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How Do LEDs Work?
- Grades 5-8
- 15 minute activity
- NGSS 4-PS3-2
Quick answer
An LED, or light-emitting diode, is a tiny chip made of two kinds of semiconductor material. When electricity flows through it the right way, electrons cross from one layer to the other and give off their extra energy as light. Flip the LED around and the electricity cannot cross, so it stays dark. The material in the chip decides the color.
What is inside an LED?
The colored plastic dome is just a lens. It protects the parts inside and aims the light forward. The real work happens in a chip smaller than a grain of sand, sitting in a tiny metal cup.
- Anvil: the larger metal piece that holds the chip. It joins the short leg, the negative side.
- Post: the smaller metal piece. A hair-thin wire connects it to the top of the chip, and it joins the long leg, the positive side.
- Flat edge: the rim of the lens has a flat spot on the negative side, another way to tell which way the LED goes.
Why LEDs only work one way
The word diode means a part that lets electricity flow in only one direction. The LED chip has two layers. One layer, called the N side, has extra electrons. The other, the P side, has spaces where electrons are missing, called holes.
With the long leg toward the battery's + side, the battery pushes electrons and holes toward the middle, where they meet and give off light. Turned backward, the battery pulls them apart instead, so nothing crosses and nothing lights. Learn more in What Is Polarity?.
What makes the colors?
An LED's color comes from the material in its chip, not from the plastic. Each flash of light carries a set amount of energy. Red light carries the least and blue light carries the most, so blue LEDs need a bigger push, more voltage, to light up.
White LEDs are usually blue LEDs with a yellow coating inside. The blue and yellow light mix and look white to your eyes. Color-changing LEDs hold several tiny chips and a little controller that switches between them.
Try it with dough
Which color fades first?
Use a long dough snake to slowly take away the push, and see which LED color gives up first.
You need
- Battery holder with 4 AA batteries
- Conductive (salty) play dough
- 1 red LED and 1 blue LED
- Ruler
Steps
- Roll a dough snake about as thick as your finger and 10 cm (4 inches) long. Set a small lump of dough next to one end with a gap.
- Push the red battery wire into the far end of the snake and the black wire into the lump.
- Bridge the gap with the red LED: long leg in the snake, short leg in the lump. Note the brightness.
- Swap in the blue LED and note its brightness.
- Make the snake longer, 20 cm and then 40 cm, keeping it the same thickness. Test both colors at each length.
- Finally, flip one LED around. What happens?
What happens
Both LEDs dim as the snake gets longer, but the blue LED fades first. Blue light needs more voltage, so it runs out of push sooner. Backward, neither LED lights at all, because a diode only lets electricity through one way.
Not working?
Neither LED lights even with a short snake? Make sure the snake does not touch the lump and the long leg is on the red-wire side. See Ohm's Law Explained for the math behind the dimming.
Why LEDs are everywhere
An old-fashioned light bulb makes light by heating a thin wire until it glows, and most of its energy is wasted as heat. An LED makes light directly from electricity, so it uses a fraction of the energy, stays cool and can last for years.
- The first practical visible LED, a red one, was made in 1962.
- Bright blue LEDs took until the 1990s. Their inventors won the 2014 Nobel Prize in Physics, because blue made white LED lighting possible.
- Today LEDs light homes, screens, traffic lights, cars and flashlights.
Key words
- LED
- Light-emitting diode: a part that gives off light when electricity flows through it the right way.
- Diode
- A part that lets electricity flow in only one direction.
- Semiconductor
- A material, like silicon, that is partly a conductor and partly an insulator. Computer chips and LED chips are made of semiconductors.
- Anode
- The positive side of an LED: the long leg.
- Cathode
- The negative side of an LED: the short leg, on the flat edge.
- Forward voltage
- The push an LED needs before it turns on, about 2 volts for red and 3 volts for blue.
For teachers
In the P-N junction, electrons from the N-type layer recombine with holes in the P-type layer. The energy released matches the band gap of the semiconductor, which sets the photon energy and therefore the color. Red LEDs commonly use aluminum gallium indium phosphide; blue and green LEDs use indium gallium nitride.
An LED is not a resistor: below its forward voltage almost no current flows, and above it current rises steeply. That is why LEDs need something to limit the current. In a dough circuit, the dough itself does that job. Never connect an LED straight across the battery holder's wires, since the extra current can damage it. Find more lessons on our education page.
LED questions
How does an LED work in simple terms?
Electricity pushes electrons across a tiny chip. When each electron falls into a gap on the other side, it gives off its extra energy as light.
What does LED stand for?
Light-emitting diode. A diode is a part that lets electricity flow in only one direction.
Why does an LED only work one way?
The chip has two layers. Facing the right way, the battery pushes electrons and holes together and they make light. Backward, it pulls them apart, so no electricity flows.
What determines the color of an LED?
The material in the chip. Different materials release different amounts of energy, and the amount of energy sets the color. The plastic lens only tints it.
Why do blue LEDs need more voltage than red?
Blue light carries more energy than red light, so it takes a bigger push to make it. Red LEDs need about 2 volts and blue LEDs about 3 volts.
Which leg of an LED is positive?
The long leg is positive (the anode) and goes toward the battery's red wire. The short leg, next to the flat edge of the lens, is negative (the cathode).
Next, try a project
Build the Lightbulb Project A free step-by-step LED project with printable instructions.