Light Emitting Diodes

A Light-Emitting Diode (LED) in essence is a P-N junction solid-state semiconductor diode that emits light when a current is applied though the device.

To understand the principle, let’s consider an unbiased pn+ junction. The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n side to the Ec on the p-side, called the built-in voltage, V0. This potential barrier prevents the excess free electrons on the n+ side from diffusing into the p side.

When a Voltage V is applied across the junction, the built-in potential is reduced from V0 to V0 – V. This allows the electrons from the n+ side to get injected into the p-side. These electrons injected into the p-side recombine with the holes. This recombination results in spontaneous emission of photons (light). This effect is called injection electroluminescence. The diode produces a monochromatic (one colour) 

light on a single wavelength ranging from red (˜700 nanometres) to blue-violet (˜400 nanometres). Because LEDs produce a pure colour of light, tinted lenses are not needed to filter the light to the desired colour. As a result, all of the visible light is projected from the LED.

LEDs consume very little power – they are up to 90 percent efficient, which means that only a small proportion of the input energy is consumed to produce heat. In comparison, traditional light sources (e.g., incandescent bulbs) are 5 to 10 percent efficient, with 90 percent or more of the input energy wasted in the form of heat.

LED Materials:

Important classes of commercial LEDs that cover the visible spectrum are the ternary alloys based on alloying GaAs and GaP which are denoted by GaAs1- yPy. InGaAlP is an example of a quarternary (four elements) alloy with a direct band gap.

The LEDs realized using two differently doped semiconductors that are the same material is called a homojunction. When they are realized using different band gap materials they are called a heterostructure device. A heterostructure LED is brighter than a homoJunction LED.

Advantages of using LEDs

• LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
• LEDs can emit light of an intended colour without the use of colour filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• When used in applications where dimming is required, LEDs do not change their colour tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
• LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or High Intensity Discharge (HID) lamps that require a long time before restarting.
• LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
• LEDs can have a relatively long useful life. A Philips LUXEON k2 LED has a life time of about 50,000 hours, whereas Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
• LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• LEDs can be very small and are easily populated onto printed circuit boards.
• LEDs do not contain mercury, unlike compact fluorescent lamps.

Disadvantages:

• LEDs are expensive than conventional lighting technologies.
• LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure.
• LEDs must be supplied with the correct current. This can involve series resistors or current-regulated power supplies.