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.
What are diodes?
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.
Et voila, a diode is born.
What is an LED?
An LED is a diode that emits light. Inside the diode’s p-n junction, the electrons have more energy than the holes. When an electron meets and occupies a hole, it releases energy into its surroundings. If the frequency of this energy is in the visible part of the electromagnetic spectrum, the diode will be seen to emit light. The overall phenomenon is called electroluminescence.
The energy of a wave is proportional to its frequency. So making sure the light emitted by an LED is visible light is a matter of making sure the electron-hole recombination releases a certain amount of energy, not more and not less. This is possible to achieve thanks to the band gap.
What is the band gap?
Particles like electrons can only have specific energy values. They can occupy only particular energy levels. When a group of electrons comes together in a system – say, in a collection of atoms like a small piece of metal – they’re required to follow some rules. One of them is that no two electrons can occupy the same energy level at the same time.
These electrons generally prefer to have lower energy, and thus prefer to occupy the lowest available energy level. If that level is taken, they occupy the next available level.
Sometimes they can acquire more energy, tear free from their atoms, and flow around the material. In these circumstances, we say the material is an electrical conductor. When the electrons don’t have enough energy to flow around, the material is an insulator.
Electrons can acquire such extra energy when an electric field is applied to the material. The field will accelerate the electrons and energise them, and the electrons will be ‘kicked’ from lower to higher energy levels.
In some materials, there is an energy gap between these lower and higher levels – i.e. between when the electrons can’t and can flow around the material. An electron can’t have an amount of energy that would place it in one of these levels. It’s the reason why electrons in these materials can’t conduct an electric current unless they receive a minimum amount of energy – the energy required to jump across this gap.
This gap is called the band gap.
In LEDs, the energy emitted when an electron and a hole recombine is the energy of the band gap.
By carefully choosing the materials that make up the p-layer and the n-layer, researchers can engineer the composite p-n junction to have a band gap that corresponds to visible light. Electron-hole recombination can be triggered by passing an electric current through the diode, which creates the electric field that ‘kicks’ the electrons.
What colours can an LED produce?
Since LEDs can produce all three primary colours – red, green, and blue – different LEDs can be combined on a display board to produce a large variety of colours. (There are other ways as well.)
This said, scientists were able to create red and green LEDs more than 40 years before they created blue LEDs. The reason: scientists had identified a compound, gallium nitride, that was electroluminescent and whose band gap could yield blue light, but they didn’t know how to create crystals of this compound with the precise physical, electronic, and optical properties. Gallium nitride was also fragile, quickly becoming a powder in the process used to create crystals.
Inventing the blue LED eventually required a series of breakthroughs in epitaxy, the process by which p-type and n-type materials are built layer by layer. In the late 1980s, three Japanese researchers, Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura, led teams that at long last produced a bright blue LED with gallium nitride. For this feat they received the physics Nobel Prize in 2014.
What are the advantages of LEDs?
According to Moore’s law, specified by American engineer Gordon Moore in the 1970s, the number of transistors on a chip would double every two years. Similarly, improvements to LEDs since 1970 have followed Haitz’s law. Named for scientist Roland Haitz, it states that for a given frequency of light, the cost per unit of light of an LED will drop 10x and the amount of light it produces will increase 20x every decade.
But even before Haitz’s law, researchers prized LEDs because they were more efficient than incandescent bulbs and fluorescent lamps. Per watt of power consumed, LEDs can produce up to 300 lumen (amount of visible light emitted per second) versus incandescent bulbs’ 16 lumen and fluorescent lamps’ 70 lumen. Together with their greater durability and light contrast, LEDs’ advantages translated to higher cost savings and less material waste.
LEDs have several applications in industry, consumer electronics, and household appliances: from smartphones to TV screens, from signboards to ‘feeding’ plants light in greenhouses, from barcode scanners to monitoring air quality.
Today, LEDs can also produce a variety of colours or emit energy at higher and lower frequencies; LEDs can be ‘embedded’ in skin; and organic LEDs emit more light (albeit by a different mechanism). Researchers are also exploring more efficient LEDs made of materials called perovskites.
The author thanks Adhip Agarwala, assistant professor at IIT Kanpur, for his inputs.
