How an LED Produces Light
Electrons falling across the band gap release energy as photons.
The junction is where light starts
An LED chip is built from two layers of semiconductor joined together. One layer is doped so it has spare electrons (n-type). The other is doped so it has gaps, called holes, where electrons could sit (p-type). The boundary between them is the p-n junction.
Step by step
- Connect the positive supply to the p side (anode) and the negative to the n side (cathode). This is called forward bias.
- Once the voltage reaches the LED's forward voltage, electrons are pushed across the junction toward the p side.
- Electrons meet holes and drop into them. This is recombination.
- An electron falling into a hole loses a small packet of energy, and in an LED that energy leaves as a photon of light.
Why different LEDs have different colours
The colour depends on the energy gap of the semiconductor material. A small gap releases low-energy photons, which appear red or infrared. A larger gap releases higher-energy photons, which appear green, blue or ultraviolet. Manufacturers choose and blend materials to get the colour they want.
How white LEDs are made
Most white LEDs use a blue chip covered with a yellowish phosphor coating. Some blue light passes through, the phosphor turns the rest into yellow and red light, and together your eye sees white. A thicker or different phosphor changes whether the white looks warm or cool.
Why brightness follows current
Each electron that crosses the junction can produce at most one photon, so more current means more recombinations and more light. That is why LED circuits are designed around current rather than voltage.
Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.