Diode (6) Why can LEDs emit light, but ordinary diodes cannot

LEDs are semiconductors known as "light-emitting diodes".

The name is taken from the initials of "Light Emitting Diode". In 1993, a high-brightness blue LED based on gallium nitride was commercialized, followed by the manufacture of a white LED, which attracted attention as the fourth illumination source.

A light-emitting diode (LED) is a semiconductor light source that emits light when an electric current is passed through it; That is, an electroluminescent semiconductor electronic device, in which electrons are recombined with electron holes to release energy in the form of photons.

The core part of the LED structure is the p-n junction, and the perimeter part is sealed with epoxy resin to protect the inner core. When the p-n junction is connected with a forward current, it can emit visible or invisible radiation, which is a composite light source composed of trivalent and pentavalent elements.

LEDs can only be turned on (energized) in one direction, which is called forward bias; When an electric current flows, electrons and holes (electron holes) recombine within it to emit monochromatic light, which is called the "electroluminescence effect". The wavelength and color of light are related to the type of semiconductor material used and the intentional incorporation of elemental impurities. Light-emitting diodes have the advantages of high efficiency, long life, not easy to break, fast response speed, high reliability, and other traditional light sources.

When light-emitting diodes appeared in 1962, they could only emit low-luminosity red light, and HP bought a patent to use them as indicator lights. Later, other monochromatic versions of light were developed, and today, the light emitted is permeable to visible, infrared and ultraviolet light, and the luminosity has also increased to a considerable level. With the advent of white light-emitting diodes, the use of LEDs has gradually developed from the initial indication of indicators and display boards to the lighting applications in recent years. The luminous efficiency of white LEDs has also improved recently, and their cost per 1,000 lumens has become more and more popular in lighting applications in recent years due to the fact that the price has been reduced due to the large amount of capital investment.

How LEDs were invented

The invention of LEDs (light-emitting diodes) was a multi-stage process that involved the contributions of many scientists. Here are some of the key moments in the history of the invention of LEDs:

1. Early Theories and Experiments:

• 1907: United Kingdom scientist H.J. Round first observed the semiconductor material silicon carbide (SiC) emitting light when energized. This is the first time that electroluminescence of a semiconductor material has been recorded.
• 1920s: Russian scientist Oleg Losev further studied this phenomenon and published a paper on the principle of LEDs in 1927, but it did not attract much attention at the time.

Development of Practical LEDs:

• 1962: Nick Holonyak Jr., then an engineer working at General Electric (GE), invented the first practical visible LED (red LED). Holonyak is known as the "father of LEDs".
• 1972: M. George Craford, a student of Holonyak, invented the first yellow LED and greatly increased the brightness of red and orange LEDs. He improved on the gallium nitride phosphorus (GaAsP) material to increase the brightness of LEDs by a factor of ten.
• 1970s and 1980s: Evolving technology led to the advent of more colors of LEDs, including green, yellow, and orange LEDs.

Breakthrough of blue LEDs:

• 1990s: Scientists from Hitachi and Nichia, notably Shuji Nakamura, invented high-brightness blue LEDs. This is a major breakthrough in the use of gallium nitride (GaN) materials. The invention of blue LEDs made full-color displays and white LEDs possible.
• 2014年:Shuji Nakamura、Ismu Akaski和hiroshi Amano因在蓝光LED方面的贡献而获得诺贝尔物理学奖.

Development of White LEDs:

• White LEDs are typically achieved by combining blue LEDs with phosphors. The blue light emitted by the blue LEDs excites the phosphor, which in turn emits yellow light, which combines to produce white light.

Continuous advancements in LED technology have not only produced LEDs in multiple colors in the visible range, but also extended to the ultraviolet and infrared ranges. Today, LEDs are used in a wide range of applications such as displays, lighting, indicators, and communications. If you are more interested in a particular stage or the work of a scientist, you can let me know and I will provide you with more detailed information.

2. Why do LEDs emit light?

A light-emitting diode is a special type of diode. Like ordinary diodes, light-emitting diodes are composed of semiconductor chips that are pre-injected or hybridized to produce the p,n architecture. Like other diodes, current can easily flow from the p-pole (anode) to the n-pole (cathode) in the light-emitting diode, but not in the opposite direction. Two different carriers: holes and electrons flow from the electrode to the p and n structures under different electrode voltages. When a hole and an electron meet to form a recombination, the electron falls to a lower energy level and releases energy in the form of a photon (photon, also known as light).

The wavelength (color) of the light it emits is determined by the bandgap energy of the semiconductor materials that make up the p,n architecture. Since silicon and germanium are indirect bandgap materials, at room temperature, the recombination of electrons and holes in these materials is a non-radiative transition, which does not release photons, but converts energy into heat, so silicon and germanium diodes cannot emit light (they will emit light at a specific temperature at very low temperatures, which must be found at a special angle, and the brightness of the luminescence is not obvious). The materials used in light-emitting diodes are direct bandgap, so the energy is released in the form of photons, and these bandgap energies correspond to the light energy in the near-infrared, visible, or near-ultraviolet bands.

In the early days, light-emitting diodes using gallium arsenide (GaAs) could only emit infrared or red light. With the advancement of materials science, newly developed light-emitting diodes are capable of emitting light waves with higher and higher frequencies. Nowadays, light-emitting diodes can be made into various colors.

Diodes are typically constructed on an N-type substrate with a layer of P-type semiconductors deposited on its surface and linked together with electrodes. P-type substrates are less common, but they are used. Many commercial light-emitting diodes, especially GaN/InGaN, also use sapphire substrates.

Most of the substances used to make light-emitting diodes have a very high refractive index. This means that most of the light waves will be reflected back to matter at the interface between matter and air, so light wave extraction is an important topic for light-emitting diodes, and a lot of research and development has focused on this topic.

The main difference between LEDs (light-emitting diodes) and ordinary diodes is their materials and structures, which result in significant differences in the efficiency with which electrical energy is converted into light energy. Here are some key points that explain why LEDs emit light, while normal diodes cannot:

1. Different materials: LEDs use III-V semiconductor materials, such as gallium arsenide (GaAs), gallium phosphide (GaP), gallium nitride (GaN), etc. These materials have a direct bandgap that allows electrons to directly transition and release photons (light). Ordinary diodes typically use silicon or germanium, which have an indirect bandgap and the transition of electrons occurs primarily in the form of the release of heat energy, rather than light.
2. Different structures: LEDs are designed to optimize the generation and emission of light. LEDs typically incorporate specific dopants and layer structures at the p-n junction to facilitate the generation and release of photons. Ordinary diodes, on the other hand, are designed to optimize the rectification function of the current and do not focus on the generation of light.
3. Energy band gap: The material of LEDs has a large band gap energy, which means that the energy released by the electrons during the transition is high enough to be manifested in the form of light. The material band gap energy of ordinary diodes is small, and the electrons are mainly released in the form of heat energy during the transition.
4. Luminescence mechanism: When the p-n junction of the LED is positively biased, the electrons move from the n-region to the p-region, recombine with the holes, and the energy is released in the form of photons, producing light. Whereas, in ordinary diodes, the recombination of electrons and holes is mainly carried out in the form of non-radiative recombination, i.e., the energy is released in the form of heat.

These differences allow LEDs to emit light while operating, while ordinary diodes cannot.

3. How to improve the luminous efficiency of LEDs?

Methods for improving the luminous efficiency of LEDs (light-emitting diodes) involve a number of aspects, including material selection, structural design, and manufacturing processes. Here are some of the main strategies:

1. Optimize material selection

• High-efficiency light-emitting materials: Use semiconductor materials with high luminous efficiency, such as gallium nitride (GaN), gallium arsenide (GaAs), and gallium phosphide (GaP). These materials have a large band gap that efficiently converts electrical energy into light energy.
• Doping technology: The optical properties of the material are adjusted by doping specific elements (e.g., aluminum, gallium, nitrogen) to improve the luminous efficiency of LEDs.

2. Improve structural design

• Optimize the design of the p-n junction: By improving the structure of the p-n junction, the recombination efficiency of electrons and holes is optimized, so as to improve the luminous efficiency.
• Optical design: Design efficient optical structures, such as optical lenses and optical cavities, to reduce light loss and improve light output.

3. Improve packaging technology

• Optical encapsulation: Use a high light transmittance encapsulation material, such as silicone or epoxy, to reduce the absorption or scattering of light during the encapsulation process.
• Reflective layer: A reflective layer is added to the LED package to reflect the light that is not emitted effectively, improving the overall output of light.

4. Improve the manufacturing process

• Reduced defect density: By optimizing the manufacturing process, crystal defects and material impurities are reduced, thereby improving the luminous efficiency and stability of LEDs.
• Improve quantum efficiency: Increase the quantum efficiency (QE) of LED chips, that is, the proportion of electrical energy converted into light energy, and reduce non-radiative recombination.

5. Thermal management

• Improved thermal design: Effective thermal management can reduce the temperature of LEDs when they are operating, preventing light degradation due to overheating. Use high thermal conductivity materials and design effective heat dissipation structures such as heat sinks and cooling fans.

6. Adjust the current and drive mode

• Appropriate Current: Controls the current to avoid loss of efficiency caused by excessive current. The constant current drive can keep the LED in the best working condition.
• Driver circuit optimization: Optimize the design of LED driver circuit, improve the conversion efficiency of electric energy, and reduce energy loss.

7. Quantum dots and phosphors technology

• Quantum dot technology: The introduction of quantum dots into LEDs can improve color purity and light efficiency.
• Phosphor coating: For white LEDs, optimizing the formulation and coating process of phosphor can improve the light efficiency and color quality.

4. Yellow LED

On the road of innovation, there is no end, Jiang Fengyi never dares to stop, from "following" to "chasing", and then to "super". In 2011, he and his team expanded the research direction - the research and development of LED high-end equipment (MOCVD system), and in 2014, the successful development of production MOCVD equipment (37 chips and 61 chips), in which the heart of the equipment - the reaction tube to achieve independent innovation, the silicon substrate blue LED produced with this equipment, its electro-optical conversion efficiency is the same as the international advanced level of sapphire substrate blue LED produced by imported equipment; In 2016, he and his team made a historic breakthrough in the yellow LED developed with this device, and its electro-optical conversion efficiency reached 21.5%, much higher than the highest level (9.63%) that can be publicly reported or queried abroad, and in 2019 they increased this light efficiency to 27.9%; In 2020, the light efficiency of the reddish LED developed by them is internationally leading. These inventions and creations have made the mainland LED technology in the international "partial leading" position.