Light Emitting Diodes (LEDs)

The best known of all opto-electronic devices is the so called LED (light-emitting diode), which emits a fairly narrow bandwidth of visible (usually red, orange, yellow or green) or invisible (infrared) light when its internal diode junction is stimulated by a forward electric current/voltage (power). LEDs have typical power-to-light energy conversion efficiencies some 10 to 50 times greater than of a simple tungsten lamp and have very fast response times (approximately 0.1 us compared with tens or hundreds of milliseconds for a tungsten lamp), and are thus widely employed as visual indicators and as moving light displays.

Operation

The operation of LED is based on the phenomenon of electroluminance, which is the emission of light from a semiconductor under the influence of an electrical field. The recombination of charge carriers takes place in a forward P-N junction as the electrons cross from the N-region and recombine with holes existing in P-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band. Therefore, the electrons  are at high energy levels than the holes. For the electrons to recombine with the holes,they must give some of their energy. Typically, these electrons give up energy in the form of heat and light.

In silicon and germanium diodes, most of the electrons give up their energy in the form of heat. However, with GaAsP, and GaP semiconductors, the electrons give up their energy by emitting photons. If the semiconductor is transluscent, the light will be emitted and the junction becomes a source of light, i.e., a light-emitting diode (LED). LEDs emit no light when reverse biased. In fact, operating LEDs emit no light when reverse biased. In fact, operating LEDs in reverse bias mode will quickly destroy them.

LEDs are manufactured with domed lenses so as to reduce the reabsorption problem. LEDs are always encased in order to protect their wires. LEDs made from GaAs emit invisible infrared light, LEDs constructed of GaAsP tend to emit either red or yellow light. The GaP LEDs give either red or green light. LEDs can be manufactured to produce light of virtually any colour. Table 1 provides  a list of common compound semiconductors and the light they produce. In addition, the typical range of forward bias voltage for each is listed.

Table 1. Light-Emitting Diodes
Colour	Construction	Typical Forward Voltage Voltage in Volts
Amber	AllnGaP	2.1
Blue	GaN	5.0
Green or red	GaP	2.2
Yellow	GaAsP	2.2
Red	GaAsP	1.8
White	GaN	4.1
Yellow	AllnGaP	2.1

Light Circuit Symbol

The below figure shows a circuit symbol for a LED. The arrows indicate radiation emitted by the diode.

LED Operating Characteristics

The below figure give two curves used to determine LED operating characteristics. The V-I curve is practically applicable in burglar alarms. Forward bias of approximately one volt is needed to give significant forward current. The second figure is used to represent a radiant power-forward current curve. The output power produced is very small and thus the efficiency in electrical-to-radiant energy conversion is very less.

Advantages of LEDs:

1. LEDs are miniature in size and can be stacked together to form numeric and alphanumeric displays in high density matrix.
2. Smooth control of light intensity from an LED because light output from an  LED is a function of current flowing through it.
3. LEDs are economical and have a high degree of reliability.
4. LEDs are rugged and can, therefore, withstand shocks and vibrations.
5. LEDs can be operated over a with temperature range (0-70oC).
6. The switching time is less than 1 ns. So they are very suitable where dynamic operation of large number of arrays is involved.
7. LEDs are available in different colours like red, yellow, green and amber.
8. They need moderate power. They are used where low dc power is available.
9. They occupy small area.
10. LED can be driven by transistor-transistor logic (TTL).

The drawbacks of LEDs are that they may get damaged by overvoltage or overcurrent. They have wide optical bandwidth compared to the LASER. Their temperature depends on the radiant output power and wavelength. LEDs are not suited for large area displays, primarily because of their high cost. For large displays, devices using gas filled plasma are used.