Transcript LIGHT EMITTING DIODE - Universiti Sains Malaysia

LIGHT EMITTING
DIODE – Design
Principles
EBB 424E
Lecture 2 – LED 1
Dr Zainovia Lockman
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
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
4 Main Issues
1.
2.
3.
4.
The device configuration
Materials requirements
Materials selection
Material issues
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
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
What is LED?
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.
Applications of LEDs
Your fancy telephone, i-pod, palm
pilot and digital camera
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 diode’s 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
LED chip is the part
that we shall deal
with in this course
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
Excitation
E
Electron (excited by the biased
forward voltage) is in the conduction
band
k
Hole is in valance band
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)
How does it work?
P-n junction
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
Junction is biased to produce even more
e-h and to inject electrons from n to p for
recombination to happen
Recombination
produces light!!
Electrical
Contacts
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 pregion.
 The recombination causes light to be emitted
 Such process is termed radiative recombination.
Recombination and Efficiency
(a)
p
n+
ECEg
(b)
p
n+
Eg
h =Eg
EF
EV
eVo
Electrons in CB
Holes in VB
◘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
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
Calculate
• If GaAs has Eg = 1.43ev
• What is the wavelength, g it emits?
• What colour corresponds to the
wavelength?
Construction of Typical LED
Light output
Al
SiO2
p
n
Electrical
contacts
Substrate
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
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
Correct band gap
Direct band gap
Materials
Requirements
Efficient radiative
pathways must exist
Material can be
made p and n-type
Direct band gap
materials
e.g. GaAs not Si
 UV-ED  ~0.5-400nm
Eg > 3.25eV
 LED -  ~450-650nm
Eg = 3.1eV to 1.6eV
 IR-ED-  ~750nm- 1nm
Eg = 1.65eV
Candidate
Materials
Materials with refractive
index that could allow light
to ‘get out’
Readily doped n or p-types
Typical Exam Question
Describe
the
principles
of
operation of an LED and state the
material’s requirements criteria to
produce an efficient LED.
(50 marks)
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:
Colour of an
LED should
emits
Violet
Blue
Green
Yellow
Orange
Red
~ 3.17eV
~ 2.73eV
~ 2.52eV
~ 2.15eV
~ 2.08eV
~ 1.62eV
The band gap, Eg
that the
semiconductor
must posses to
emit each light
Electromagnetic Spectrum
Visible lights
V ~ 3.17eV
B ~ 2.73eV
G ~ 2.52eV
Y ~ 2.15eV
O ~ 2.08eV
R ~ 1.62eV
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
Candidate Materials for LED’s
Question 1
• Indicate the binary compounds
that can be selected for red,
yellow, green and blue LED.
Candidate Materials
Group III-V & Group II-VI
Group II
Group III
Group IV
iii iv
Group V
v
N
ii
Al
P
Ga
As
In
Periodic Table to show group III-V and II-V binaries
Group III-V (1950)
The era of III–V compound semiconductors
started in the early 1950s when this class of
materials was postulated and demonstrated by
Welker (1952, 1953). The class of III–V
compounds had been an unknown substance
prior to the 1950s that does not occur
naturally. The novel man-made III–V
compounds proved to be optically very active
and thus instrumental to modern LED
technology.
Group III-V LED materials
Al
N
Ga
P
GaN, GaP, GaAs
In
As
InN, InP, InAs
GaAs
AlN, AlP,AlAs
GaP
GaAsP
GaAl
GaAsAl
Binary
compounds
Ternary
compounds
Questions to ask when choosing the right material:
1. Can it be doped or not?
2. What wavelength it can emit?
3. Would the material able to allow radiative recombiation?
4. Direct or indirect semiconductor?
Announcement
Evening classes