LIGHT EMITTING DIODE

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LIGHT EMITTING DIODE Design Principles

• EBB 424E
• Lecture 2 LED 1
• Dr Zainovia Lockman

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1907 Publication report on Curious Phenomenon
On applying a potential to a crystal of
carborundum (SiC), the material gave out a
yellowish light
H.J. Round, Electrical World, 49, 309, 1907
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3 Lectures on LED
OBJECTIVES

• To learn the basic design principles of LED
• To relate properties of semiconductor material to
  the principle of LED
• To be able select appropriate materials for
  different types of LED
• To be able to apply knowledge of band gap
  engineering to design appropriate materials for a
  particular LED
• To acknowledge other materials that can and have
  been used in LED

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4 Main Issues

• The device configuration
• Materials requirements
• Materials selection
• Material issues

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By the end of this lecture you must be able to

• Draw a typical construction of an LED.
• Explain your drawing.
• State all the issues regarding the materials
  selection of an LED.
• State all of the possible answers regarding your
  materials issues.
• Explain band gap engineering
• Explain the isoelectronic doping in GaAsP system
• State examples of materials that emit, UV, Vis,
  IR lights

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For the LED lectures you need

1. Complete set of notes (3 lecture presentation and
   lecture notes)
2. A photocopy from Kasap (p.139-150)
3. A photocopy from Wilson (p-141-155)
4. Some reading materials

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What is LED?

• Semiconductors bring quality
• to light!

LED are semiconductor p-n junctions that under
forward bias conditions can emit radiation by
electroluminescence in the UV, visible or
infrared regions of the electromagnetic spectrum.
The qaunta of light energy released is
approximately proportional to the band gap of the
semiconductor.
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Applications of LEDs
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Your fancy telephone, i-pod, palm pilot and
digital camera
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Getting to know LED
Advantages of Light Emitting Diodes
(LEDs) Longevity The light emitting element in
a diode is a small conductor chip rather than a
filament which greatly extends the diodes life
in comparison to an incandescent bulb (10 000
hours life time compared to 1000 hours for
incandescence light bulb) Efficiency Diodes
emit almost no heat and run at very low
amperes. Greater Light Intensity Since each
diode emits its own light Cost Not too
bad Robustness Solid state component, not as
fragile as incandescence light bulb
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LED chip is the part that we shall deal with in
this course
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Luminescence is the process behind light emission

• Luminescence is a term used to describe the
  emission of radiation from a solid when the solid
  is supplied with some form of energy.
• Electroluminescence ? excitation results from the
  application of an electric field
• In a p-n junction diode injection
  electroluminescence occurs resulting in light
  emission when the junction is forward biased

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Excitation
E
Electron (excited by the biased forward voltage)
is in the conduction band
k
Normally the recombination takes place between
transition of electrons between the bottom of the
conduction band and the top of the valance band
(band exterma). The emission of light is
therefore hc/? Ec-Ev Eg(only direct band gap
allows radiative transition)
Hole is in valance band
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How does it work?
A typical LED needs a p-n junction
There are a lot of electrons and holes at the
junction due to excitations
Electrons from n need to be injected to p to
promote recombination
Recombination produces light!!
Junction is biased to produce even more e-h and
to inject electrons from n to p for recombination
to happen
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Injection Luminescence in LED

• Under forward bias majority carriers from both
  sides of the junction can cross the depletion
  region and entering the material at the other
  side.
• Upon entering, the majority carriers become
  minority carriers
• For example, electrons in n-type (majority
  carriers) enter the p-type to become minority
  carriers
• The minority carriers will be larger ? minority
  carrier injection
• Minority carriers will diffuse and recombine with
  the majority carrier.
• For example, the electrons as minority carriers
  in the p-region will recombine with the holes.
  Holes are the majority carrier in the p-region.
• The recombination causes light to be emitted
• Such process is termed radiative recombination.

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Recombination and Efficiency
EC
EF
EV

• Ideal LED will have all injection electrons to
  take part in the recombination process
• In real device not all electron will recombine
  with holes to radiate light
• Sometimes recombination occurs but no light is
  being emitted (non-radiative)
• Efficiency of the device therefore can be
  described
• Efficiency is the rate of photon emission over
  the rate of supply electrons

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Emission wavelength, ?g

• The number of radiative recombination is
  proportional to the carrier injection rate
• Carrier injection rate is related to the current
  flowing in the junction
• If the transition take place between states
  (conduction and valance bands) the emission
  wavelength, ?g hc/(EC-EV)
• EC-EV Eg
• ?g hc/Eg

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Calculate

• If GaAs has Eg 1.43ev
• What is the wavelength, ?g it emits?
• What colour corresponds to the wavelength?

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Construction of Typical LED
Al
Light output
SiO2
p
n
Electrical contacts
Substrate
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LED Construction

• Efficient light emitter is also an efficient
  absorbers of radiation therefore, a shallow p-n
  junction required.
• Active materials (n and p) will be grown on a
  lattice matched substrate.
• The p-n junction will be forward biased with
  contacts made by metallisation to the upper and
  lower surfaces.
• Ought to leave the upper part clear so photon
  can escape.
• The silica provides passivation/device isolation
  and carrier confinement

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Efficient LED

• Need a p-n junction (preferably the same
  semiconductor material only different dopants)
• Recombination must occur ? Radiative transmission
  to give out the right coloured LED
• Right coloured LED ? hc/? Ec-Ev Eg
• ? so choose material with the right Eg
• Direct band gap semiconductors to allow efficient
  recombination
• All photons created must be able to leave the
  semiconductor
• Little or no reabsorption of photons

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Materials Requirements
Correct band gap
Direct band gap
Material can be made p and n-type
Efficient radiative pathways must exist
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Candidate Materials
? UV-ED ? 0.5-400nm Eg gt 3.25eV ? LED - ?
450-650nm Eg 3.1eV to 1.6eV ? IR-ED- ?
750nm- 1nm Eg 1.65eV
Direct band gap materials e.g. GaAs not Si
Readily doped n or p-types
Materials with refractive index that could allow
light to get out
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Typical Exam Question

• Describe the principles of operation of an LED
  and state the materials requirements criteria to
  produce an efficient LED.
• (50 marks)

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Visible LED
Definition LED which could emit visible light,
the band gap of the materials that we use must be
in the region of visible wavelength 390- 770nm.
This coincides with the energy value of 3.18eV-
1.61eV which corresponds to colours as stated
below
The band gap, Eg that the semiconductor must
posses to emit each light
Violet 3.17eV Blue 2.73eV Green 2.52eV
Yellow 2.15eV Orange 2.08eV Red 1.62eV
Colour of an LED should emits
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Electromagnetic Spectrum
The appearance of the visible light will be the
results of the overlap integral between the eye
response curve and the spectral power of the
device ? the peak of the luminous curve will not
in general be the same as the peak of the
spectral power curve
V 3.17eV B 2.73eV G 2.52eV Y 2.15eV O
2.08eV R 1.62eV
Visible lights
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Candidate Materials for LEDs
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Question 1

• Indicate the binary compounds that can be
  selected for red, yellow, green and blue LED.

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Candidate Materials Group III-V Group II-VI
Group II
Group III
Group IV
Group V
N P As
ii
Al Ga In
Periodic Table to show group III-V and II-V
binaries
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Group III-V (1950)

• The era of IIIV compound semiconductors started
  in the early 1950s when this class of materials
  was postulated and demonstrated by Welker (1952,
  1953). The class of IIIV compounds had been an
  unknown substance prior to the 1950s that does
  not occur naturally. The novel man-made IIIV
  compounds proved to be optically very active and
  thus instrumental to modern LED technology.

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Group III-V LED materials
Al Ga In
N P As
AlN, AlP,AlAs GaN, GaP, GaAs InN, InP, InAs
Binary compounds
GaP GaAl
GaAsP GaAsAl
Ternary compounds
GaAs

• Questions to ask when choosing the right
  material
• Can it be doped or not?
• What wavelength it can emit?
• Would the material able to allow radiative
  recombiation?
• Direct or indirect semiconductor?

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Announcement

• Evening classes