Introduction
The structural tolerance of semiconductor light-emitting diodes is not as strict as that of lasers, and there is no resonant cavity. Therefore, the emitted light is not laser, but fluorescence. LED is a device that works with an external forward voltage. Under the action of the positive bias, the electrons in the N region will diffuse in the positive direction and enter the active layer, and the holes in the P region will also diffuse in the negative direction and enter the active layer. The electrons and holes entering the active layer are enclosed in the active layer due to the heterojunction barrier, forming a population inversion distribution. These electrons whose populations are inverted and distributed in the active layer will generate spontaneous radiation when they recombine with holes through transitions.
Advantages
Semiconductor light-emitting diodes are simple in structure, small in size, low in working current, easy to use, and low in cost, so they are extremely popular in optoelectronic systems.
Under forward bias, a semiconductor PN junction or similar structure can emit visible light or near-infrared light. This device that directly converts electrical energy into light energy is called a light-emitting diode, or LED for short.
Luminescence is the process by which energy stored in a certain way inside an object is converted into light radiation. The light radiation of a light-emitting object is generated when the excited electrons in the material transition to the ground state. Semiconductors (mainly compound semiconductors composed of Group III and Group V elements in the periodic table) light-emitting diodes are electroluminescent devices excited by current.
The phenomenon of electroluminescence was discovered in 1923 and did not attract people's attention at that time. With the development of modern technology, new requirements are put forward for light-emitting devices, and it is hoped that the light-emitting tube is simple, reliable, long-lived, low-cost, and miniaturized. Therefore, the research on electroluminescence has been very active since the 1960s.
Luminescence mechanism
Atoms, molecules and certain semiconductor materials can absorb and emit light or electromagnetic waves of a certain wavelength, respectively. According to the energy band theory of solids, the energy state of electrons in semiconductors is divided into valence band and conduction band. When an electron transitions (transfers) from an energy state E1 in one band to an energy state E2 in another band, it will emit or absorb a certain frequency. (υ) the light. υ is proportional to the energy difference (ΔE=E2-E1), that is,
υ=ΔE/h (Hz)
This formula is called the Bohr condition. In the formula, h=6.626×10-34J·s. When the light-emitting diode is working, under the forward bias, the empty conduction band of the semiconductor is usually occupied by electrons injected into it through the junction. These electrons recombine with the holes in the valence band to emit photons, which generates light. The emitted photon energy is approximately the band gap energy between the conduction band and the valence band of a specific semiconductor. This natural emission process is called spontaneous radiation recombination (Figure 1). Obviously, the radiation transition is the basis of recombination luminescence. The recombination of injected electrons may also be non-luminous, that is, non-radiative recombination. In the case of non-radiative recombination, the energy lost by the conduction band electrons can become multiple phonons, causing the crystal to heat up. This process is called multi-phonon transition; it can also recombine with valence band holes and transfer the energy to the conduction band. Another electron in the band puts it in a high-energy state, and then transfers the excess energy to the lattice through the process of thermal equilibrium. This process is called Auger recombination. As the electron concentration increases, this process will become more important. During the transition between bands, the two processes of radiative recombination and non-radiative recombination compete with each other. Some luminescent materials are predominant in radiation recombination.
LED structure
The structure of the LED varies depending on the application and material doping. LEDs used for visible light indication and display require structural optimization to obtain high efficiency; LEDs used for optical communications require high radiance to couple the maximum power into the fiber, and greater modulation capabilities are also desired. The basic structure of LED used as indicator light and display is shown in Figure 2.
Semiconductor light-emitting diodes
The emission wavelength of the LED for optical communication must be in the window area where the optical fiber exhibits low loss. The 0.8-0.9 micron GaAlAs-GaAs light-emitting tube and the 1.3-1.6 micron InGaAsP-InP light-emitting tube, the wavelength falls on the first and second transparent windows of the quartz fiber, respectively. In order to couple with the fiber, light can be extracted from one side or one side of the LED.
Requirements for LEDs
①Improve the internal quantum efficiency and reduce crystal defects and harmful impurities as much as possible; ②Improve the external quantum efficiency, and the structure should be convenient for light collection, extraction and emission; ③ The information-carrying output current can be used to directly modulate the optical output at a high rate; ④The structure should be conducive to heat dissipation and reduce the drop in optical power caused by the rise in junction temperature; ⑤It must have high radiance, so direct band gap semiconductors must be used And a structure that can be driven at high current density.
Characteristics of LED
Work at low voltage (less than 2 volts) and low current (tens of milliamps to 200 milliamps), low power consumption, small size, and direct Used in connection with solid-state circuits; stable, reliable, and long life (105-106 hours); convenient modulation, modulating the light output by modulating the current of the LED; the light output response speed is relatively fast (1-100 MHz); the price is cheap. "Application" LED can be used as indicator light, text-digital display, optical coupling device, light source of optical communication system, etc.
Calculation of LED luminous flux
φ=2πÌ(1-cos½α)
Device characteristics
①Output light power and current and temperature The relationship is shown in Figure 3. For surface light emitting devices, when the driving power is small, the output power increases linearly with the current, and when the current is large, it becomes sub-linear, and the quantum efficiency also decreases, which is caused by the temperature rise of heat. Due to the considerable gain of the super luminescent diode, the device has a strong super-linear behavior, and the temperature sensitivity increases with the increase of the gain; ②spectral characteristics, the surface-emitting LED spectrum belongs to the spontaneous emission spectrum.
GaAlAs (gallium aluminum arsenic) device with an emission wavelength of 0.85μm, the spectral width is about 40nm. The 1.3μm InGaAsP (Indium Gallium Arsenide Phosphorus) device is 110nm. The end-emission LED, due to self-absorption along the length of the active area, has a narrower spectrum width than the surface-emission LED. For super luminescent diodes, the appearance of optical gain makes the spectrum further narrower; ③The output power and modulation bandwidth affect the output power of the LED. The factors that adjust the bandwidth include interface non-radiative recombination, self-absorption and carrier leakage. These factors are related to device parameters, such as doping concentration, minority carrier diffusion length, absorption coefficient, active layer thickness, band gap energy difference between active layer and confinement layer in double heterojunction, and injection current density. These parameters are related again. The modulation bandwidth has a reciprocal relationship with the output power, and the product of power and bandwidth is also different for a specific material and process level.
Indicator light
LEDs used as indicator lights have two structures: radial lead structure and axial lead structure. The former is small in size and low in price, and is suitable for installation on a printed circuit board; the latter can be installed on the instrument panel or directly on the printed circuit board. The smallest LED indicator designed specifically for printed circuit boards, compatible with transistors and integrated circuits, to indicate circuit status and faults. LED can be used as a TV channel tuning indicator, and can also be used in high-fidelity radios and tape recorders, as well as in the automotive, aircraft, and electromechanical industries. Most indicator lights are single-tube cores, and new developments include two-color and multi-color lights.
Alphanumeric
LED is used as a pocket calculator. The digital display of digital watches and electronic instruments is generally a seven-segment display. But for desktop calculators, the more flexible display method (to produce full text-numbers) is a 35-dot matrix, where the LEDs are arranged in a 7×5 array. The 35-dot matrix is expensive and the drive circuit is complicated, and its application is not as wide as the seven-segment display device. Optical coupling device The combination of GaAs (or GaAsP) LED and Si-PN junction detector can be made into many new devices for optical-electricity and electric-optical transmission, usually called optical coupling devices. The well-known is an opto-coupled isolator, which is high-speed, reliable, and provides electrical isolation as high as 2.5 kV. GaAsLED and Si transistors are combined to form a punch card or tape optoelectronic reading head, which is more reliable, stable, shock-resistant and less power consumption than the radial distribution of tungsten filament lamps. The automobile ignition device using optical coupling device is fuel-efficient, easy to start, and works smoothly. GaAlAs negative resistance light-emitting diodes can be used as light-emitting switches, controllable light-emitting rectifiers and light wavelength conversion.
Light source
Applications such as communications, information processing, and optical coupling require LEDs to have good directivity. The two main light sources suitable for optical communication applications are high-radiance LEDs and semiconductor injection lasers. LED is stable, reliable, long life, simple drive circuit, power is not sensitive to temperature, and is widely used as the light source of medium and short distance (railroad, electric power, transportation, public security, etc.) optical communication systems. The bandwidth of the GaAlAs-GaAs surface light emitting tube is 10-20 MHz, which is suitable for the secondary group optical communication system (which can transmit 120 telephones), and the transmission distance is greater than 5 kilometers. The GaAlAs-GaAs fast edge luminous tube has a bandwidth of 50-100 MHz, which is suitable for the three-level group optical communication system (which can transmit 480 telephones), and the transmission distance is several kilometers. InGaAsP-InP LEDs can be used in longer-distance (more than 10 kilometers) transmission systems. In addition, LEDs are also used for information processing, image transmission, ranging and sensing.