(LED), a semiconductor device that emits radiation in the visible, infrared, or ultraviolet regions of the electromagnetic spectrum when an electric current passes through it. Diodes are electronic devices that allow current to pass in only one direction. The first practical LED producing visible light was developed by U.S. scientist Nick Holonyak (1928– ) while working at General Electric in 1962. It gave off red light. Other colors later became available.Today there are many types of useful LEDs.

An LED produces light by electroluminescence (see Luminescence). In its basic form, the semiconductor chip in an LED consists of two adjacent regions: one is a so-called p-type semiconductor, containing an excess of positively charged “holes” that can accept electrons, which have a negative charge; the other is an n-type semiconductor, containing extra electrons. The two regions are separated by a “junction,” which serves as a barrier to the flow of current between them. If a sufficient voltage is applied across the chip in the right direction, it causes a current to flow: electrons move across the junction to fill holes in the p-region. When an electron “recombines” with a hole, a photon of electromagnetic radiation is emitted. The color (frequency) and brightness of the light produced depend on the materials of which the LED is made; phosphors may be incorporated in the LED to yield a specific desired color. The device's design can also affect the light output: for example, the chip and its housing may be structured so as to focus the light.

Compared to such light sources as incandescent lamps (see Electric Lighting), LEDs are typically more robust, longer lived, and smaller sized, as well as more efficient, producing relatively little heat. They also turn on more quickly. Because of these characteristics they have found a wide variety of applications. Visible-light LEDs are used as indicator lights in instrument panels and automobile dashboards and on devices such as cameras, computer hardware, household appliances, and telephones. They are employed as light sources for both small and large displays using alphabetical and/or numeric characters, such as in watches, clocks, calculators, message signs, and video displays, and they are used to provide backlighting for liquid-crystal displays (LCDs). They also may be found in traffic signals and in automobile taillights. Their illumination applications, however, are limited by the fact that in some situations conventional lighting devices remain cheaper to produce. Notable examples of infrared LED applications include home-entertainment remote controls, optical-fiber communication systems, and, in combination with a photodetector, devices used as optical interfaces (“optoisolators”) between electric circuits.

In the special type of LED known as an organic light-emitting diode (OLED), the interaction between holes and electrons takes place in a thin film of certain organic, or carbon-containing, compounds. OLEDs tend to be relatively light in weight and are employed for such purposes as light sources and display screens, often in applications where LCDs were once commonplace, such as personal digital assistants (PDAs) and cell phones. OLEDs generally provide brighter output, show faster response time, and are more efficient and cheaper to make than LCDs, which in contrast to OLEDs require a backlight and cannot yield a clear image when viewed from an angle. Extremely thin and flexible displays can be made with so-called polymer light-emitting diodes (PLEDs), which use a thin film of certain very large organic molecules.