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 Si by cali0998

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									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.   The device configuration
 2.   Materials requirements
 3.   Materials selection
 4.   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



                                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
  Hole is in valance band
                                radiative transition)
How does it work?




                                                   P-n junction   Electrical
                                                                  Contacts


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       Recombination
e-h and to inject electrons from n to p for
recombination to happen
                                              produces light!!
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.
Recombination and Efficiency
(a)                                     (b)
            p              n+                     p               n+

       ECEg                                     Eg
                                                                  h =Eg
       EF                  eVo
       EV



        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

                              Al
             Light output
                                   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   Material can be
pathways must exist   made p and n-type
                                UV-ED  ~0.5-400nm
    Direct band gap
                                      Eg > 3.25eV
       materials
                                LED -  ~450-650nm
    e.g. GaAs not Si                  Eg = 3.1eV to 1.6eV
                                IR-ED-  ~750nm- 1nm
                                      Eg = 1.65eV


                     Candidate
                     Materials

Materials with refractive      Readily doped n or p-types
index that could allow light
to ‘get out’
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:

                                                           The band gap, Eg
                          Violet          ~ 3.17eV             that the
                          Blue            ~ 2.73eV          semiconductor
                          Green           ~ 2.52eV          must posses to
    Colour of an                                            emit each light
                          Yellow          ~ 2.15eV
    LED should
       emits              Orange          ~ 2.08eV
                          Red             ~ 1.62eV
Electromagnetic Spectrum


                                    The appearance of the
      Visible lights   V ~ 3.17eV
                                    visible light will be the results
                       B ~ 2.73eV   of the overlap integral
                                    between the eye response
                       G ~ 2.52eV
                                    curve and the spectral power
                       Y ~ 2.15eV   of the device  the peak of
                                    the luminous curve will not in
                       O ~ 2.08eV
                                    general be the same as the
                       R ~ 1.62eV   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           Group V


                                     iii iv   v
                                              N
                      ii                      P
                                    Al
                                    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            AlN, AlP,AlAs
 Ga           P                                   Binary
                           GaN, GaP, GaAs
                                                  compounds
 In           As           InN, InP, InAs

                  GaP        GaAsP          Ternary
 GaAs
                  GaAl       GaAsAl         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

								
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