All You Need to Know About LED

Ⅰ. What is a light-emitting diode?

A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in semiconductors recombine with electron holes, releasing energy in the form of photons. The color of light (corresponding to the energy of the photon) is determined by the energy required for electrons to cross the semiconductor band gap. It obtains white light by using multiple semiconductors or coating a semiconductor device with a layer of light-emitting phosphor.

Ⅱ. LED symbols

The symbol for an LED is similar to a normal PN junction diode, except it includes an arrow pointing away from the diode, indicating that the diode is emitting light. LED are available in different colors. The most common colors of LED are orange, yellow, green and red. The schematic symbol for an LED does not represent the color of the light. The schematic symbols are the same for all colors of LED. Therefore, we cannot identify the color of an LED by its symbol.

Ⅲ. History of LED

In 1961, Robert Biard and Gary Pittman of the American company Texas Instruments first discovered the infrared radiation of gallium arsenide and other semiconductor alloys.

In 1962, General Electric consulting engineer Nick Holoniak invented the first visible light LED. It is a red LED that uses gallium arsenide phosphide as the substrate for the diode. Holoniak has earned the honor of being called the "Father of Light-Emitting Diodes" for his contributions. He also holds 41 patents, and his other inventions include the laser diode and the first dimmer.

In 1976, Thomas Pearsall invented an efficient and extremely bright LED for use in fiber optics and fiber optic communications. He also invented new semiconductor materials optimized for the wavelengths transmitted by optical fibers.

In 1993, Shuji Nakamura, who worked at Nichia Corporation in Japan, successfully doped magnesium to create blue light based on wide-bandgap semiconductor materials gallium nitride and indium gallium nitride (InGaN), which has commercial value.

In 2002, 5W light-emitting diodes began to appear on the market, and its efficiency was about 18-22 lm (lumen) per watt.

In September 2003, Cree Inc. demonstrated its new blue light-emitting diode, which has an efficiency of 35% at 20 mW. They also produced a commercial white LED that reaches 65 lm/W (lumens per watt). This was the brightest white LED on the market at the time. In 2005 they showed a prototype of a white light-emitting diode. It achieves a record efficiency of 70 lm per watt at 350mW.

In February 2009, Japanese light-emitting diode manufacturer Nichia released a light-emitting diode with an efficiency as high as 249 lm/W.

In April 2012, American light-emitting diode manufacturer Cree launched a luminous effect of 254 lm/W, which once again refreshed the power.

In 2014, Japanese engineer Hiroshi Amano, Isamu Akasaki and Shuji Nakamura jointly won the Nobel Prize in Physics for their "invention of high-brightness blue light-emitting diodes that brought energy-saving and bright white light sources". Some critics argued that it was unfair for the Nobel Prize to skip the inventor of the red and green LEDs. But Per Delsing, Chairman of the Nobel Committee, refuted it in an exclusive interview with the Yomiuri Shimbun. He insisted that the award was made after carefully studying the contribution of the three people to the invention.

Ⅳ. How does a LED Work?

Just like regular diodes, LED only work in forward bias, where the anode is held at a higher voltage than the cathode, or the anode is connected to the positive terminal and the cathode is connected to the negative terminal. The n region is dominated by electrons and the p region is dominated by holes. Besides that, the n-type layer is heavily doped compared to the p-type layer.

When the LED is forward biased, the applied potential starts to push the P and N layers. As a result, the depletion region or active layer begins to shrink. Thus, electrons from the n-region and holes from the p-region start to pass through the junction. It starts to recombine in the active or depleted zone. During recombination, electrons from a higher energy band (conduction band) fall into a lower energy band (valence band) by recombining with holes (without electrons in the valence band), releasing energy in the form of light. After several times of recombination, the width of the depletion region further decreases and the intensity of light increases. The figure below shows the light emission due to the combination of electron-hole pairs under forward bias.

Ⅴ. The IV characteristics of LED

Before an LED can "emit" any form of light, it needs to have current flowing through it. Because it is a current-dependent device, the light output intensity is directly proportional to the forward current flowing through the LED. Since the LED will be connected to the power supply in a forward biased condition, we should use a series resistor to current limit it to prevent excessive current flow. We should never connect the LED directly to a battery or power supply, as too much current will flow through it and burn it, so the LED will fail almost immediately.

It can be seen from the above table that each light emitting diode has its own PN junction voltage drop. This parameter depends on the semiconductor material used, and it represents the forward voltage drop at a certain forward conduction current. The current is 20mA.

In most cases, the LED is powered from a low-voltage DC source and the forward current is limited to a safe value (from 5mA for simple LED indicators to 30mA or more where high brightness light output is required) using a series resistor R S .

Ⅵ. The characteristics of LED

LED lights are light-emitting diodes. It uses solid semiconductor chips as light-emitting materials. Compared with traditional lamps, LED lamps are energy-saving, environmentally friendly, and have better color rendering and response speed.

1. Energy saving

This is the most prominent feature of LED lights. In terms of energy consumption, the energy consumption of LED lamps is one-tenth of that of incandescent lamps and one-fourth of that of energy-saving lamps. It is precisely because of this characteristic of energy saving that the application range of LED lights is very wide.

2. Environmental protection

LED lamps do not contain any heavy metal materials such as mercury, but fluorescent lamps do. This reflects the environmental protection characteristics of LED lights. Now people attach great importance to environmental protection, so more people are willing to choose environmentally friendly LED lights.

3. Fast response

Another outstanding feature of LED lights is that the response speed is relatively fast. As long as the power is turned on, the LED light will light up immediately. Compared with the energy-saving lamps we usually use, its response speed is faster. Traditional light bulbs tend to take a long time to light a room. After the bulb is completely heated, it can light up.

4. Compared with other light sources, LED lights are more "clean"

The so-called "clean" does not refer to the cleanliness of the surface and interior of the lamp, but that the lamp is a cold light source that will not generate too much heat and will not attract those insects that love light and heat. Especially in summer, there will be a lot of bugs in the countryside. Some bugs are heat-loving by nature, and incandescent lamps and energy-saving lamps will generate heat after a period of use. This heat is easy to attract insects. This will undoubtedly bring a lot of pollutants to the surface of the lamp. Moreover, the excrement of bugs can also make the room dirty. However, LED light is a cold light source, so it will not attract bugs. This way there will be no bug excrement. So LED lights are more "clean".

5. It can work in high speed switching state

When we usually walk on the road, we will find that every screen or picture composed of LED is unpredictable. This shows that LED lights can perform high-speed switching. However, the incandescent lamps we usually use cannot reach such a working state. In life, if the number of switches is too many, it will directly cause the filament of the incandescent lamp to break. This is also an important reason for the popularity of LED lights.

Ⅶ. The main parameters of LED

1. Luminous Intensity (Iv)

It represents the luminous flux emitted per unit solid angle in the vertical direction of the die when the light-emitting diode passes through the specified forward current. It is generally measured in millicandles (mcd). This is the parameter that represents the brightness of the light-emitting diode and is therefore the most important parameter.

2. Forward Voltage

It refers to the voltage generated between the anode A and the cathode K when a forward current flows. Its unit is V (volts). When the forward voltage is less than a certain value (called threshold), the current is extremely small and no light is emitted. When the voltage exceeds a certain value, the forward current increases rapidly with the voltage and emits light. And beyond the normal operating voltage, the LED may be broken down. Usually, LED with different luminous colors have different normal forward voltages, such as red lights are usually between 1.8~2.0V, while blue lights and white lights are between 2.8~3.3V

3. Forward Current 

It refers to the current generated between the anode A and the cathode K when a forward current flows. Its unit is A (Ampere). Generally, the forward current of LED should be controlled within 20mA. Excessive current can easily cause the temperature of the junction between P and N of the LED to be too high, resulting in damage to the LED or reducing the service life. In addition, LED is like ordinary diodes. When the voltage increases, the current will increase to a large extent, so generally LED light-emitting diodes are connected in series with protective resistors.

4. Maximum Forward Current 

It refers to the maximum forward DC current allowed to be added. If this value is exceeded, the LED may be damaged.

5. Maximum Reverse Voltage 

It refers to the maximum reverse voltage allowed to be added. If this voltage is exceeded, the LED may be damaged by breakdown

6. V-I Characteristics

It refers to the relationship between LED voltage and current. When the forward voltage is less than a certain value (threshold), the current is extremely small.

7. Operating Temperature

It refers to the temperature range within which normal operation is possible. If it is lower or higher than this temperature range, the LED will not work properly. It's much less efficient.

8. Maximum Power Consumption 

It refers to the maximum value allowed to be applied to the product of the forward DC voltage across the LED and the current flowing through it. If this value is exceeded, the LED will heat up and be damaged. Generally speaking, the working voltage of LED is 2-3.6V, and the working current is 0.02-0.03A. That is to say, it consumes no more than 0.1W of power. Under the same lighting effect, it can save more than 80% energy than traditional light sources.

9. Service Lifespan

People call the LED light source a longevity lamp. It is a solid cold light source, encapsulated by epoxy resin, and there is no loose part in the lamp body, and there are no disadvantages such as filament luminous and easy to burn, heat deposition, and light decay. Under the appropriate current and voltage, its service lifespan can reach 60,000 to 100,000 hours, which is more than 10 times longer than that of traditional light sources.

Ⅷ. The application of LED

Due to the advantages of high efficiency, energy saving and long life, LED has been widely used in various fields. Here are some common application scenarios:

1. Medical industry

With the gradual advancement of research on light-emitting diodes, LED can promote plant growth. UV LED is also widely used in the field of sterilization. The modern LED blue light therapy then appeared, which can release suitable blue light anytime and anywhere, which can help relieve stress, relieve mood, and effectively relieve body functions. The application of LED light sources in the medical field continues to innovate.

2. Traffic lights

Traffic lights have long used LED equipment, mainly because LED light sources can directly emit monochromatic light, and it was originally used for a long life abroad. Low-efficiency 140-watt incandescent lamps are used as traffic lights, which produce 2000 lumens of white light. After passing through the red filter, 90% of the light is lost, leaving only 200 lumens of red light. When it is changed to an LED light source, it only needs to consume 14 watts to produce the same light effect. This allows LED to quickly occupy the traffic light market.

3. Automotive lighting

Ultra-bright LED can not only be used as brake lights, tail lights and direction lights of cars, but also as instrument lighting and interior lighting. It has outstanding advantages over incandescent lamps in terms of shock resistance, power saving and long life. When used as a brake light, it has a response time of 60ms. Driving on a typical highway, it will add 4-6m safety distance. As an outstanding representative of low-carbon environmental protection, LED has low voltage and high light efficiency. This makes it quickly become the darling of the field of automotive lighting, and harvest tens of billions of market demand. Therefore, the LED automotive lighting market has a bright future.

4. Lighting

Due to the increase in brightness and price reduction of light-emitting diodes, coupled with long life and power saving, the drive and control are simpler than neon lights, not only can flash, but also change color, so single-color, multi-color and even The color-changing luminous column is matched with various luminous units of other shapes to decorate tall buildings, bridges, streets, squares and other landscape projects. Many units have already produced more than 10,000 meters of LED beams and tens of thousands of colored lights, which are being gradually promoted, and it is estimated that they will gradually expand and form a separate industry.

Due to the increase in brightness and the drop in price of light-emitting diodes, in addition, it has the characteristics of long life, power saving, easy driving and control compared with neon lights, and it can flash and change color. Therefore, the single-color, multi-color and even color-changing light-emitting columns made of ultra-high-brightness LED can play a very good decorative role with other shapes of light-emitting units of various colors. We are used to decorate landscape projects such as tall buildings, bridges, streets and squares. Now there are many units producing LED beams of more than 10,000 meters. This will grow in popularity and become an industry of its own.

5. Lighting source

With the emergence of white LED, the superiority of LED lamps has attracted the attention of various businesses in a short time. With the rapid development of technology, the luminous efficiency of today's LED lamps reaches 40%, which is 8 times that of incandescent lamps, 2.5 times that of tungsten lamps, and 2 times that of fluorescent lamps. Due to the strong directionality of light, high usage rate, and cold light source, it is called the fourth-generation lighting source and quickly occupied the market.