Light‐Emitting Diodes

Abstract

Inorganic semiconductor light‐emitting diodes (LEDs) have found widespread use in small‐area mobile displays, large‐area displays, signaling, signage, and general lighting. The entire visible spectrum can be covered by light‐emitting semiconductors: AlGaInP and AlGaInN compound semiconductors are capable of emission in the red‐to‐yellow and violet‐to‐green wavelength range, respectively. For white light sources based on LEDs, the most common approach is the combination of a blue LED chip with a yellow phosphor. White LEDs are currently used to replace incandescent and fluorescent sources. In the present review, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current flow patterns. LED structures providing high internal quantum efficiency, namely heterostructures and multiple quantum well structures, will be discussed. Techniques enhancing the external quantum efficiency will be reviewed, including chip shaping and surface roughening. Different approaches to white LEDs will be presented and figures‐of‐merit such as the color rendering index and luminous efficacy will be explained. Besides visible LEDs, the technical challenges in deep ultraviolet LEDs will be introduced. Finally, the packaging of low power and high power LED chips will be discussed.