Physics of LEDs


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Physics of LEDs

  1. 1. Physics of Light-Emitting Diodes (LEDs)
  2. 2. Objectives • Describe what a semiconductor is. • Describe the difference between n-type and p-type semiconductors. • Describe what is a diode and how its connection across a voltage supply gives rise to “forward bias” or “reverse bias” state. • Describe what a light-emitting diode (LED) is and give some examples of its application. • Recognise circuit symbols for an LED. • Connect an LED in forward bias in an electric circuit.
  3. 3. Semiconductor • Materials with varying ability to conduct electrical current • Most semiconductors are poor electrical conductors that has impurities (atoms of another material) added to it. • Process of adding impurities - doping
  4. 4. n-type vs p-type semiconductors • n-type semiconductor: – material with extra NEGATIVE charges (i.e. free electrons) – free electrons move from -vely charged area to +ly charged area • p-type semiconductor: – material with extra POSITIVE charges (i.e. “vacant spaces” for which free electrons can occupy) – “vacant spaces” are known as positive holes – As free electrons move from hole to hole from –velycharged area to +vely charged area, the positive holes appear to move from +vely-charged area to –vely charged area.
  5. 5. Doping in 2 types of semiconductors n-type semiconductor • Elements with 5 valence electrons are introduced as impurities to silicon: n-type doping. p-type semiconductor • Elements with 3 valence electrons are introduced as impurities to silicon: p-type doping. More about how n-type and p-type semiconductors are doped can be found here:
  6. 6. What is a diode? • It is an electronic component that is made up of a section of n-type material and a section of p-type material bonded together. • Each section has an electrode at its end. • The entire setup is cased in plastic.
  7. 7. When no voltage source is applied across diode… • At boundary between 2 materials (i.e. p-n junction), electrons from n-type material naturally fill holes in ptype material. • A depletion zone is formed when all the holes in this region are filled. • Charges cannot flow because there are no free electrons or positive holes available in zone.
  8. 8. When n-type section is connected to –ve terminal of voltage supply… • Free electrons in n-type material are repelled by negative electrode and move towards positive electrode. • When the potential difference between the electrodes is high enough, electrons in depletion zone get out from holes and start moving freely again. • Result: – Depletion zone disappears. – Charges move across diode. (i.e. current flows) – The diode is in “forward bias” state.
  9. 9. When p-type section is connected to –ve terminal of voltage supply… • Free electrons in n-type material are attracted to positive electrode • Positive holes in p-type material are attracted to negative electrode • Result: – Depletion zone increases. – Current will not flow. – The diode is in “reverse bias” state.
  10. 10. In summary, diodes are… • electronic components that have – very low resistance when current flows through it in one direction (forward bias) AND – very high resistance when current flows through it in the other direction (reverse bias). • It acts similarly to a one-way valve that allows water to flow through a pipe in only one direction.
  11. 11. What is a light-emitting diode (LED)? • A diode that converts electrical energy into light of a narrow frequency range when sufficient current flows through it in the forward bias direction. • Advantages of LEDs over conventional light bulbs: – More energy efficient – Longer lifetime • Uses: Traffic lights, digital alarm clocks, TV remote controls • More about how a diode can produce light as free electrons and positive holes move across the p-n juction can be found here: •
  12. 12. Circuit symbols for LED Use of LED in forward bias state in an electric circuit Current flow Connect to (+) terminal of battery Connect to (-) terminal of battery Cathode (-) OR Anode (+) Cathode (-) Anode (+)
  13. 13. References • • The physics behind light-emitting diodes. Measuring Planck’s Constant, p. 6 – 8. Perimeter Institute for Theoretical Physics (2008). • Image sources: • • • common/LEDs/1.html •