What are light-emitting diodes and why are they prized as light sources? | Explained Premium

The text explains the science behind diodes and LEDs, their applications, and the breakthroughs that led to the creation of blue LEDs.

In October 2014, the Royal Swedish Academy of Sciences issued a statement in which it said, “Incandescent light bulbs … lit the 20th century; the 21st century will be lit by LED lamps.” The occasion was the awarding of the Nobel Prize for physics for that year, for an achievement that paved the way for light-emitting diodes (LEDs) to succeed the incandescent bulbs and fluorescent lamps of previous centuries as the world’s light-source of choice.

A diode is an electronic component about 5 mm wide. It has two points of contact, or terminals, called its anode and cathode. A diode’s primary purpose is to allow current to flow in only one direction. It achieves this using a p-n junction.

A p-n junction is made of two materials laid next to each other. One material is a p-type material: its primary charge-carriers are holes. The other is an n-type material: its primary charge-carriers are electrons. You’re familiar with electrons: they are ‘places’ inside atoms that carry negative charge. A hole denotes a ‘place’ in an atom or a group of atoms where there could be an electron but isn’t. Thus, a hole is an electron placeholder but without the electron, so it has a positive charge.

A p-n junction is an interface where the surface of a p-type material and the surface of an n-type material meet. At this interface, electrons can pass easily from the n-type material to the p-type material but can’t go the other way. This asymmetry creates the diode’s ability to allow current to pass in only one direction.

Wire attached to the p-type material is called the diode’s anode; that attached to the n-type material is the cathode. These are the diode’s two terminals.

When the two materials are first placed next to each other, some electrons move from the n-side to the p-side until there is a layer, between the two sides, where there are neither (free) electrons nor holes present.

When a suitable voltage is applied across the diode, more electrons are encouraged to flow from the n-side to the p-side, implying an electric current flowing from the p-side to the n-side, i.e. from the anode terminal to the cathode terminal. But if the voltage is reversed, current won’t flow in the opposite direction.