LED is an optoelectronic device; it consists mainly of a p-n junction. LEDs are particularly designed so that the radiative recombination dominates to produce a photon. This radiative recombination process of electrically injected electrons and holes from n-type and p-type regions occurs efficiently in the active region of LEDs. The wavelength of the emitted photon depends on the bandgap energy of the semiconductor material in the active region.

The typical structure of LED is a p-n junction diode, mainly consisting of one layer uniformly p-doped in one side and one n-doped on the other side. Once the p-n junction has been created, a space charge layer and a built-in voltage (energy barrier) will automatically be formed due to the carrier balance process. By applying an external forward voltage to the diode, the electrons and holes in the conduction band and valance band respectively see a lower energy barrier and start the minority injection process. The electrons will be injected to the n-type layer and the holes to the p-type layer. When they reach the active region, the electrons in the conduction band recombine with holes from the valence band creating a photon. The wavelength of the emitted photons is associated to the bandgap energy of the active region.

The active region is the core of the LED structure, and this region in the LED determines the way that the photons created by electron–hole radiative recombination are emitted. Quantum well structure is typically employed as active region in LED structure.

A quantum well (QW) is a thin layer sandwiched between two larger–energy gap materials. The width of the QW corresponds to the thickness of the sandwiched layer, so the well width is controlled by changing the thickness of this layer. The QW structures are used as active regions in the LED structure because they have a small volume, therefore when carriers are injected into a QW structure, the free carrier concentration in QW is high so nonradiative recombination probability is less. QW structures give a freedom to design the transition energies through changing the well width.

There are two fundamental recombination mechanisms in LED, radiative recombination and nonradiative recombination. During a radiative recombination, electrons and holes recombine followed by the emission of photons, whereas in a nonradiative recombination process, the energy is converted to phonons, generating extra heat to the material.

The LED turn-on voltage is the voltage at which the LED first emits light or at which the diffusion current starts to dominate. This turn-on voltage depends on the bandgap of the active region.