The hottest lights on the market today are cool light-emitting diodes, or LEDs.

LEDs are made from semiconductors, which differ from conductors in the way their crystal structure holds electrons. Conductors are typically metals, which exist naturally as crystals consisting of positive ions held together by a sea of loosely bound, mobile electrons.

When a source of electrical energy is applied to a conductor, the electric field exerts a force on electrons in the metal. They move through the conductor, constituting an electric current.

By contrast, a semiconductor is a material that can be doped with impurities to alter its electronic properties in a controllable way. The most common semiconductors are crystalline inorganic solids that are classified according to the periodic table groups from which they are made, with silicon being the most well known.

Two types of substances are used for doping. One type adds atoms with extra negatively charged electrons (N-type). The other adds atoms with fewer electrons that create positively charged holes into which electrons can move (P-type).

A diode is constructed by bonding a P-type to an N-type. When no voltage is applied, excess electrons flow from the N-type and fill the holes of the P-type. This causes a region known as the depletion zone to form an insulating layer between the two materials.

The diode operates when a negative electrode is attached to an N-type and a positive electrode is connected to the P-type.

At a certain voltage, which is different with different materials, the repulsive force on the negative side is great enough to force the electrons in the depletion zone out of the holes. Current then flows as electrons jump from hole to hole through the diode.

Reversing the electrodes allows no current to flow because electrons are pulled away from the depletion zone.

As electrons fall into holes, they release a fixed amount of energy that depends on the properties of the two materials. The energy is emitted as a photon that corresponds to a narrow band of wavelengths, so the light is very nearly monochromatic.

In silicon semiconductor the emitted energy falls in the range of infrared photons, so it is not visible to the human eye, but infrared-emitting diodes (IRED) are ideal for remote controls, among other things.

To make visible light the electrons need to fall into deeper holes, and this requires specialized materials. Red LEDs are typically aluminum-gallium-arsenide (AlGaAs), and different materials produce different-colored LEDs. Blue LEDs are based on GaN (gallium nitride), and InGaN (indium gallium nitride), semiconductors.

There are no white LEDs because white is a composite of colors. White light can arise from equal intensities of red, green and blue LEDs, but white LEDs on the market are UV/Blue LEDs with a yellow filter.

LEDs are now the favored source of artificial light because they are more efficient, producing much less heat and more lumens per watt than either fluorescent or incandescent bulbs.

The search for new semiconductor materials and the improvement of existing materials will continue to be an important field of study in materials science in the 21st century.