The principle of LED


LEDs are electronic devices that emit photons.
LEDs have higher efficiency and longer life than bulbs.

Basically, LEDs have a structure like a p-n junction diode.
The p-n junction diode is made by attaching a p-type semiconductor and an n-type semiconductor.
Inside the P-type semiconductor, some electrons are empty, so there are 'positive holes', which are relatively positively charged. And, inside the N-type semiconductor, There ars 'free electrons' with negative charged.
The p-type and n-type semiconductors are made by doping some materials in pure silicon(Si) or germanium(Ge). Depending on the materials to be doped, they have slightly different electrical properties, such as energy levels.

Valence band

In solid state physics, the valence band refers to the highest electron energy band in which an electron exists at absolute zero temperature. Electrons are only observed within bands, and cannot be outside the band (the basic principle of quantum mechanics).
Pure semiconductors are full of electrons, so electrons cannot move. For example, imagine a road that is full of cars and can no longer move.
The p-type semiconductor allows electrons to move through the void space(electron holes) inside the valence band.
The picture below shows why the particles need empty space to move.

Conduction band

In n-type semiconductors, some of the electrons can move freely in the'conduction band'. Electrons in this conduction band are easily accelerated (=current) when an electric field is applied.

Fermi level(Fermi energy)

Let's imagine small particles like electrons filling up from the ground state. The Fermi level is the point where the probability of existence of these particles is 1/2. If compare this to a water cup, it is the water level.
In p-type semiconductors, the positive holes are located slightly below the Fermi level. In n-type semiconductors, the free electrons are a little higher than the Fermi energy.
Let's assume that no voltage is applied. If the p-type semiconductor and the n-type semiconductor are put together, the Fermi levels of the two are adjusted to the same level. This state is called 'thermal equilibrium(or no biased)'.
When power is connected to a diode in thermal equilibrium, the Fermi level of the part connected to the (+) pole increases, and the Fermi level of the part connected to the (-) pole decreases (caused by the supply and demand of electrons).

Wavelength of light according to the band gap

When a forward voltage(bias) is applied to the diode, the Fermi level changes and current flows. Free electrons cross the junction and combine with electron holes. At this time, energy is released as much as the energy band gap where free electrons and positive holes are located. When the band gap is about 2~3 eV, the energy emitted to visible light. When the band gap is E(eV), the wavelength λ(nm) of the emitted light can obtained as follows.

\[ \lambda (nm) \approx \frac{1240}{E(eV)} \]