In one line
An LED is a forward-biased p-n junction in which electrons recombine with holes and release their energy as photons; the colour (wavelength) is decided by the band gap of the semiconductor material.
Explained simply
In a diode, electrons cross the junction and fall into holes, like balls dropping down a step. In silicon, the energy of that fall becomes heat. In special materials such as gallium arsenide or gallium nitride, it comes out as light. The height of the step (band gap) sets the colour: a bigger step gives bluer light, a smaller step gives redder or infrared light.
Technically, this process is called electroluminescence. LEDs use direct band gap compound semiconductors, where recombination releases most of the energy as photons with \( E = hf = hc/\lambda \approx E_g \).
How it works
- Forward bias pushes electrons from the n-side and holes from the p-side into the junction region.
- Electrons in the conduction band recombine with holes in the valence band.
- Each recombination releases energy approximately equal to the band gap \( E_g \) as a photon.
- Light intensity increases with forward current. Reverse bias gives no light, and LEDs have a low reverse breakdown voltage (often around 5 V).
| Material | Typical colour |
|---|---|
| GaAs | infrared (remote controls) |
| GaAsP | red, orange, yellow |
| GaP | green (also red when doped) |
| GaN / InGaN | blue, green; white with phosphor |
Forward voltage depends on colour: roughly 1.8–2.2 V for red, about 3–3.3 V for blue and white.
Key formula
\( \lambda = \dfrac{hc}{E_g}, \qquad \lambda\,(\text{nm}) \approx \dfrac{1240}{E_g\,(\text{eV})} \)
\( \lambda \) = emitted wavelength (m or nm), \( h = 6.626\times10^{-34} \) J·s (Planck's constant), \( c = 3\times10^8 \) m/s, \( E_g \) = band gap (J or eV).
Quick example
GaAs has \( E_g \approx 1.43 \) eV, so \( \lambda \approx 1240/1.43 \approx 870 \) nm, which is infrared: this is the invisible LED in a TV remote control. A phone camera can actually show its flashes.
Common mistakes
- Saying LEDs are made from silicon; silicon has an indirect band gap and emits mostly heat.
- Saying an LED glows in reverse bias.
- Thinking the plastic lens colour decides the light colour; the semiconductor band gap does.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the working principle of an LED" (5 marks), or "Explain how different colours are obtained in LEDs". For full marks draw the forward-biased junction with recombination and photon emission, use the word electroluminescence, write \( \lambda = hc/E_g \), and list materials with colours.