Question

Explain how a p-n junction can act as an LED, and if configured correctly as a laser. (What are the requirements for it to act as a laser?)

Answer 1

The light emitting diode or LED is just a specialised type of diode which has very similar characteristics to a PN junction diode, i.e. an LED will pass current in its forward bias but block the flow of current in the reverse bias. Light emitting diodes are made from a fragile layer of fairly heavily doped semiconductor material. Depending on the material used and the amount of doping, an LED will emit a coloured light at a particular wavelength (). When the diode is forward biased, electrons from the conduction band recombine with holes from the valence band which give rise sufficient energy to produce photons which emit as monochromatic light.

A laser diode is also a particular type of diode, which also converts electrical energy into light energy to produce high-intensity coherent light. The working principle of the laser diode is almost similar to the LED, but The main difference between the LED and laser diode is that the LED emits incoherent light whereas the laser diode emits coherent light.

There are three primary steps to produce Laser.

1, Energy Absorption.

In laser diodes, electrical energy is used as the external energy source. When electrical energy supplies enough energy to a valence electron it breaks the bond with the atom and jumps into the higher energy level which is known as conduction band. The electrons in the conduction band are known as free electrons. When the valence electron leaves the valence shell, a free space is created which is known as a hole.

2.Spontaneous Emission.

In spontaneous emission, photons emit naturally while electrons get down to the lower energy state. The valence band electrons are in the lower energy state. Therefore, the holes generated after the valence electrons left are also in the lower energy state. The conduction band electrons or free electrons are in the higher energy state, i.e. the free electrons have more energy than holes. The free electrons lose their extra energy and recombine with the holes and releasing energy in the form of photons.

3. Stimulated Emission.

Stimulated emission is the process in which excited electrons are stimulated to fall into the lower energy state. and they releasing energy in the form of light. The stimulated emission is an artificial process. In stimulated emission, the excited electrons or free electrons need not wait for the completion of their lifetime. Before the end of their lifetime, external photons will force the free electrons to recombine with the holes. In stimulated emission, each incident photon will generate two photons.

This is the process to form a laser diode.