and the space between the crystals allow some blue light to pass
through in LEDs with partial phosphor conversion. In LEDs with PFS
phosphor, some blue light passes through the phosphors, the
Ce:YAG phosphor converts blue light to green and red (yellow)
light, and the PFS phosphor converts blue light to red light.
• The best color rendition LEDs use a mix of phosphors, resulting in
less efficiency and better color rendering.
• The first white light-emitting diodes (LEDs) were offered for sale in
the autumn of 1996.
https://en.wikipedia.org/wiki/Light-emitting_diode
https://en.wikipedia.org/wiki/Light-emitting_diode
White LED
Usually blue InGaN LEDs coated with yellow phosphor epoxy
encapsulation- cerium-doped yttrium aluminium garnet (Ce3+:YAG)
Blue light emitted by InGaN hits the coating which emits yellow
light.
Blue Light + Yellow Light = “White” Light
In terms of wavelength,
(460-490 nm) + (500-700 nm)
Using several phosphor layers of distinct colors broadens the
emitted spectrum, effectively raising the color rendering index
(CRI)
broad band emission of YAG:Ce phosphor originates from the
5d→4f transition of Ce3+
*PL : Photoluminescence
PL emission spectra of (Y 1–xCex)3Al5O 12 phosphors sintered
What determines the color of an LED?
• The material used for constructing LED determines its color. In other
words, the wavelength or color of the emitted light depends on the
forbidden gap or energy gap of the material.
• Different materials emit different colors of light.
• Gallium arsenide LEDs emit red and infrared light.
• Gallium nitride LEDs emit bright blue light.
• Yttrium aluminium garnet LEDs emit white light.
• Gallium phosphide LEDs emit red, yellow and green light.
• Aluminium gallium nitride LEDs emit ultraviolet light.
• Aluminum gallium phosphide LEDs emit green light.
More on color and efficiency
Bright LEDs – obnoxious headlights!
The first white LEDs were expensive and
inefficient. However, the light output of
LEDs has increased exponentially, with a
doubling occurring approximately every 36
months since the 1960s (similar to Moore's
law). This trend is generally attributed to the
parallel development of other
semiconductor technologies and advances
in optics and materials science and has
been called Haitz's law after Dr. Roland
Haitz.
LED construction
One of the methods used to construct
LED is to deposit three semiconductor
layers on the substrate. The three
semiconductor layers deposited on the
substrate are n-type semiconductor, ptype semiconductor and active region.
Active region is present in between the ntype and p-type semiconductor layers.
Advantages of LED
• The brightness of light emitted by LED is depends on the current flowing through the
LED. Hence, the brightness of LED can be easily controlled by varying the current. This
makes possible to operate LED displays under different ambient lighting conditions.
• Light emitting diodes consume low energy.
• LEDs are very cheap and readily available.
• LEDs are light in weight.
• Smaller size.
• LEDs have longer lifetime.
• LEDs operates very fast. They can be turned on and off in very less time.
• LEDs do not contain toxic material like mercury which is used in fluorescent lamps.
• LEDs can emit different colors of light.
Disadvantages of LED
• LEDs need more power to operate than normal p-n junction diodes.
• Luminous efficiency of LEDs is low.
Applications of LED
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The various applications of LEDs are as follows
Burglar alarms systems
Calculators
Picture phones
Traffic signals
Digital computers
Multimeters
Microprocessors
Digital watches
Automotive heat lamps
Camera flashes
Aviation lighting
White LED
Usually blue InGaN LEDs coated with yellow phosphor epoxy
encapsulation- cerium-doped yttrium aluminium garnet (Ce3+:YAG)
Blue light emitted by InGaN hits the coating which emits yellow
light.
Blue Light + Yellow Light = “White” Light
In terms of wavelength,
(460-490 nm) + (500-700 nm)
Using several phosphor layers of distinct colors broadens the
emitted spectrum, effectively raising the color rendering index
(CRI)
broad band emission of YAG:Ce phosphor originates from the
5d→4f transition of Ce3+
*PL : Photoluminescence
PL emission spectra of (Y 1–xCex)3Al5O 12 phosphors sintered
fini