Light Emitting Diodes

Inorganic semiconductor light-emitting diodes (LEDs) are environmentally benign and have found widespread use as indicator lights, mobile displays, large-area displays, signage applications, and lighting applications. 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 wavelength range 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. Alternatively, a group of red, green, and blue (RGB) LEDs can be used; such source allows for color tunability. White LEDs are currently used to replace incandescent and fluorescent sources. In this review, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current-flow patterns. Structures providing high internal quantum efficiency, namely heterostructures and multiple quantum well structures, will be discussed. Advanced techniques enhancing the external quantum efficiency will be reviewed, including die shaping (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 of newly evolving deep ultraviolet (UV) LEDs will be introduced. Finally, the packaging of low-power and high-power LED dies will be discussed.