LED Basics
In the 1920’s Oleg Vladimirovich Losev, a radio technician noticed that the diodes used in radio receivers emitted light when current was passed through them but the LED was introduced as an electronic component only in 1962. LEDs have now evolved in brightness and colour and are now considered as sources of area illumination.
TIMELINE
In 1955, Rubin Braunstein Braunstein observed infrared emission generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77 kelvin.
In 1961, experimenters Robert Biard and Gary Pittman working at Texas Instruments found that GaAs emitted infrared radiation when electric current was applied and received the patent for the infrared LED.
The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company. Holonyak is seen as the "father of the light-emitting diode".
M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972.
In 1976, T.P. Pearsall created the first high-brightness, high efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.
SCIENTIFIC DELAILS
A light-emitting diode or an LED is a small semi-conductor chip often less than one millimeter square located in the centre of the bulb.
Just as you would run a current through a filament in an incandescent light bulb, you can run current through a semi-conductor chip, and it emits light in the visible spectrum. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions.
Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region. In other words when the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the colour of the light is determined by the energy gap of the semiconductor.
In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons. This very small chip with a lead frame package that supports the wires and holds the chip in place is the substrate of an LED. It’s typically a little piece of metal with a lead coming off of it and another piece of metal with a wire that connects to the other side of the LED. That whole thing gets encapsulated in an epoxy or clear silicon. That individual package is called an LED, and it’s essentially a miniature light bulb.
WHAT IS THE DIFFERENCE?
When you pass current through the LED, it doesn’t get hot like a filament in an incandescent light bulb gets hot. An LED produces light through a completely different physical process. It’s an electronic process essentially. Here the chip itself isn’t getting white hot, it’s simply producing light directly from the current that’s passing through it. The whole thing gets encapsulated into that epoxy, and there’s no air gap inside it. If you look at an LED, there’s usually a little dome or lens of some sort as part of that package, but the whole thing is solid. That’s where we get the term solid-state lighting.
Most of the time we cluster that individual LED with other similar LEDs on a circuit board, and that package is called anLED array. To power the array, we need an electronic driver. The electricity can come straight from your standard wall jack, and the electronic driver conditions the 120V AC electrical power and sends it to the LED array in a form that’s appropriate, usually DC. The whole thing — the LED array, the electronic driver and any reflectors or lenses that are added to provide a very specific light distribution — gets put into some sort of housing that brings everything together.
WHAT CAUSES THE LED TO EMIT LIGHT AND WHAT DETERMINES THE COLOUR OF THE LIGHT?
When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions. In the p region there are many more positive than negative charges. In the n region the electrons are more numerous than the positive electric charges.
When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region. Once in the p region the electrons are immediately attracted to the positive charges due to the mutual Coulomb forces of attraction between opposite electric charges. When an electron moves sufficiently close to a positive charge in the p region, the two charges "re-combine".
Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus).
Only photons in a very narrow frequency range can be emitted by any material. LED's that emit different colours are made of different semi-conductor materials, and require different energies to light them.
HOW MUCH ENERGY DOES AN LED EMIT?
The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different coloured LEDs emit predominantly light of a single colour.
TYPES OF LED
There are four common LED types:-
The first and most common is the 'Pinned LED'. Most popular being 5mm round types. They are constructed from a metalleadframe onto which the light emitting die is placed a body is then moulded around the leadframe forming the completed ;LED.There are many variations to choose from for example Diffused which gives a wide angle of light or waterclear which gives a more directional beam. They are good general purpose LED’s suitable for a wide range of applications, easy to use and simple to assemble.
The second type is the 'Surface Mount LED'. These generally have the smallest body a die chip can be mounted into. They need to be fitted onto a PCB which can either be rigid or flexible. They offer wide view angles of light and are ideal where space is at a premium.
The third is the 'Power LED' these are high light output LEDs. They will be mounted on some form of heat conductive material which is needed to draw heat away from the LED die. These are the brightest LED’s manufactured and use large die chips or multiple chips placed in a tight cluster to achieve the brightness.
The fourth type called 'C.O.B (Chip on Board)' uses the LED die bonded directly to a PCB or substrate. It eliminates the need for an LED body or leadframe to soldered to. It is very low cost in high volume applications and allows very small and intricate layouts to be produced.
LED FACTS
Efficiency: LEDs emit more light per watt than incandescent bulbs. Their efficiency is not affected by shape and size, unlike Fluorescent light bulbs or tubes.
Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.
On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.
Cycling:LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting.
Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.
Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.
Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.
Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
Low toxicity: LEDs do not contain mercury, unlike fluorescent lamps.
TECHNOLOGY COMPARISON
No Perfect Artificial Light Source Exists (yet)
LED lifetime and Lumen Maintenance
• Operating under reasonable design conditions LEDS have a projected lifetime of 50,000 hours at 70% lumen maintenance
• LEDs inherently fail “gracefully” – no burn out, catastrophic failure
• Up to 100,000 hours (>11 years continuous life) can be projected
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