Physics of Light-Emitting Diodes (LEDs)

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.

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

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 –vely-charged

area to +vely charged area, the positive holes appear to move from +vely-charged area to –vely charged area.

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/

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.

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 p-type 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.

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.

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.

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.

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

Circuit symbols for LED

Anode (+) Cathode (-)

OR

Connect to (+) terminal of battery

Connect to (-) terminal of battery

Use of LED in forward bias state in an electric circuit

Anode (+)Cathode (-)

Current flow

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-lcd-tvs/• http

://www.education.rec.ri.cmu.edu/content/electronics/common/LEDs/1.html

• http://www.physics-chemistry-class.com/electricity/diodes.html