Tech led ppt(pptplanet.com)
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The document discusses light emitting diodes (LEDs). It begins by stating the objectives of learning about LED design principles, relating semiconductor properties to the LED principle, and selecting appropriate materials for different LED types. It then provides background on LEDs, noting they are semiconductor p-n junctions that emit light through electroluminescence when forward biased. The document outlines key topics that will be covered including device configuration, materials requirements, selection, and issues. It poses questions for the reader on topics like LED construction, materials selection issues, band gap engineering, and examples of materials that emit different wavelengths of light. References and materials that will be provided are also listed.
Tech led ppt(pptplanet.com)
- 1. Submitted To Submitted By www.pptplanet.com www.pptplanet.com
- 2. Objectives What is LED? 4 Main Issues By the end of this lecture you must be able to … For the LED lectures you need: Construction of Typical LED Injection Luminescence in LED LED Construction References
- 3. 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. 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.
- 5. H.J. Round, Electrical World, 49, 309, 1907 On applying a potential to a crystal of carborundum (SiC), the material gave out a yellowish light
- 6. 1. The device configuration 2. Materials requirements 3. Materials selection 4. Material issues
- 7. 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
- 8. 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
- 11. 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
- 12. LED chip is the part that we shall deal with in this course
- 13. 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
- 14. P-n junction Electrical Contacts A typical LED needs a p-n junction Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen 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!!
- 15. 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.
- 16. eVo Eg p n+ h =Eg Eg p n+ (a) (b) Electrons in CB Holes in VB EC EV EF ◘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
- 17. ◘ 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
- 18. If GaAs has Eg = 1.43ev What is the wavelength, g it emits? What colour corresponds to the wavelength?
- 20. 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
- 21. 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
- 22. Correct band gap Direct band gap Material can be made p and n-type Efficient radiative pathways must exist
- 23. 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 Readily doped n or p-typesMaterials with refractive index that could allow light to ‘get out’
- 24. Describe the principles of operation of an LED and state the material’s requirements criteria to produce an efficient LED. (50 marks)
Editor's Notes
- #23: Dr Zainovia Lockman L2