A LED, or light-emitting diode, is a semiconductor device that emits infrared or visible light when charged with an electronic current. LEDs have been used as indicator lamps on many electronic devices, billboards, and even car brake lights for years. Recently LEDs have infiltrated the general lighting market as well. A traditional light bulb emits light through incandescence, where an electric current heats a wire filament, causing the wire to emit photos. LEDs, on the other hand, operate by electroluminescence. Photons are emitted during electroluminescence, the electronic excitation of a material. Electroluminescence was discovered in 1907 by British radio researcher Henry Joseph Round while experimenting with silicon carbide. In the early 1920s, Russian radio researcher Oleg Vladimirovich Losev studied electroluminescence in the diodes used in radio sets. In 1927 he published a paper called Luminous carborundum
[silicon carbide] detector and detection with crystals. No LED was created at that time but his research influenced future inventors. Robert Biard and Gary Pittman, from Texas Instrument,s patented the first infrared LED in 1961. A year later Nick Holonyack, a consulting engineer for General Electric Company, invented the first visible light LED. Holonyack used gallium arsenide phosphide as the substrate to create his red LED.[i] Gallium arsenide is the most commonly used material for LEDs, but variations, including aluminum gallium arsenide or aluminum gallium indium phosphide, are also used. These compounds are members of the III-V group of semiconductors. The III-V group consists of elements listed in the columns III and V of the periodic table of elements. Changing the precise composition of the semiconductor varies the wavelength, or color of the emitted light. LED emission is generally in the visible area of the spectrum, with wavelengths from 0.4 to 0.7 micrometers. Infrared wavelengths are in the range of 0.7 to 2.0 micrometers. The diode refers to the twin-terminal structure of the LED. Similarly to other semiconductor devices, the terminals are two different semiconductor materials with different composition and electronic properties that are brought together to form a junction. The negative, or n-type, semiconductor charge carries the electronics, while the positive, or p-type, semiconductor charge carries the holes created by the lack of electrons. When an electronic field is on, current flows across the p-n junction, providing the excitation that causes the material to emit light.[ii]
LEDs Thermal Management
While LEDs are cool to the touch and do not produce heat in the form of infrared radiation, they do generate heat in the diode semiconductor structure, known as the junction temperature. Three factors affect junction temperature: ambient air temperature, the thermal path between the junction and the outside environment, and the LED’s efficiency. The LEDs junction temperature (Tj) can be calculated by adding the product of the LEDs overall thermal resistance (R) and the power dissipated (Pd) to the ambient temperature (Ta). See equation below.[iii]
Tj = (R x Pd) + Ta
Knowing a LEDs junction temperature is important because it determines the operating life of the device and effects the heat dissipation and airflow design of the system. While a high junction temperature is required to create the appropriate difference to transfer the heat from the die to the ambience, the long term operative life a LED is decreased with increased junction temperature. Integrated circuits can be come unreliable with elevated junction temperatures. The recommended junction temperature is below 1250 C.[iv]
Benefits of LEDs to the Commercial Lighting Industry
LED technology has come a long way since their creation in the early 1900s. While the initial cost of an LED may be higher then the traditional incandescent light bulb, the operating cost of LED lights is much lower and they last approximately five times longer. LEDs also emit less carbon then the traditional light bulb, therefore benefiting the environment as well.[v] As LEDs continue to gain in popularity on the commercial level, their price tag will continue to trend downward while their efficiency level continues to rise.