This article gives a vivid description of the principle and working procedure of a Light Emitting Diode. It provides a comprehensive understanding of how this very important optical device is useful in our daily applications, its types, structure and other related information.

1. OPTOMETRY – Part X
LIGHT EMITTING DIODE
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
1. Basic Theory
2. Light Generation Process
3. Types of Recombination of Carriers
4. Direct Recombination
5. Indirect Recombination
6. Light Extraction Process
7. LED Structure
8. LED Device Structure
9. LED Material
10. LED Efficiency
11. Advantages of LED
12. Disadvantages of LED
13. Applications of LED
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3. Basic Theory:
 A Light emitting diode (LED) is essentially a pn junction diode.
 When carriers are injected across a forward-biased junction, it emits incoherent light.
 Most of the commercial LEDs are realized using a highly doped n and a p Junction.
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4. Theory:
 To understand the principle, let us consider an unbiased pn+ junction (Figure in the next
slide shows the pn+ energy band diagram).
 The depletion region extends mainly into the p-side.
 There is a potential barrier from 𝐸 𝐶 on the n-side to the 𝐸 𝐶 on the p-side, called the
built-in voltage, 𝑉0.
 This potential barrier prevents the excess free electrons on the n+ side from diffusing
into the p side.
 When a Voltage V is applied across the junction, the built-in potential is reduced from
𝑉0 to 𝑉0 – V.
 This allows the electrons from the n+ side to get injected into the p-side. Since electrons
are the minority carriers in the p-side, this process is called minority carrier injection.
 But the hole injection from the p side to n+ side is very less and so the current is
primarily due to the flow of electrons into the p-side.
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5. Theory:
 These electrons injected into the p-side recombine with the holes.
 This recombination results in spontaneous emission of photons (light).
 This effect is called injection electroluminescence.
 These photons should be allowed to escape from the device without being reabsorbed.
 Recombination of an electron hole pair (EHP) involves electron in the conduction band
occupying a hole in the valence band.
 This results in the annihilation of the electron-hole pair.
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6. Light Generation Process:
 When a p - n junction is biased in the forward direction, the resulting current flow
across the boundary layer between the p and n regions has two components: holes are
injected from the p region into the n region and electrons are injected from the n region
into the p region.
 This so-called minority-carrier injection disturbs the carrier distribution from its
equilibrium condition.
 The injected minority carriers recombine with majority carriers until thermal
equilibrium is re established.
 As long as the current continues to flow, minority carrier injection continues.
 On both sides of the junction, a new steady-state carrier distribution is established such
that the recombination rate equals the injection rate.
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7. Light Generation Process:
 Minority-carrier recombination is not instantaneous.
 The injected minority carriers have to find proper conditions before the recombination
process can take place.
 Both energy and momentum conservation have to be met.
 Energy conservation can be readily met since a photon can take up the energy of the
electron-hole pair, but the photon doesnot contribute much to the conservation of
momentum.
 Therefore, an electron can only combine with a hole of practically identical and
opposite momentum.
 Such proper conditions are not readily met , resulting in a delay. In other words, the
injected minority carrier has a finite lifetime τr before it combines radiatively through
the emission of a photon.
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8. Light Generation Process:
 This average time to recombine radiatively through the emission of light can be
visualized as the average time it takes an injected minority carrier to find a majority
carrier with the right momentum to allow radiative recombination without violating
momentum conservation.
 Unfortunately, radiative recombination is not the only recombination path.
 There are also crystalline defects, such as impurities, dislocations, surfaces etc. that can
trap the injected minority carriers.
 This type of recombination process may or may not generate light.
 Energy and momentum conservation are met through the successive emission of
phonons.
 Again, the recombination process is not instantaneous because the minority carrier first
has to dif fuse to a recombination site.
 This non radiative recombination process is characterized by a lifetime τn.
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9. Light Generation Process:
 It is therefore important to develop conditions where radiative recombination occurs
fairly rapidly compared with nonradiative recombination.
 The effectiveness of the light – generation process is described by the fraction of the
injected minority carriers that recombine radiatively compared to the total injection.
 The internal quantum efficiency, ηi can be calculated from τr and τn.
 The combined recombination processes lead to a total minority – carrier lifetime τ given
by
1
𝜏
=
1
𝜏 𝑟
+
1
𝜏 𝑛
 ηi is simply computed as a fraction of carriers recombining radiatively.
𝜂𝑖 =
𝜏 𝑛
𝜏 𝑟 + 𝜏 𝑛
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10. Types of Recombination of carriers:
 The recombination can be classified into the following two kinds
1. • Direct recombination
2. • Indirect recombination
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11. Direct Recombination:
 In direct band gap materials, the minimum energy of the conduction band lies directly
above the maximum energy of the valence band in momentum space energy (Figure 2
shows the E-k plot of a direct band gap material).
 In this material, free electrons at the bottom of the conduction band can recombine
directly with free holes at the top of the valence band, as the momentum of the two
particles is the same.
 This transition from conduction band to valence band involves photon emission (takes
care of the principle of energy conservation).
 This is known as direct recombination. Direct recombination occurs spontaneously.
 GaAs is an example of a direct band-gap material.
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12. Direct Recombination:
 Energy- k plot: E-k plot is a different way of describing the material characteristics.
The effects of the crystal lattice are included by defining effective mass m*.
 From the plot the effective mass can be calculated
𝑚∗ = Ђ2 𝑑2 𝐸 𝑑𝑘2
where Ђ is the Plank’s constant (h/2π) and d2E/dk2 gives the curvature.
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13. Direct Recombination:
 The effective mass can be positive, negative or infinity.
 Infinity means particle cannot be accelerated by external forces
 Negative means the object reacts to an attractive force as if it would experience a
repulsive force.
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14. Indirect Recombination:
 In the indirect band gap materials, the minimum energy in the conduction band is
shifted by a k-vector relative to the valence band.
 The k-vector difference represents a difference in momentum.
 Due to this difference in momentum, the probability of direct electron hole
recombination is less.
 In these materials, additional dopants (impurities) are added which form very shallow
donor states.
 These donor states capture the free electrons locally; provides the necessary momentum
shift for recombination.
 These donor states serve as the recombination centres.
 This is called Indirect (non-radiative) Recombination.
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15. Indirect Recombination:
 Figure 3 shows the E-k plot of an indirect band gap material and an example of how
Nitrogen serves as a recombination center in GaAsP.
 In this case it creates a donor state, when SiC is doped with Al, it recombination takes
place through an acceptor level.
 The indirect recombination should satisfy both conservation energy, and momentum.
 Thus besides a photon emission, phonon emission or absorption has to take place.
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16. Indirect Recombination:
 Phonons: Phonons are a quantum mechanical version of a special type of vibrational
motion, known as normal modes in classical mechanics, in which each part of a lattice
oscillates with the same frequency.
 These normal modes are important because any arbitrary vibrational motion of a lattice
can be considered as a superposition of normal modes with various frequencies
according to classical mechanics.
 In this sense, the normal modes are the elementary vibrations of the lattice.
 Although normal modes are wave-like phenomena in classical mechanics, they acquire
certain particle like properties when the lattice is analysed using quantum mechanics.
 They are then known as phonons.
 The properties of long-wavelength phonons give rise to sound in solids -- hence the
name phonon from the Greek φωνή (phonē) = voice
 GaP is an example of an indirect band-gap material.
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17. Indirect Recombination:
 The wavelength of the light emitted, and hence the colour, depends on the band gap
energy of the materials forming the p-n junction.
 The emitted photon energy is approximately equal to the band gap energy of the
semiconductor.
 The following equation relates the wavelength and the energy band gap.
ℎν = 𝐸𝑔
ℎ𝑐
λ
= 𝐸𝑔
λ =
ℎ𝑐
𝐸𝑔
 Where h is Plank’s constant, c is the speed of the light and 𝐸𝑔 is the energy band gap
Thus, a semiconductor with a 2 eV band-gap emits light at about 620 nm, in the red. A 3
eV band-gap material would emit at 414 nm, in the violet.
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18. Light Extraction Process:
 Generating light efficiently within a semiconductor material is only one part of the
problem to build an efficient light source .
 The next challenge is the extraction of light from within the LED chip to the outside.
 The designer must consider total internal reflection.
 According to Snell’s law, light can escape from a medium of high index of refraction n 1
into a medium of low index refraction n0 only if it intersects the surface between the two
media at an angle from normal less than the critical angle θc with θc being defined by
𝜃 𝐶 = 𝑎𝑟𝑐 sin
𝑛0
𝑛1
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19. Light Extraction Process:
 Most semiconductor LEDs have an isotropic emission pattern as seen from within the
light-generating material.
 Assuming a cubic shape for the LED chip, because of internal reflections, only a small
fraction of the isotropically emitted light can escape any of the six surfaces.
 As a case in point, let us calculate the emission through the top surface. For typical
light-emitting semiconductors, n1 is in the range of 2.9 to 3.6. If n1 = 3.3 and n0 = 1.0
(air), we find θc = 17.68ᵒ.
 The emission from an isotropic source into a cone with a half angle of θc is given by (1-
cos θc)/2 .
 After correcting for Fresnel reflections, only 1.6 % of the light generated escapes
through the LED top surface into air.
 Depending on chip and p - n junction geometry, virtually all of the remaining light (98.4
%) is reflected and absorbed within the LED chip.
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20. Light Extraction Process:
 The fraction of light coupled from chip to air is a function of the number of surfaces
through which the chip can transmit light effectively. Most LED chips are called
‘‘absorbing substrate’’ (AS) chips.
 In such a chip, the starting substrate material has a narrow bandgap and absorbs all the
light with energy greater than the bandgap of the substrate.
 Consider the case of a GaAsP LED grown on a GaAs substrate. The emitted light (Eg >
1.9 eV) is absorbed by the GaAs substrate (Eg = 1.4 eV).
 Thus, a GaAsP-emitting layer on a GaAs substrate can transmit only through its top
surface.
 Light transmitted toward the side surfaces or downward is absorbed.
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21. Light Extraction Process:
 To increase light extraction, the substrate or part of the epitaxial layers near the top of
the chip has to be made of a material transparent to the emitted light.
 The ‘‘transparent substrate’’ (TS) chip is designed such that light transmitted towards
the side surfaces within θc half-angle cones can escape.
 Assuming that there is negligible absorption between the point of light generation and
the side walls, this increases the extraction efficiency by a factor of five (five instead of
one escape cones).
 A common approach is to use a hybrid chip with properties between AS and TS chips.
These chips utilize a thick, transparent window layer above the light-emitting layer.
 If this layer is sufficiently thick, then most of the light in the top half of the cones
transmitted towards the side surfaces will reach the side of the chip before hitting the
substrate.
 In this case of hybrid chips, the efficiency (4.5 %, no. of cones 3) is between that of AS
(1.5 %, no. of cone 1) and TS (7.5%, no. of cones 5) chips.
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22. LED Structure:
 The LED structure plays a crucial role in emitting light from the LED surface.
 The LEDs are structured to ensure most of the recombinations take place on the surface
by the following two ways.
1. By increasing the doping concentration of the substrate, so that additional free
minority charge carrier electrons move to the top, recombine and emit light at the
surface.
2. By increasing the diffusion length 𝐿 = 𝐷𝜏, where D is the diffusion coefficient
and τ is the carrier life time. But when increased beyond a critical length there is a
chance of re-absorption of the photons into the device.
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23. LED Structure:
 The LED has to be structured so that the photons generated from the device are emitted
without being reabsorbed.
 One solution is to make the p layer on the top thin, enough to create a depletion layer.
 There are different ways to structure the dome for efficient emitting.
 Following picture shows the layered structure.
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24. LED Structure:
 LEDs are usually built on an n-type substrate, with an electrode attached to the p-type
layer deposited on its surface.
 P-type substrates, while less common, occur as well.
 Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
 LED dome shapes: The LED domes are constructed such most of the light gets emitted
efficiently.
 Following picture shows the two different kinds of domes.
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25. LED Device Structure:
 LED devices come in a broad range of structures.
 Each material system requires a different optimization.
 The only common feature for all LED structures is the placement of the p - n junction
where the light is generated.
 The p - n junction is practically never placed in the bulk-grown substrate material for
the following reasons:
 The bulk-grown materials such as GaAs, GaP, and InP usually do not have the right
energy gap for the desired wavelength of the emitted light.
 The light-generating region requires moderately low doping that is inconsistent with
the need for a low series resistance.
 Bulk-grown material often has a relatively high defect density, making it difficult to
achieve high efficiency.
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26. LED Device Structure:
 For these reasons, practically all commercially important LED structures utilize a
secondary growth step on top of a single-crystal bulk-grown substrate material.
 The secondary growth step consists of a single-crystal layer lattice matched to the
substrate.
 This growth process is known as epitaxial growth.
 The commonly used epitaxial structures can be classified into the following categories:
 Homojunctions
1. grown
2. diffused
 Heterojunctions
1. single confinement
2. double confinement
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27. LED Material:
 An important class of commercial LEDs that cover the visible spectrum are the III-V
ternary alloys based on alloying GaAs and GaP which are denoted by GaAs1-yPy.
 InGaAlP is an example of a quarternary (four element) III-V alloy with a direct band
gap.
 The LEDs realized using two differently doped semiconductors that are the same
material is called a homo junction.
 When they are realized using different bandgap materials they are called a hetero
structure device.
 A hetero structure LED is brighter than a homo junction LED.
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28. LED Material:
 Semiconductors in the periodic table:
 An example of III-V components is GaP or GaAs.
 The following table shows the semiconductors in the periodic table.
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II III IV V VI
B C
Al Si P S
Zn Ga Ge As Se
Cd In Sn Sb Te

29. LED Material:
 Following is a list of semiconductor materials and the corresponding colours:
 Aluminium Gallium Arsenide (AlGaAs) — red and infrared
 Aluminium Gallium Phosphide (AlGaP) — green
 Aluminium Gallium Indium Phosphide (AlGaInP) — high-brightness orange-red, orange, yellow, and green
 Gallium Arsenide Phosphide (GaAsP) — red, orange-red, orange, and yellow
 Gallium Phosphide (GaP) — red, yellow and green
 Gallium Nitride (GaN) — green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum
Barrier)
 Indium Gallium Nitride (InGaN) — 450 nm - 470 nm — near ultraviolet, bluish green and blue
 Silicon Carbide (SiC) as substrate — blue
 Silicon (Si) as substrate — blue (under development)
 Sapphire (Al2O3) as substrate — blue
 Zinc Selenide (ZnSe) — blue
 Diamond (C) — ultraviolet
 Aluminium Nitride (AlN), Aluminium Gallium Nitride (AlGaN), Aluminium Gallium Indium Nitride (AlGaInN)
— near to far ultraviolet (down to 210 nm)
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30. LED Efficiency:
 A very important metric of an LED is the external quantum efficiency ηext.
 It quantifies the efficiency of the conversion of electrical energy into emitted optical
energy.
 It is defined as the light output divided by the electrical input power.
 It is also defined as the product of Internal radiative efficiency and Extraction efficiency.
η 𝑒𝑥𝑡 = 𝑃 𝑜𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝐼 ∗ 𝑉
 For indirect band gap semiconductors ηext is generally less than 1%, whereas for a direct
band gap material it could be substantial.
η𝑖𝑛𝑡 = 𝑅𝑎𝑡𝑒 𝑜𝑓 𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑅𝑒𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑡𝑖𝑜𝑛 𝑇𝑜𝑡𝑎𝑙 𝑅𝑒𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑡𝑖𝑜𝑛
 The internal efficiency is a function of the quality of the material and the structure and
composition of the layer.
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31. Advantages of using LED:
 LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered
or energy-saving devices.
 LEDs can emit light of an intended colour without the use of colour filters.
 The solid package of the LED can be designed to focus its light.
 When used in applications where dimming is required, LEDs do not change their colour tint
as the current passing through them is lowered.
 LEDs are ideal for use in applications that are subject to frequent on-off cycling,
 LEDs, being solid state components, are difficult to damage with external shock.
 LEDs can have a relatively long useful life.
 LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent
bulbs.
 LEDs light up very quickly. A typical red indicator LED will achieve full brightness in
microseconds.
 LEDs can be very small and are easily populated onto printed circuit boards.
 LEDs do not contain mercury, unlike compact fluorescent lamps.
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32. Disadvantages of LED:
 LEDs are currently more expensive, price per lumen.
 LED performance largely depends on the ambient temperature of the operating
environment. Over-driving the LED in high ambient temperatures may result in
overheating of the LED package, eventually leading to device failure.
 LEDs must be supplied with the correct current.
 LEDs do not approximate a "point source" of light, so they cannot be used in
applications needing a highly collimated beam.
 There is increasing concern that blue LEDs and white LEDs are now capable of
exceeding safe limits of the so-called blue-light hazard as defined in the eye safety
specifications.
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33. Applications of LED:
 Devices, medical applications, clothing, toys
 Remote Controls (TVs, VCRs)
 Lighting
 Indicators and signs
 Opto-isolators and opto-couplers
 Sensors: Transmissive, Reflective, Scattering
 Fibre Optic Source
 Alpha Numeric / Numeric Display
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