Light Emitting Diode Sumitesh Majumder

What are the Light Sources ? The most common light sources in this systems are Light Emitting Diode (LED) and Laser Diode (LD) Convert the electrical signal into a corresponding light signal Small size Solid structure Low power requirements

Light-Emitting Diodes The combination between a free electron and a hole, returning the atom to its neutral atom, releases energy. Light is a form of this energy.

Light-Material Interaction Spontaneous Emission Stimulated Emission Absorption

A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction. LEDs converts electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not.

l c t r i C B H (a) The energy band diagram of a p-n (heavily u ­ E g ( b ) V a p n + e o F l c t r i C B H v y D s d (a) The energy band diagram of a p-n (heavily -type doped) junction without any bias. Built-in potential prevents electrons from diffusing from to side. (b) The applied bias reduces and thereby allows electrons to diffuse, be injected, into the -side. Recombination around the junction and within the diffusion length of the electrons in the -side leads to photon emission.

How Does A LED Work? (1/2) When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions. In the p region there are many more positive than negative charges. When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region.

How Does A LED Work? (2/2) Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus)..

Properties of LED Emits incoherent light through spontaneous emission. Used for Multimode systems w/ 100-200 Mb/s rates. Broad spectral width and wide output pattern. 850nm region: GaAs and AlGaAs 1300–1550nm region: InGaAsP and InP Two commonly used types: ELEDs and SLEDs

Double hetero junction structure Two different bandgap energies and refractive indice The change in different bandgap energies create potential energy for holes and electrons The electrons can meet and recombine in the well-defined active layer. [ double hetero junction ]

SLEDs – Surface Emitting LEDs Primary active region is a small circular area located below the surface of the semiconductor substrate, 20-50µm diameter and up to 2.5µm thick. Emission is isotropic and in lambertian pattern. A well is etched in the substrate to allow the direct coupling of emitted light to the optical fiber Emission area of substrate is perpendicular to axis of optical fiber Coupling efficiency optimized by binding fiber to the substrate surface by epoxy resin with matching refractive index

Surface Emitting LED

[surface-emitting LED ]

ELEDs – Edge Emitting LEDs Primary active region is a narrow strip that lies beneath the semiconductor substrate Semiconductor is cut and polished so emission strip region runs between front and back. Rear face of semiconductor is polished so it is highly reflective while front face is coated with anti-reflective, light will reflect from rear and emit through front face Active Regions are usually 100-150µm long and the strips are 50-70µm wide which are designed to match typical core fibers of 50-100µm. Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

Edge Emitting LED

L i g (a) Surface emitting LED (b) Edge emitting LED D o u b l e h t r

2. LED Operating characteristics

2. LED Operating characteristics

Planar LED

Dome LED L i g h t o u p E l e c r d s P a m D n J ( ) b + (a) Some light suffers total internal reflection and cannot escape. (b) Internal reflections can be reduced and hence more light can be collected by shaping the semiconductor into a dome so that the angles of incidence at the semiconductor-air surface are smaller than the critical angle. (b) An economic method of allowing more light to escape from the LED is to encapsulate it in a transparent plastic dome. S

g h p G R I N . M u l t i m o d e f b r L n s ( a ) E D - S c v y g h p G R I N . - S © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Characteristics of commercial LEDs

1. Light-Emitting Diodes

Applications Sensor Applications Mobile Applications Signal Applications LED Signals Illuminations Indicators