1. Physics of Light-Emitting Diodes (LEDs)

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. 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. 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. 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:
http://www.halbleiter.org/en/fundamentals/doping/

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. 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. 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. 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. 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. 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:
• http://electronics.howstuffworks.com/led2.htm

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. References
• http://electronics.howstuffworks.com/led1.htm
• The physics behind light-emitting diodes. Measuring
Planck’s Constant, p. 6 – 8. Perimeter Institute for
Theoretical Physics (2008).
• Image sources:
• http://blog.oscarliang.net/use-led-in-robot-projects/
• http://www.digitaltrends.com/home-theater/led-vs-lcdtvs/
• http://www.education.rec.ri.cmu.edu/content/electronics/
common/LEDs/1.html
• http://www.physics-chemistryclass.com/electricity/diodes.html