In an electronics circuit, an LED behaves very much like any other diode. LEDs are often used to indicate the presence of a voltage at a particular point and are often used as a supply-rail indicator.
When used in this fashion, there must be a current-limiting resistor placed in the circuit. The resistor’s value should be calculated to give the required level of current. For many devices, a current of around 20mA is suitable, although it is often possible to run them at a lower current. If less current is drawn, the device will obviously be dimmer.
When calculating the amount of current drawn, voltage across the LED itself may need to be taken into consideration. The voltage across an LED in its forward-biased condition is just over a volt, although the exact voltage is dependent upon the diode and, in particular, its colour. Typically, a red one has a forward-voltage of just below two volts, and it is around 2.5 volts for green or yellow.
Great care must be taken not to allow a reverse bias to be applied to the diode. Usually, LEDs have a reverse breakdown of very few volts. If breakdown occurs, then the LED is destroyed. To prevent this from happening, an ordinary silicon diode can be placed across the LED in reverse direction to prevent any reverse bias being applied.
Development of LEDs
LED, in electronics, is a semiconductor device that emits infra-red or visible light when charged with an electric current. LEDs operate by electro-luminescence, a phenomenon in which emission of photons is caused by electronic excitation of a material. The material used most often in LEDs is gallium-arsenide, though there are many variations on this basic compound, such as AlGaAs or aluminium-gallium-indium-phosphide.
These compounds are members of the so-called III-V group of semiconductors, that is, compounds made of elements listed in columns III and V of the periodic table. By varying the precise composition of the semiconductor, the wavelength (and therefore the colour) of the emitted light can be changed.
LED emission is generally in the visible part of the spectrum (with wavelengths from 0.4 to 0.7 micrometre) or in the near infra-red (with wavelengths between 0.7 and 2.0 micrometres). The brightness of the light observed from an LED depends on the power emitted by the LED and on the relative sensitivity of the eye at the emitted wavelength. Maximum sensitivity occurs at 0.555 micrometre, which is in the yellow-orange and green region. The applied voltage in most LEDs is quite low, in the region of two volts. The current depends on the application and ranges from a few milliamperes to several-hundred milliamperes.
The term diode refers to the twin-terminal structure of an LED. In a flashlight, for example, a wire filament is connected to a battery through two terminals—one (the anode) bearing the negative electric charge and the other (the cathode) bearing the positive charge. In LEDs, as in other semiconductor devices such as transistors, the terminals are actually two semiconductor materials of different composition and electronic properties brought together to form a junction.
In one material (the negative, or n-type semiconductor), the charge carriers are electrons, and in the other (the positive, or p-type semiconductor), the charge carriers are holes created by the absence of electrons. Under the influence of an electric field (supplied by a battery, for instance, when the LED is switched on), current can be made to flow across p-n junction, providing the electronic excitation that causes the material to luminance.
In a typical LED structure, the clear epoxy dome serves as a structural element to hold the lead frame together, as a lens to focus the light and as a refractive-index match to permit more light to escape from the LED chip. The chip, typically 250×250×250 micrometres in dimension, is mounted in a reflecting cup formed in the lead frame.
The p-n-type GaP:N layers represent nitrogen added to gallium-phosphide to give green emission, the p-n-type GaAsP:N layers represent nitrogen added to gallium-arsenide-phosphide to give orange and yellow emission, and the p-type GaP:Zn,O layer represents zinc and oxygen added to gallium-phosphide to give red emission.
Two further enhancements, developed in the 1990s, are LEDs based on aluminium-gallium-indium-phosphide, which emit light efficiently from green to red-orange, and also blue LEDs based on silicon-carbide or gallium-nitride. Blue LEDs can be combined on a cluster with other LEDs to give all colours, including white, for full-colour moving displays.