About LEDsAbout LED Devices

Why do LEDs Light up?

LEDs are used in a vast range of applications in our daily lives, such as indicators and lighting, but how do they light up?
For example, light bulbs and fluorescent lamps emit light by indirectly converting electrical energy into light through filament heating and discharge phenomena. In contrast, LEDs convert electrical energy directly into light, so they achieve a very high electricity-to-light conversion efficiency.
This article describes the principles of light emission from the semiconductor elements used in LEDs.
* In this article, a product housed in a package is called an LED (or LED device) and is distinguished from a light-emitting element.

Principle of LED Light Emission

The element of LED basically comprises a structure called a “p-n junction,” which is a junction between a p-type and an n-type semiconductor.
　*p-type semiconductor (+: positive): hole-rich semiconductor
　 n-type semiconductor (– :negative): electron-rich semiconductor
When a forward voltage is applied to a light-emitting element, holes (positive) and electrons (negative) move toward the p-n junction, where they recombine and disappear. The energy generated in this process is converted into emitted light. This phenomenon is called light emission by recombination.

　* When electrons (negative) and holes (positive) recombine, the excess energy is released in the form of light.

Materials of Light-Emitting Elements

Ordinary diodes are mainly made of Si (silicon) or Ge (germanium), but these single-element semiconductors do not emit light easily when electrons and holes recombine. For this reason, compound semiconductors such as GaN, GaAs, and InP are used in LEDs, because these materials do efficiently emit light by electron-hole recombination.
A combination of these compound semiconductors (examples of combinations include GaAsP, AlGaAs, InGaAlP, and AlGaN) can be grown on a given substrate with the crystal planes aligned (epitaxial growth) to form a light-emitting element.

Emission Color Varies with Material

Compound semiconductors, the materials used in the element of LED emit light at various wavelengths, in the infrared, visible (e.g., red, green) and ultraviolet regions, depending on their composition.
The emission color is determined by the difference in the energy of the electrons before and after recombination. The energy difference (bandgap energy (Eg)) is converted into light, and the larger this difference, the shorter the wavelength of the light.