The fundamental theory for design and construction of LED and LASER optical sources is based on electron hole recombination within a semiconductor material.
The electron hole recombination results in generation of photons that have frequencies that are determined by the physical parameters of the semiconductor material.
The phenomena which results from interaction of between matter and light are absorption, spontaneous emission and stimulated emission.
When light is incident on a semiconductor material, some photons get absorbed in the material which then transfer their energy to the electrons in the ground state and cause them to migrate to the excited state. This phenomenon is called absorption.
The transition of an electron from the excited state to the ground state can happen as a result of the natural tendency of the electron without the action of any external agent. The radiation produced as a result of such transitions is called as spontaneous radiation.
Stimulated emission of radiation is the process whereby photons are used to generate other photons that have exact phase and wavelength as that of parent photon. Thus stimulated emission is caused by external stimuli.
LED operation is based on spontaneous emission and LASER operation is based on stimulated emission.
LASER receives the optical output or the required external photon for stimulated emission from a low output optical source and then amplifies it by its internal processes.
If we compare it to that of an LED, LED is truly an optical source which emits light via radiative recombination.
The main difference between both is the active area. LASER has small active layer where massive electrons are injected to get a sharp beam of light as shown in the figure above whereas in LED’s are electro-luminescent device that are composed of semiconductor materials and are capable of generating light when they are forward biased by a current source.